a  Guidelines for the Environmentally Optimized Design of Low Volume Roads @ 2018 The International Bank for Reconstruction and Development/The World Bank 1818 H Street NW, Washington DC 20433, USA; Telephone: 202-473-1000 Internet: www.worldbank.org E-mail: feedback@worldbank.org All rights reserved This volume is the product of the staff of the International Bank for Reconstruction and Development/The World Bank. The findings, interpretations, and conclusions expressed in this volume do not necessarily reflect the views of the Executive Directors of the World Bank or the Governments they represent. The World Bank does not guarantee the accuracy of the data included in this work. The boundaries, colors, denominations, and other information shown on any map in this work do not imply any judgment on the part of The World Bank concerning the legal status of any territory or the endorsement or acceptance of such boundaries. 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Ltd. | www.macrographics.com Table of Contents FOREWORD vii ACKNOWLEDGEMENTS ix TERMINOLOGY x LIST OF ABBREVIATIONS xv 1 INTRODUCTION 1 1.1 Background 1 1.2 Purpose 1 1.3 Scope 1 1.4 Structure 3 1.5 Benefits 4 1.6 Sources of Information 4 2 LOW VOLUME ROADS IN PERSPECTIVE 5 2.1 Introduction 5 2.1.1 Background 5 2.1.2 Purpose and Scope 5 2.2 Characteristics of Low Volume Roads 5 2.2.1 Definition 5 2.2.2 Special features 6 2.3 Approach to Design 6 2.3.1 General approach 6 2.3.2 Influence of road environment 7 2.3.3 Road deterioration factors 7 2.3.4 Towards Sustainable Roads 8 2.3.5 Environmentally optimized design 9 2.4 Risk Factors 11 3 SURVEYS AND INVESTIGATIONS 13 3.1 Introduction 13 3.1.1 Background 13 3.1.2 Purpose and Scope 13 3.2 Traffic 14 3.2.1 General 14 3.2.2 Traffic Surveys 15 3.2.3 Procedure for Determining Design Traffic 18 3.3 Route Survey 25 3.3.1 General 25 3.3.2 DCP surveys 25 Table of Contents  iii  3.3.3 Test pits and Sampling 28 3.3.4 Moisture and Drainage 28 3.3.5 Problem subgrades 29 3.4 Construction Materials Survey 42 3.4.1 General 42 3.4.2 In situ materials 45 3.4.3 Borrow pit materials 45 3.4.4 Aggregate sources 46 3.4.5 Typical Local Materials and Industry By-Products 47 3.4.6 Material improvement 51 3.4.7 Construction water 55 3.5 Materials Testing 56 3.5.1 General 56 3.5.2 Test methods 56 4 PAVEMENT DESIGN 59 4.1 Introduction 59 4.1.1 Background 59 4.1.2 Purpose and Scope 59 4.2 Design Principles 60 4.2.1 Approach to design 60 4.2.2 Pavement structure and function 60 4.3 Design Methods 61 4.3.1 General 61 4.3.2 DCP Design Method 63 4.3.3 CBR Design Method 70 4.3.4 Comparison of DCP-DN and IRC DCP Catalogues 70 4.4 Practical Considerations 70 4.4.1 General 70 4.4.2 Compaction 70 4.4.3 Drainage 71 4.4.4 Shoulders 72 5 SURFACINGS 75 5.1 Introduction 75 5.1.1 Background 75 5.1.2 Purpose and Scope 75 5.2 Bituminous Surfacings 75 5.2.1 General 75 5.2.2 Surfacing Types 76 5.3 Non-Bituminous Surfacings 76 5.3.1 General 76 5.3.2 Surfacing Types 77 5.4 Surfacing type selection 77 5.4.1 Selection criteria 77 5.4.2 Gradient 78 5.4.3 Turning actions/external stresses 78 5.4.4 Macrotexture 78 5.4.5 Preliminary selection guideline 79 iv  Guidelines for the Environmentally Optimized Design of Low Volume Roads References 82 LIST OF ANNEXES ANNEX- I EXAMPLE OF DESIGN TRAFFIC DETERMINATION 85 ANNEX- II: DETERMINATION OF UNIFORM SECTIONS FROM CUSUM ANALYSIS 87 ANNEX- III: DETERMINATION AND CHOICE OF DN PERCENTILE VALUES 89 ANNEX-IV: COMPARISON OF DCP-DN AND IRC CBR CATALOGUES 91 ANNEX-V: DCP DESIGN EXAMPLE 95 LIST OF FIGURES Figure 1: Structure of Guidelines 3 Figure 2: Various road environment factors affecting design 7 Figure 3:  Traffic loading versus dominant mechanism of pavement distress (schematic) [1] 8 Figure 4: LVR implementation within an EOD context 9 Figure 5: Environmentally optimized and spot improvement design [schematic] 11 Figure 6: Possible errors in ADT estimates from random counts of varying duration [12] 14 Figure 7: Seasonal variations in rural traffic [6] 17 Figure 8: Procedure for establishing design traffic category 19 Figure 9: Traffic development over time on a new or improved road [schematic] 20 Figure 10: DCP effects where large stones are present 27 Figure 11: Cracking on expansive clay 31 Figure 12: Identification of expansive clay soils and estimate of expansion [16] 32 Figure 13: Possible solutions for the construction of roads on active clays 33 Figure 14: Typical moisture movement regime under roads on expansive clays 34 Figure 15: Dispersive soil showing formation of “pipes” 35 Figure 16: Dispersive soil (crumb) test showing suspension that does not settle out 35 Figure 17: Mechanism and manifestation of soluble salt damage to bituminous surfacings 37 Figure 18: Typical manifestation of collapsible subgrade 41 Figure 19: Illustrative soil strength/suction relationship 44 Figure 20: Change in strength (DN in mm/blow) as a result of blending 53 Figure 21: Determination of laboratory DN value 57 Figure 22: DN/density/moisture relationship 58 Figure 23: Dispersion of surface load through a granular pavement structure 61 Figure 24: Flow diagram of DCP design procedure [16] 64 Figure 25: Example of uniform sections obtained from CUSUM analysis of DCP results 65 Figure 26: Average maximum and minimum strength profiles for a uniform section 67 Figure 27: DCP layer-strength diagram for different traffic classes 68 Figure 28: Comparison of DCP design and in situ strength profiles 68 Figure 29: Benefits of compaction to refusal 71 Figure 30: Minimum drainage requirements 72 Figure 31: Bituminous Surfacings for LVRs 76 Table of Contents  v  Figure 32: Examples of typical non-bituminous surfacings for LVRs 77 LIST OF TABLES Table 1: Vehicle classification system 17 Table 2: Equivalency Factors for different axle loads 22 Table 3: Indicative VDF per vehicle type 23 Table 4: Lane width adjustment factors for design traffic loading 24 Table 5: Traffic categories 24 Table 6: Frequency of DCP testing 26 Table 7: Pavement material types and characteristics 43 Table 8: Variation of CBR with moisture content 45 Table 9: Classification and summary of uses and properties of by-product materials 48 Recommended percentiles of minimum in situ strength profile Table 10:  to be used 66 Table 11: DCP design catalogue for different traffic classes 67 Recommended crown height in relation to drain type and Table 12:  longitudinal gradient 72 Table 13: Construction gradient 78 Table 14: External stresses 78 Table 15: Texture depth by type of macrotexture 79 Table 16: Guide for selection of LVR bituminous surfacings 81 vi  Guidelines for the Environmentally Optimized Design of Low Volume Roads ACKNOWLEDGEMENTS The “Guidelines for the Environmentally Optimized Design of Low Volume Roads” have been prepared by the World Bank Transport & ICT (T&I) Global Practice of the World Bank Group. The study was funded by the Korea Green Growth Partnership (KGGP) and the Department of Foreign Affairs and Trade (DFAT), Australia. The activity was led by Ashok Kumar, and the team comprised Arnab Bandyopadhyay, Reenu Aneja, D.P. Gupta, Rashi Grover, and Lakshmi Narayan. The international panel of experts engaged to prepare these Guidelines included Philip Paige-Green, Mike Pinard, Gerrie Van Zyl and Dr. Arun Kumar. The team gratefully acknowledges the useful insights and guidance received from Karla Gonzalez Carvajal (Practice Manager) and Shomik Raj Mehndiratta (Practice Manager) as well as the support and guidance provided by Eun Joo Allison Yi (KGGTF Program Manager) and Laurent Durix (Consultant). The team gratefully acknowledges the cooperation and support provided by Ministry of Rural Development (MORD), National Rural Roads Development Agency (NRRDA), and the State Rural Road Development Agencies of Bihar, Rajasthan, Karnataka, and Uttarakhand throughout this assignment especially in organizing consultations, workshops and providing insightful inputs. Special thanks are due to Mr Rajesh Bhushan, Additional Secretary, Government of India, Ms Alka Upadhyay, Joint Secretary, MORD, Dr I.K. Pateriya, Director Technical, NRRDA, and Mr Vinay Kumar, Secretary, Rural Works Department, Government of Bihar. ACKNOWLEDGEMENTS  ix  TERMINOLOGY Aggregate Hard mineral elements of construction material mixtures, for example: sand, gravel (crushed or uncrushed) or crushed rock. Asphalt Same as Bitumen. Average The total yearly traffic volume in both directions divided by the number Annual Daily of days in the year. Traffic (AADT) Average Daily The total traffic volume during a given period in whole days greater Traffic (ADT) than one day and less than one year divided by the number of days in that time period. Base Course The main component of the pavement contributing to the spreading of the traffic loads. In many cases, it will consist of crushed stone or gravel, or of good quality gravelly soils or decomposed rock. Materials stabilised with cement or lime can also be used. Bitumen The residue from the refining of crude oil after the more volatile material has been distilled off. It is a viscous liquid comprising many long-chain organic molecules. For use in roads it is practically solid at ambient temperatures but can be heated sufficiently to be poured and sprayed. Borrow Area An area within designated boundaries, approved for obtaining borrow material. A borrow pit is the excavated pit in a borrow area. Borrow Any gravel, sand, soil, rock or ash obtained from borrow areas, dumps Material or sources other than cut within the road prism and which is used in the construction of the specified work for a project. Does not include crushed stone or sand obtained from commercial sources. Boulder A rock fragment, usually rounded by weathering or abrasion, with an average dimension of 0.30 m or more. Capping layer The top of embankment or bottom of excavation prior to construction of the (selected subgrade) pavement structure. Where very weak soils and/or expansive soils (such as black cotton soils) are encountered, a capping layer is sometimes necessary. This consists of better quality subgrade material imported from elsewhere or subgrade material improved by stabilisation (usually mechanical), and may also be considered as a lower quality sub-base. Carriageway That portion of the roadway including the various traffic lanes and auxiliary lanes but excluding shoulders. x  Guidelines for the Environmentally Optimized Design of Low Volume Roads Cross fall The difference in level measured transversely across the surface of the roadway. Culvert A structure, other than a bridge, which provides an opening under the carriageway or median for drainage or other purposes. Cutting Cutting shall mean all excavations from the road prism including side drains, and excavations for intersecting roads including, where classified as cut, excavations for open drains. Chippings Stones used for surface dressing (treatment). Deformed Bar A reinforcing bar for rigid slabs conforming to “Requirements for Deformations” in AASHTO Designations M 31 M. Design Period The period of time that an initially constructed or rehabilitated pavement structure is intended to perform before reaching a level of deterioration requiring more than routine or periodic maintenance. Diverted Traffic Traffic that diverts from another route (or mode of transport) to the project road because of the improved pavement, but still travels between the same origin and destination. Dowel A load transfer device in a rigid slab, usually consisting of a plain round steel bar. Unlike a tie bar, a dowel may permit horizontal movement. Equivalent A measure of the potential damage to a pavement caused by a vehicle Standard Axle load expressed as the number of 8.2 metric tonnes single axle loads (ESA) that would cause the same amount of damage. The ESA values of all the traffic are combined to determine the total design traffic for the design period. Equivalency Used to convert traffic volumes into cumulative equivalent standard Factors axle loads. Equivalent Summation of equivalent 8.16 tonne single axle loads used to combine Standard Axle mixed traffic to calculate the design traffic loading for the design Load (ESA) period. Escarpment Escarpments are geological features that are very steep and extend laterally for considerable distances, making it difficult or impossible to construct a road to avoid them. They are characterised by more than 50 five-metre contours per km and the transverse ground slopes perpendicular to the ground contours are generally greater than 60%. Expansion A joint located to provide for expansion of a rigid slab without damage Joint to itself, adjacent slabs, or structures. Fill Material of which a man-made raised structure or deposit such as an embankment is composed, including soil, soil-aggregate or rock. Material imported to replace unsuitable roadbed material is also classified as fill. Flexible Include primarily those pavements that have a bituminous (surface Pavements dressing or asphalt concrete) surface. The terms "flexible and rigid" are somewhat arbitrary and were primarily established to differentiate between asphalt and Portland cement concrete pavements. TERMINOLOGY  xi  Formation Level Level at top of subgrade. Generated Additional traffic which occurs in response to the provision of Traffic improved road. Heavy Vehicles Those having an unloaded weight of 3000 kg or more. Hot Mix This is a generic name for all high quality mixtures of aggregates and Asphalt (HMA) bitumen that use the grades of bitumen that must be heated in order to flow sufficiently to coat the aggregates. It includes Asphaltic Concrete, Dense Bitumen Macadam and Hot Rolled Asphalt. Longitudinal A joint normally placed between traffic lanes in rigid pavements to Joint control longitudinal cracking. Maintenance Routine work performed to keep a pavement as nearly as possible in its as-constructed condition under normal conditions of traffic and forces of nature. Mountainous Terrain that is rugged and hilly with substantial restrictions in both (Terrain) horizontal and vertical alignment. It is defined as having 26-50 five-metre contours per km. The transverse ground slopes perpendicular to the ground contours are generally above 25%. Normal Traffic Traffic which would pass along the existing road or track even if no improved pavement were provided. Overlay One or more courses of asphalt construction on an existing pavement. The overlay often includes a levelling course, to correct the profile of the old pavement, followed by a uniform course or courses to provide needed thickness. Pavement The layers of different materials which comprise the pavement Layers structure. Project The specifications relating to a specific project, which form part of the Specifications contract documents for such project, and which contain supplementary and/or amending specifications to the standard specifications. Pumping The ejection of foundation material, either wet or dry, through joints or cracks, or along edges of rigid slabs resulting from vertical movements of the slab under traffic. Quarry An area within designated boundaries, approved for the purpose of obtaining rock by sawing or blasting. Reconstruction The process by which a new pavement is constructed, utilizing mostly new materials, to replace an existing pavement. Recycling The reuse, usually after some processing, of a material that has already served its first-intended purpose. Rehabilitation Work undertaken to significantly extend the service life of an existing pavement. This may include overlays and pre overlay repairs, and may include complete removal and reconstruction of the existing pavement, or recycling of part of the existing materials. Reinforcement Steel embedded in a rigid slab to resist tensile stresses and detrimental opening of cracks. xii  Guidelines for the Environmentally Optimized Design of Low Volume Roads Rigid Pavement A pavement structure which distributes loads to the subgrade having, as the main load bearing course, a Portland cement concrete slab of relatively high-bending resistance. Road base A layer of material of defined thickness and width constructed on top of the sub-base, or in the absence thereof, the subgrade. A road base may extend to outside the carriageway. Road Bed The natural in situ material on which the fill, or in the absence of fill, any pavement layers, are to be constructed. Road Bed The material below the subgrade extending to such depth as affects Material the support of the pavement structure. Road Prism That portion of the road construction included between the original ground level and the outer lines of the slopes of cuts, fills, side fills and side drains. It does not include sub-base, road base, surfacing, shoulders, or existing original ground. Roadway The area normally travelled by vehicles and consisting of one or a number of contiguous traffic lanes, including auxiliary lanes and shoulders. Rolling (Terrain) Terrain with low hills introducing moderate levels of rise and fall with some restrictions on vertical alignment. Defined as terrain with 11-25 five-metre contours per km. The transverse ground slopes perpendicular to the ground contours are generally between 10 and 25%. Side Fill That portion of the imported material within the road prism which lies outside the fills, shoulders, road base and sub-base and is contained within such surface slopes as shown on the Drawings or as directed by the Engineer. A distinction between fills and side fill is only to be made if specified. Side Drain Open longitudinal drain situated adjacent to and at the bottom of cut or fill slopes. Stabilisation The treatment of the materials used in the construction of the road bed material, fill or pavement layers by the addition of a cementitious binder such as lime or Portland Cement or the mechanical modification of the material through the addition of a soil binder or a bituminous binder. Concrete and asphalt shall not be considered as materials that have been stabilised. Sub-base The layer of material of specified dimensions on top of the subgrade and below the road base. The secondary load-spreading layer underlying the base course. Usually consisting of a material of lower quality than that used in the base course and particularly of lower bearing strength. Materials may be unprocessed natural gravel, gravel-sand, or gravel-sand-clay, with controlled gradation and plasticity characteristics. The sub-base also serves as a separating layer preventing contamination of the base course by the subgrade material and may play a role in the internal drainage of the pavement. TERMINOLOGY  xiii  Subgrade The surface upon which the pavement structure and shoulders are constructed. It is the top portion of the natural soil, either undisturbed (but recompacted) local material in cut sections, or soil excavated in cut or borrow areas and placed as compacted embankment. Subsurface Covered drain constructed to intercept and remove subsoil water, Drain including any pipes and permeable material in the drains. Surface The sealing or resealing of the carriageway or shoulders by means Treatment of one or more successive applications of bituminous binder and crushed stone chippings. Surfacing This comprises the top layers(s) of the flexible pavement and consists of a bituminous surface dressing or one or two layers of premixed bituminous material (generally asphalt concrete). Where premixed materials are laid in two layers, these are known as the wearing course and the binder course. Surfacings can also be non-bituminous. Tie Bar A deformed steel bar or connector embedded across a joint in a rigid slab to prevent separation of abutting slabs. Traffic Lane Part of a travelled way intended for a single stream of traffic in one direction, which has normally been demarcated as such by road markings. Traffic Volume Volume of traffic usually expressed in terms of Average Annual Daily Traffic (AADT). Typical Cross- A cross-section of a road showing standard dimensional details and section features of construction. Unbound Naturally occurring or processed granular material which is not Pavement held together by the addition of a binder such as cement, lime or Materials bitumen. Wearing The top course of an asphalt surfacing or, for gravel roads, the Course uppermost layer of construction of the roadway made of specified materials. Welded Wire Welded steel wire fabric for concrete reinforcement. Fabric xiv  Guidelines for the Environmentally Optimized Design of Low Volume Roads LIST OF ABBREVIATIONS AADT Average Annual Daily Traffic AASHO American Association of State Highway Officials (old designation) AASHTO American Association of State Highway and Transportation Officials (current designation) ASTM American Society for Testing Materials CBR California Bearing Ratio DCP Dynamic Cone Penetrometer EOD Environmentally Optimized Design FMC Field Moisture Content ESA Equivalent Standard Axles GGBS Ground Granulated Blastfurnace Slag HCV Heavy Commercial Vehicle HMA Hot Mixed Asphalt HVR High Volume Road ISD Initial Stabiliser Demand LSP Layer Strength Profile LVR Low Volume Road MCV Medium Commercial Vehicle MDD Maximum Dry Density MESA Million Equivalent Standard Axles OMC Optimum Moisture Content PCC Portland Cement Concrete PFA Pulverised Fly Ash PMGSY Pradhan Mantri Gram Sadak Yojana UCS Unconfined Compressive Strength VDF Vehicle Damage Factor VPD Vehicle Per Day, also used in the text as ‘vpd’ LIST OF ABBREVIATIONS  xv  1. INTRODUCTION 1.1 Background  The provision of all-weather road access to all citizens is a key development priority facilitating both accessibility and road connectivity to the many habitations in the rural areas of India, which is being implemented under a national level program, Pradhan Mantri Gram Sadak Yojana (PMGSY) and many sub-national programs. The traffic levels on most of these roads are very low, typically below 300 motorised vehicles per day, consisting of mostly farm tractors and light commercial vehicles with very few heavy commercial vehicles. Whilst there are potentially significant life-cycle benefits to be achieved from upgrading the existing unpaved low volume rural roads to a paved standard, the cost of doing so following traditional approaches to road design and materials utilization can be prohibitive. This is because such approaches tend to be overly conservative for application to Low Volume Roads (LVRs). This has led to a need to develop an alternative design procedure with the objective of enhancing the efficiency and effectiveness of LVR provision and, by extension, reducing the cost of providing much needed connectivity in the rural areas of the country. 1.2 Purpose  The main purpose of these Guidelines are to provide practitioners with the requisite tools for undertaking an environmentally optimized approach to the design of LVRs in India that takes account of the many locally prevailing road environment factors that impact on the design of such roads. Such an approach is aimed at providing appropriate and cost-effective designs for LVRs, bearing in mind their practical implementability within the available resources and level of expertise in rural areas. To the extent possible, the use of locally available materials in their natural state, or after suitable processing, must be maximized to not only reduce construction costs but, also, to minimize adverse environmental impacts regarding the use of non-renewable resources (aggregate and gravel). The availability of plant and equipment for construction and maintenance, as well as the level of quality control that can be effectively exercised in the field should also be considered. 1.3 Scope  The Guidelines take account of the many advances in LVR technology based on a number of international research and investigation projects that have been carried out in environments similar to those prevailing in India [1,2,3,4,5]. The corroborative findings 1. INTRODUCTION  1  of this work provide a wealth of performance-based information that has advanced previous knowledge on various aspects of LVR technology. This has allowed state-of-the- art guidance to be provided in the Guidelines including more extensive use of local and by-product materials, simplified, environmentally optimized pavement design methods, optimization of pavement composition and use of a range of potentially suitable bituminous surfacing options. The Guidelines incorporate the latest approaches to the provision of LVRs that mirror the sequential activities that are typically undertaken in designing such roads, i.e. activities that progress from the surveys and investigations stage, through to the structural design of existing and new roads, selection of appropriate surfacing type, attention to drainage as well as aspects of construction dealing with quality assurance and control. The Guidelines complement and link to relevant aspects of the latest versions of other manuals in India including: ™™ IRC Guidelines for the Design of Flexible Pavements for Low Volume Rural Roads [6]. ™™ New Technology Initiatives under PMGSY [7]. ™™ IRC Guidelines of Road Drainage [8]. ™™ MORD Specifications for Rural Roads [9]. ™™ MORD Quality Assurance Handbook for Rural Roads [10]. ™™ IRC Recommendations for road construction in areas affected by water-logging, flooding and/or salt infestation [11]. The main differences between these Guidelines for Environmentally Optimized Design of Low Volume Roads and the above guidelines are: ™™ It moves away from the more traditional, empirically developed, CBR design approach, which provides an indirect measure of the strength of a material, to a more direct method of measuring in situ shear strength based on the use of the Dynamic Cone Penetrometer (DCP). ™™ It focuses on the use of the DCP for evaluating in situ road conditions and, by integrating the design strength profile optimally with the in situ strength profile, for designing LVR pavement structures in a cost-effective manner that minimizes the use of imported materials. ™™ It facilitates the greater use of local, more abundant, and therefore less expensive, locally available and by-product materials in the road pavement by a variety of techniques for improving strength of these materials and for evaluating their properties in the laboratory using the DCP. ™™ It offers a wider array of relatively low-cost bituminous surfacings in addition to the more traditionally used surface dressing and premix carpet. ™™ It also allows improved consideration of factors such as soaked or unsoaked conditions of pavements. In essence, therefore, the Guidelines provide an alternative approach to the design of LVRs that, in some respects complements, and in other respects enhances, traditional approaches adopted in other IRC guidelines. 2  Guidelines for the Environmentally Optimized Design of Low Volume Roads 1.4 Structure  The Guidelines are structured as shown in Figure 1. Figure 1: Structure of Guidelines ™™ Background ™™ Purpose Chapter 1 ™™ Scope Introduction ™™ Structure ™™ Benefits ™™ Sources of Information ™™ Introduction Chapter 2 ™™ Characteristics of LVRs Low Volume Roads ™™ Approach to Design in Perspective ™™ Risk Factors ™™ Introduction Chapter 3 ™™ Traffic Surveys and ™™ Route Survey* Investigations ™™ Materials Survey ™™ Materials Testing ™™ Introduction Chapter 4 ™™ Design Principles Pavement ™™ Design Methods Structural Design ™™ Practical Considerations ™™ Introduction Chapter 5 ™™ Bituminous Surfacings Surfacings ™™ Non-Bituminous Surfacings ™™ Surfacing Type Selection * Includes moisture and drainage. A brief description of each chapter is given below: Chapter 1 – Introduction: Discusses the background, purpose and scope of the Guidelines. Chapter 2 – Low Volume Roads in Perspective: Places in broad perspective the various factors that affect the provision of LVRs, including the definition and particular characteristics of LVRs, the environmentally optimized design philosophy and various sustainability and implementation considerations. Chapter 3 – Surveys and Investigations: Details the procedures to be followed in obtaining the basic inputs to the design of the road pavement, including traffic, route and materials surveys as well as moisture and drainage investigations and materials testing. 1. INTRODUCTION  3  Chapter 4 – Pavement Structural Design: Includes the principles of LVR pavement design and the methods available, with a focus on the use of the Dynamic Cone Penetrometer method. Also considers several related practical considerations including compaction, drainage and shoulders. Includes an Annex with a typical design example. Chapter 5 - Surfacings: Provides an overview of the various types of bituminous and non-bituminous discrete-element surfacings that are potentially suitable for use on LVRs, their performance characteristics, and the factors that may govern their selection. Concrete surfacings are dealt with in a separate Guidelines. The Guidelines do not address other complementary aspects of road design such as geometric and drainage design, road safety, construction issues or special engineering measures required for hill roads or roads in flood-prone areas, which are addressed in other IRC guidelines. A number of explanatory Annexes are also included in the Guidelines as follows: ™™ Annex I: Determination of Design Traffic. ™™ Annex II: Determination of Uniform Sections from CUSUM Analysis. ™™ Annex III: Determination and Choice of DN Percentile Values. ™™ Annex IV: Comparison of DCP-DN and IRC CBR Catalogues. ™™ Annex V: DCP Design Example. 1.5 Benefits  There are several benefits to be gained from adopting the approaches advocated in the Guidelines. These include providing LVRs that: ™™ Are less expensive, in life-cycle cost terms, to build and to maintain through the adoption of more appropriate, environmentally optimized designs. ™™ Maximise the use of locally available, often unprocessed materials, including industrial by-product materials and, by so doing, avoiding long haulage of more expensive processed materials. ™™ Provide well balanced pavement structures that are relatively less sensitive to vehicle overloading. ™™ Offer improved design reliability due to the much larger data set of DCP-DN measurements for statistical analysis and pavement design based on discrete uniform sections rather than general blanket designs. ™™ Integrate sustainability aspects (social, environmental and economic success) into the design, delivery and operation of road infrastructure assets (ref. Sections 2.3.4 and 2.3.5). 1.6 Sources of Information  In addition to providing general information and guidance, the Guidelines also serve as a valuable source document because of its comprehensive list of references from which readers can obtain more detailed information to meet their particular needs. A bibliography can be found at the end of the Guidelines. 4  Guidelines for the Environmentally Optimized Design of Low Volume Roads 2. LOW VOLUME ROADS IN PERSPECTIVE 2.1 Introduction  2.1.1 Background The traditional approaches to the provision of LVRs in many tropical and sub-tropical countries tend to be based on technology and research carried out in external environments that are not reflective of those that prevail in these countries. While these “standard” approaches might still be appropriate for much of the main trunk road network, they remain conservative, inappropriate and too costly for application on much of India’s rural road network. Thus, in facing the challenges of improving and expanding the country’s LVR network, more appropriate approaches need to be considered. The approach to the design of LVRs follows the general principles of any good road design. However, there are several important differences from the traditional approaches that need to be appreciated by the designer to provide designs that will meet with the multiple social, economic and environmental requirements in a sustainable manner. 2.1.2 Purpose and Scope The main purpose of this chapter is to place in broad perspective the various factors that may govern the provision of LVRs. To this end, the chapter addresses the following topics: ™™ The particular characteristics of LVRs. ™™ The LVR design philosophy. ™™ Various implementation considerations. 2.2 Characteristics of LOW VOLUME ROADS  2.2.1 Definition A common understanding of the definition of an LVR is crucially important as it will dictate the approach to undertaking the design of such roads in relation to their characteristics and the related criteria to be used in providing them at an appropriate level of service and minimum life cycle cost. There is no internationally accepted definition of an LVR. In developed countries such as the USA, roads carrying about 400 Vehicles Per Day (Vpd) are defined as very low volume roads. The figure that is currently, typically used is about 300 VPD or a design traffic 2. LOW VOLUME ROADS IN PERSPECTIVE  5  loading not exceeding about 1 Million Equivalent Standard Axles (MESA). However, neither of these definitions provides a complete picture of the unique characteristics of an LVR in that there are many other characteristics that need to be considered in their design as discussed below. For traffic loading of more than 1 MESA, standard pavement design procedures should be followed. 2.2.2 Special features The following specific features of LVRs affect the manner of their provision and need to be fully appreciated by the designer: ™™ They are constructed mostly from naturally-occurring, often “non-standard”, moisture-sensitive materials. ™™ Pavement deterioration is driven primarily by environmental factors, particularly moisture, with traffic loading being a relatively lesser influential factor, and drainage being of paramount importance. ™™ The alignment may not necessarily be fully “engineered”, especially at very low traffic levels, with most sections following the existing alignment. ™™ A need to cater for a significant amount of non-motorized traffic, especially in urban/ peri-urban areas, coupled with a focus on the adoption of a range of low-cost road safety measures. ™™ ariable travelling speeds that will seldom exceed 60 km/h, as dictated by local V vehicle characteristics and prevailing topography. A holistic appreciation of the attributes that characterize LVRs will guide designers in producing more appropriate designs with an emphasis on using a fit-for-purpose, context sensitive, environmentally optimized approach to their design and construction. This will place an onus on the design engineer to provide a road that meets the expected level of service at least life-cycle cost, based on a full understanding of the local environment and its demands, and to turn these to a design advantage. 2.3 Approach to Design  2.3.1 General approach Whilst the approach to the design of LVRs follows the general principles of any good road design practice, the level of attention and engineering judgement required for optimal provision of such roads tends to be higher than that required for the provision of other roads. This is because optimizing a design requires a multi-dimensional understanding of all of the project elements and in this respect all design elements become context specific. This will require: ™™ A full understanding by the design engineer of the local environment (physical and social). ™™ Recognition and management of risk. ™™ Innovative and flexible thinking through the application of appropriate engineering solutions rather than following traditional thinking related to road design. 6  Guidelines for the Environmentally Optimized Design of Low Volume Roads 2.3.2 Influence of road environment The term “road environment” is an all-encompassing one that includes both the natural or bio-physical environment and the human environment. It includes the interaction between the different environmental factors and the road structure. Some of these factors are uncontrollable, such as those attributable to the natural environment, including the interacting influence of climate (e.g. wind, rainfall and intensity), local hydrology and drainage, terrain and gradient. Collectively, these will influence the performance of the road and the design approach needs to recognize such influence by providing options that minimize the negative effects. Other factors, such as the construction and maintenance regime, safety and environmental demands, and the extent and type of traffic, are largely controllable and can be more readily built into the design approach. Typical road environment factors that impact on the LVR design process are presented in Figure 2 and are covered in more detail in various parts of the Guidelines. Figure 2: Various road environment factors affecting design Road Safety Climate Regime Maintenance Surface/sub- Regime Surface Hydrology Road Construction Environment Subgrade Regime Impact Factors "Green" Terrain Environment Controllable Construction Uncontrollable Traffic Factors Materials Factors 2.3.3 Road deterioration factors Research carried out in many countries has shown that the relative influences of road deterioration factors are significantly different for LVRs compared with HVRs. A critical observation is that for LVRs carrying less than 1.0 MESA, pavement deterioration is controlled mainly by how the road responds to environmental factors, such as moisture changes in the pavement layers, fill and subgrade, rather than to traffic, as illustrated in Figure 3. Thus, particular attention needs to be paid to the influence of moisture in the design of LVR pavements and the adoption of appropriate drainage. 2. LOW VOLUME ROADS IN PERSPECTIVE  7  Traffic loading versus dominant mechanism of pavement Figure 3:  distress (schematic) [1] 100 Environment 80 Percentage contribution 60 Area of Traffic interest 40 20 0 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 Traffic (million ESA) 2.3.4 Towards Sustainable Roads The expectation to integrate sustainability aspects (social, environmental and economic success) into the design, delivery and operation of road infrastructure assets is growing rapidly. Moreover, a sustainable approach could lead to more cost-effective and environmentally sound solutions. Efficient material use, lean design and finding the shortest lead distances for construction can be important design criteria. Designs should consider the use of industry “waste”, local materials, and reuse of building materials. Construction materials In constructing LVRs (for several thousand kilometres as planned in India), large quantities of materials, energy and water are required. The resources of most concern are those that are non-renewable. Consideration should be given to the following while designing roads and pavements: ™™ Water: Fresh water is becoming increasingly scarce and excessive consumption threatens ecosystem function. The aim of road assets at the design and construction cycle should thus be first to minimise water consumption, and then to replace potable water with effective reuse and recycling of locally appropriate alternative water sources. ™™ Materials: Road construction typically involves the consumption of large quantities of materials, a significant portion of which are derived from natural resources. The supplies of some of these resources are limited and are becoming increasingly scarce. The design and practice should encourage selection of materials that minimizes the consumption of precious resources such as crushed rock, which are generally transported over long distances. An example could be the use of locally available materials and their improvement on site for construction. 8  Guidelines for the Environmentally Optimized Design of Low Volume Roads ™™ aste materials: Waste generation is increasing and recycling and reuse of W the products are not increasing at the same rate. Waste from construction and demolition is significant and considerable amount is disposed to landfill. In the design, consideration should be given to the use of waste material through recycling, re-use and design optimisation. ™™ T o achieve the best environmentally sustainable outcomes from investments in road infrastructure, sustainability implications should be considered at each phase of design, construction and maintenance of LVR. 2.3.5 Environmentally optimized design To obtain optimal results from investments in road infrastructure in India, it is important to adopt an approach that is guided by appropriate local standards and conditions. In this regard, international research has highlighted the benefits of applying the principles of “Environmentally Optimized Design” (EOD) to the provision of LVRs in a manner that is compatible with the local road environment as outlined below and illustrated in Figure 4. The essence of the EOD approach is that it is: 1. Task based: LVRs must suit their identified function and the nature of the traffic (the people as well as the vehicles) which will pass along them, by applying appropriate standards. 2. Environmentally compatible: Suitable for, and where necessary, adapted to the local road environment factors. 3. Local resource based: The design of the LVR must be compatible with the construction materials that are readily available within appropriate specifications, Figure 4: LVR implementation within an EOD context Road Task Operational Environment SUSTAINABLE LVR DESIGN Local Resources Engineering Environment 2. LOW VOLUME ROADS IN PERSPECTIVE  9  and within the capacities of the engineers and technicians who will design the roads, and the contractors who will construct them, and within the means of the roads agency to maintain them, involving local communities, where possible. EOD can be described as a strategy for utilising the available resources of budget and materials in the most cost-effective manner to counter the variable factors of traffic, terrain, materials and subgrade that may exist along an alignment. To be successful and sustainable, LVR technology needs to be implemented within the framework of an EOD strategy. Moreover, if the LVR project is to be sustainable in the long run, several strategic objectives should be satisfied, including: ™™ Practical implementability of the recommended designs within the available resources and level of expertise in rural areas. ™™ Use of design standards and materials specifications that should aim at achieving an appropriate level of serviceability which should not fall below the minimum acceptable level during the design life. ™™ vailability of equipment/plant for construction and maintenance as well as the A level of quality control that can be effectively exercised in rural areas. ™™ Maximum use of local labour and skills. ™™ Maximum use of locally available or processed materials. ™™ In-built maintenance considerations in the design such as provision of adequate drainage, resistance to soil erosion along the side slopes, adequate lateral support from shoulders etc. as would minimize subsequent maintenance requirements. The EOD strategy should be applied with the overall aim of ensuring that each section of a road is provided with the most suitable pavement type for the specific circumstances prevailing along the road. This requires analysis of a broad spectrum of solutions to improve different road sections, depending on their individual requirements, ranging from engineered natural surfaces to bituminous pavements. The chosen solution must be achievable with materials, plant and contractors available locally. The EOD approach ensures that specifications and designs support the functions of different road sections - assessing local environment and limited available resources. EOD assesses whether the standard design is sufficient for problematic areas and whether it is necessary for good areas. An under-design of poor sections can lead to premature failure and an over-design will often be a waste of resources which would be better applied on the problematic sections. Short problematic sections should be handled as described in Section 4.3.2.2, Step 7. The EOD principle is illustrated in Figure 5. The use of the DCP method of design, which is highlighted in these Guidelines, combines the concepts of environmentally optimized design with in situ environmental and material conditions in a manner whereby specifications are adapted to suit the local road environment. This approach facilitates greater use of local, more abundant, and therefore less expensive, materials. By so doing, it reduces the need to import large quantities of virgin material by only adding a new layer (s), if necessary, to cater for the design traffic. This often results in reduced life cycle costs of LVR provision. 10  Guidelines for the Environmentally Optimized Design of Low Volume Roads Figure 5: Environmentally optimized and spot improvement design [schematic] Main Road Village Steep Standard Marshy Good Steep Good Good EOD offers a spectrum of options and solutions for providing low- Standard volume rural road access ranging from a Spot Improvement to an entire link length Problematic 2.4 Risk Factors  The departure from well-established, conservative material quality specifications may carry some increased level of risk of failure for an LVR. However, such a risk should be a calculated one and not a gamble and must consider not just materials but the whole pavement and its environment. Thus, in any pavement design strategy, it is necessary to be aware of the main risk factors which could affect the performance of LVRs so that appropriate measures can be taken to minimize them. These factors are summarized below: ™™ Quality of the materials (strength and moisture susceptibility). ™™ Construction control (primarily compaction standard and layer thicknesses). ™™ Environment (particularly drainage). ™™ Maintenance standards (drainage, surfacing and shoulders). ™™ Vehicle loads (overloading). The risk of premature failure will depend on the extent to which the above factors are negative – the greater the number of factors that are unsatisfactory, the greater the risk of failure. However, this risk can be greatly reduced by adhering to the prescribed material specifications, by ensuring that the construction quality is well controlled and that drainage measures are strictly implemented and, probably most importantly, that maintenance is carried out in a timely manner and vehicle overloading is reasonably well controlled. 2. LOW VOLUME ROADS IN PERSPECTIVE  11  3. SURVEYS AND INVESTIGATIONS 3.1 Introduction  3.1.1 Background Various surveys and investigations are required to collect and process the information required for the actual pavement design. These include the main input parameters for the pavement design: ™™ Traffic ™™ Subgrade conditions ™™ Material types and availability ™™ Moisture/drainage conditions Each of these is critical to the final pavement design and must be carried out accurately and comprehensively to ensure the most cost-effective pavement structure is designed and to minimize the potential for construction delays due to unforeseen circumstances. 3.1.2 Purpose and Scope The process and requirements of the traffic survey, subgrade investigation and details regarding material availability and moisture/drainage conditions are discussed in this chapter. The purpose of this chapter is to outline the procedures to be followed in determining these parameters as a basis for designing the road pavement. The chapter considers types of surveys that provide the inputs for determining the design traffic loading, which requires the data to be sufficiently accurate to select the correct traffic category for structural design from the six classes appropriate to LVRs. Simplified methods of accomplishing this are described. The chapter also covers the process of defining the subgrade conditions using the DCP apparatus to provide the input into the structural design decisions. The potential construction materials required for layer works in all classes of LVR are discussed. These can be obtained from borrow sources and “waste dumps” and hauled for use on the roads. Mechanisms for improving the quality of local materials are also highlighted. Finally, the need to identify sources of construction water during the site survey is briefly discussed and issues regarding material testing highlighted. 3. SURVEYS AND INVESTIGATIONS  13  3.2 Traffic  3.2.1 General Reliable data on traffic volumes and characteristics is essential for both pavement and geometric design and assists in the planning of road safety measures as summarized below: ™™ P avement design: The deterioration of the pavement is influenced by both the magnitude and frequency of individual axle loads. The large number of bicycles, motor cycles and pneumatic-tired animal drawn carts are of little consequence, and only commercial vehicles of gross laden weight > 3 tonnes are to be considered. ™™ Geometric design: The volume and composition of traffic, both motorized and non-motorized, influence the cross-section design (carriageway and shoulders). ™™ Road safety: The volume, type and characteristics of the traffic using the road will all influence the type of road safety measures required to ensure a safe road environment. In view of the above, a reliable estimate of existing (base line) and future traffic statistics is required to undertake the design of the road in an appropriate manner. Aspects related to geometric design and road safety are not covered in this document. Different traffic survey requirements are necessary for upgrading of an existing road and for a new road. For an existing road, the timing, frequency and duration of traffic surveys should be given very careful consideration in terms of striking a balance between cost and accuracy. As indicated approximately in Figure 6, short duration traffic counts in low traffic situations can lead to large errors in traffic estimation.  ossible errors in ADT estimates from random counts of Figure 6: P varying duration [12] ± 70 < 75 Vehicle/day ± 60 76-200 Vehicle/day 201-600 Vehicle/day ± 50 601-1000 Vehicle/day Error (percentage) >1001 Vehicle/day ± 40 ± 30 ± 20 ± 10 1 week 2 weeks 4 weeks 6 weeks 0 1 2 3 4 5 10 15 20 25 30 35 40 45 Week days Duration of count in days 14  Guidelines for the Environmentally Optimized Design of Low Volume Roads In the case of a new road, an approximate estimate should be made of traffic that would use the road considering the number of villages and their population along the road environment and other socio-economic parameters. This can be achieved by carrying out traffic counts on an existing road, as described above, in the vicinity with similar conditions and knowing the population served as well as agricultural/industrial produce to be transported. Likely traffic on the new road can also be estimated from Origin- Destination (O-D) surveys along the nearby existing roads which presently serve the villages proposed to be connected (see section 3.2.2.2). However, such surveys for LVRs might be restricted to special areas such as industrial or business hubs. For either a new road or an existing road, due consideration also needs to be given to the anticipated “Diverted” and “Generated” traffic because of the development of the proposed road, land use of the area served, the probable growth of traffic and the design life (see Section 3.2.3.3). 3.2.2 Traffic Surveys The following types of traffic survey are typically carried out in the project area where the road is located: ™™ Classified Traffic Surveys ™™ Origin-Destination Surveys ™™ Axle Load Surveys 3.2.2.1 Classified traffic survey A classified traffic count is one of the most important items of data for both geometric and pavement structural design as well as for planning purposes in terms of evaluating social and economic benefits derived from construction of LVRs. In most cases of LVRs, it would be sufficient to carry out only the classified traffic counts. It is necessary to ascertain the volume and composition of current and future traffic in terms of motorcycles, cars, light, medium and heavy goods vehicles, buses, and, importantly, non-motorized vehicles and pedestrians. The most common types of surveys for counting and classifying the traffic in each class are: ™™ Manual Traffic Survey ™™ Automatic Traffic Surveys ™™ Moving Observer Methods Although the methods of traffic counting may vary, the objective of each method remains the same - essentially to obtain an estimate of the Annual Average Daily Traffic (AADT) using the road, disaggregated by vehicle type. Prediction of such traffic is notoriously imprecise, especially where the roads serve a predominantly developmental or social function and when the traffic level is low. 3. SURVEYS AND INVESTIGATIONS  15  Reducing errors in estimating traffic for LVRs Errors in estimating traffic flow can be reduced, where possible, by: ™™ Counting for seven consecutive days. ™™ On some days counting for a full 24 hours, preferably with one 24-hour count on a weekday and one during a weekend; on other days, 16 hour counts (typically 06.00–22.00 hours) should be made and expanded to 24-hour counts using a previously established 16:24 hour expansion ratio. It is suggested that each state develop indices for this ratio, as these are likely to vary from state to state. ™™ Avoiding counting at times when road travel activity increases abnormally; for example, just after the payment of wages and salaries, or at harvest time, public holidays or any other occasion when traffic is abnormally high or low. However, if the harvest season is during the wet season (often the case, for instance, in the timber industry), it is important to obtain an estimate of the additional traffic typically carried by the road during these periods. This should also be adapted for local conditions as different crops can be harvested in different seasons. Care should be exercised in selecting appropriate locations for conducting the traffic counts to ensure a true reflection of the traffic using the road and to avoid under- or over- counting. Local knowledge should be used to help with this. Ideally, the accuracy of traffic counts can be improved by increasing the count duration or by counting in more than one period of the year. Improved accuracy can also be achieved by using local knowledge to determine whether there are days within the week or periods during the year when the flow of traffic is particularly high or low. It may also be possible to develop data relating the cumulative standard axles to the population and economic activities served in different districts or states. Adjustments for season An appropriate, weighted average adjustment will need to be made according to the season in which the traffic count was undertaken and the length of the wet and dry seasons, as illustrated in Figure 7. Although the number and duration of harvesting seasons can vary from one region to another, typically two harvesting seasons each year are shown in Figure 7. If T is the average number of commercial vehicles of a given category, plying per day during the lean season, the enhanced traffic during the peak season can be denoted by nT, over and above the lean season traffic T, the value of n varying widely from one region to the other. Typically, it takes about 40% of the duration of a harvesting season (t) to build up from lean season level T to the peak. The peak traffic may continue for about 20% of the duration of the harvesting period before returning to the lean season traffic level. This usually takes about 40% of the total duration of the harvesting season. Vehicle classification Table 1 shows the vehicle classification system used for compiling the results of the traffic survey described above. 16  Guidelines for the Environmentally Optimized Design of Low Volume Roads Figure 7: Seasonal variations in rural traffic [6] HARVESTING HARVESTING SEASON-1 SEASON-2 NO. OF VEHICALES PER DAY (VEHICLE TYPE A) nt 1.2 nTt nt AADT = T + 365 t t T 365 Days Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan MONTH OF THE YEAR Table 1: Vehicle classification system Class Type of Vehicle Use 1 Car, Jeep, Van 2 Auto Rickshaw Capacity analysis for geometric (cross 3 Scooter/Motor Bike section) design 4 Mini-bus (LCV) 5 Bus (loaded, unloaded, overloaded) 6 Truck Traffic loading analysis for pavement design 7 Tractor with trailer 8 Tractor without trailer (LCV) 9 Cycle Capacity analysis for geometric (cross 10 Cycle Rickshaw/Hand Cart section) design 11 Horse Cart/Bullock Cart 12 Pedestrian 3.2.2.2 Origin-Destination (O-D) surveys Origin-Destination (OD) surveys in case of rural roads are not normally required except for special areas such as industrail, business hubs etc. Such surveys can be undertaken using a variety of survey techniques. They are carried out to establish the nature of travel patterns in and around the area of enquiry and would normally be carried out as part of a regional planning exercise rather than for an individual road project. 3. SURVEYS AND INVESTIGATIONS  17  3.2.2.3 Axle load surveys Axle load surveys provide critical and essential information that is required for both cost-effective pavement design as well as preservation of existing roads, and are recommended particularly in areas where additional heavy vehicles are likely to use the new road. The importance of this parameter is highlighted by the well-known “fourth power law” which exponentially relates increases in axle load to pavement damage (e.g. an increase in axle load of 20 per cent produces an increase in damage of more than 200 per cent). Information about the loading of vehicles is essential for pavement design and for overload control. Simplified methods of acquiring vehicle load data are described below. Full axle load surveys The type of equipment which may be used for axle load surveys varies widely and includes: ™™ Static or dynamic weighing equipment. ™™ Manual or automatic recording of loads. ™™ Portable or fixed installation. The quality of the data obtained will depend on the type of equipment used, the duration of the survey and the degree of quality control performed. In general, the higher the quality of the data, the greater will be the resources required to collect it. However, axle load surveys can be expensive and are unlikely to be undertaken for an individual LVR project for which simplified methods are required. Simplified axle load surveys If a full axle load survey is not being carried out, information about the vehicle loading can be obtained by observation during the traffic counting survey. The enumerator merely records, for every heavy vehicle in the heavy vehicle classes, the state of loading (full, partial or empty), and the type of load (heavy, medium, or light). These are particularly important in areas where the traffic carries bulk quantities such as sugar cane, brick-works, quarry areas, etc. 3.2.3 Procedure for Determining Design Traffic 3.2.3.1 General The procedure for determining the traffic loading for pavement design purposes is summarized in Figure 8. 3.2.3.2 Select Design Period A structural design period must be selected over which the cumulative axle loading is determined as the basis of designing the road pavement. The design period is defined as the time span in years considered appropriate for the road pavement to function before reaching a terminal value of serviceability after which major rehabilitation or 18  Guidelines for the Environmentally Optimized Design of Low Volume Roads Figure 8: Procedure for establishing design traffic category STEP 1 STEP 4 STEP 7 Select Design Period Estimate mean Establish traffic ESA per vehicle class lane distribution STEP 2 STEP 5 STEP 8 Estimate traffic growth Estimate Vehicle Select design per vehicle class Damage Factor (VDF) traffic category per all vehicle classes STEP 3 STEP 6 Estimate initial traffic Estimate Cumulative ESALs volume (AADT) for all vehicle classes over per vehicle class Design Period (in one direction) reconstruction would be required. It will be required to carry out a condition survey at least every 2 years, so that the nature and rate of change of condition will help identify as to when the pavement will require strengthening. A design life of 10 years has been recommended to ensure that neither the strengthening will be needed to be carried out too soon nor will the design for a very long design period be unduly expensive by way of high initial investment required. This can go a long way in connecting more of the unconnected habitations with the same investment. 3.2.3.3 Estimate Traffic Growth per Vehicle Class Following the establishment of the baseline traffic, further analysis is required to establish the total design traffic based on forecast of traffic growth in each vehicle class. To forecast such growth, it is first necessary to sort traffic in terms of the following categories (Figure 9): ™™ Current traffic: Existing traffic on the road before its upgrading to a higher standard. ™™ Diverted traffic: This is traffic that diverts from another route to the new road, because of the better riding condition on an existing route or shorter access on a new route, but still travels between the same origin and destination points. Unless origin-destination surveys have been carried out (very unlikely) this can only be estimated based on judgement of the traffic on nearby roads that could benefit from a shorter or more comfortable route. Local historic precedent can sometimes assist in estimating this, otherwise a rule of thumb is that diverted traffic is typically 20% of the existing traffic but it can be considerably higher. 3. SURVEYS AND INVESTIGATIONS  19  Traffic development over time on a new or improved Figure 9:  road [schematic] Total traffic Generated traffic Normal traffic growth Traffic Diverted traffic Current traffic Time ™™ Normal traffic growth: Traffic that results from the normal growth in current traffic after the road is upgraded. ™™ Generated traffic: This is the additional traffic that occurs in response to the new or improved road. This traffic is essentially created traffic resulting from increased economic activity in the area. Values for this of 6 or 7% have been routinely used in India. Diverted traffic moves to the road quickly after completion of construction, while generated traffic is in response to the creation of new economic activities and opportunities arising from the improved or new road, and builds up over time. Estimating traffic growth over the design period is very sensitive to economic conditions and prone to error. It is therefore prudent to assume low, medium and high traffic growth rates as an input to a traffic sensitivity analysis for pavement design purposes. The growth rate of each traffic class may differ considerably. Motor cycles and motor cycle driven vehicle modes, for example, usually grow at a much faster rate than other traffic classes. Therefore, growth rate for all traffic classes should be considered. There are several methods for estimating the traffic growth. IRC SP 72 suggests values but each one should be assessed on its own merit. Local historic precedent Evidence of traffic growth on roads recently upgraded in the area is a good guide as to what to expect. 20  Guidelines for the Environmentally Optimized Design of Low Volume Roads Government predictions of economic growth Economic growth is closely related to the growth of traffic. Economic growth rates can be obtained from government plans and government estimated growth figures. The growth rate of traffic should preferably be based on regional growth estimates because there are usually large regional differences. It should be borne in mind that both geometric design classes and structural design classes are quite wide in terms of traffic range, typically a range of 100% or more, hence broad estimates of traffic projections would be sufficient in respect of low volume roads. A common method of choosing the design traffic is simply to estimate the initial traffic, including diverted and generated traffic, and to accommodate traffic growth by choosing the next higher traffic class for both geometric and structural design. 3.2.3.4 Estimate Initial Traffic volume (AADT) per Vehicle Class Based on the traffic surveys, the initial traffic volume for each vehicle class can be determined. For structural design purposes, it is only the commercial vehicles (traffic classes 5 to 7 in Table 1) that will make any significant contribution to the total number of equivalent standard axles. In contrast, for geometric design purposes it is necessary to count non-motorized and intermediate means of transport including pedestrians, bicycles, motorcycles, tractors and trailers and, possibly, animal transport. Even for such cases, single lane carriageway would suffice for LVRs. Taking account of the seasonal variations in rural traffic (Figure 7), the total number of repetitions (N) of a given vehicle type during a year, and the Average Annual Daily Traffic (AADT) are determined as follows [6]: N = T x 365 + 2nT [0.6t] AADT = T + 1.2nTt/365 Average number of commercial vehicles of a given traffic category per day Where: T =  during the lean season t = duration of harvesting season n = number of years 3.2.3.5 Estimate Mean ESA per Vehicle Class For purposes of pavement design, only commercial vehicles with a gross laden weight of 3 tonnes or more along with their axle loading are considered. These may include inter alia the following: ™™ Trucks (heavy, medium) ™™ Buses ™™ Tractor-Trailers The traffic parameter is generally evaluated in terms of a Standard Axle Load of 80 kN and the cumulative repetitions of the Equivalent Standard Axle Load (ESAL) are calculated over the design life. 3. SURVEYS AND INVESTIGATIONS  21  Vehicles with single axle loads different from 80 kN, and tandem axles different from 148 kN can be converted into standard axles using the Axle Equivalence Factor, as follows: Axle Equivalency Factor = (W/Ws)n Where W = Axle load in kN of the vehicle in question Ws = Standard axle load of 80 kN for a single axle and 148 kN for a tandem axle power exponent (lies between 2.5 and 4.5. A value of 4 is recommended for n= LVRs). The likely average Equivalency Factors for converting the Standard Axle Load of 80 kN and the Tandem Axle Load of 148 kN are given in Table 2. 3.2.3.6 Estimate Vehicle Damage Factor The Vehicle Damage Factor (VDF) is a multiplier for converting the number of commercial vehicles of different axle loads to the number of standard axle load repetitions. It is defined as the “equivalent number of standard axles per commercial vehicle”. Whilst Table 2: Equivalency Factors for different axle loads Axle Load Load Equivalency Factors (Tonnes) kN Single Axle Tandem Axle Group 3.0 29.4 0.02 0.01 4.0 39.2 0.06 0.01 5.0 49.1 0.14 0.02 6.0 58.8 0.29 0.03 7.0 68.7 0.54 0.05 8.0 78.5 0.92 0.08 8.16 80.0 1.00 0.09 9.0 88.3 1.48 0.13 10.0 98.1 2.25 0.20 10.2 100.0 2.46 0.21 11.0 107.9 3.30 0.29 12.0 117.7 4.70 0.40 13.0 127.5 6.40 0.56 14.0 137.3 8.66 0.75 15.0 147.1 11.42 1.05 16.0 157.0 - 1.27 17.0 166.8 - 1.62 18.0 176.6 - 2.03 19.0 186.4 - 2.52 20.0 196.2 - 3.09 22  Guidelines for the Environmentally Optimized Design of Low Volume Roads the VDF value is arrived at from axle load surveys on the existing roads, the project size and traffic volume in the case of rural roads may not warrant conducting an axle load survey. It may be adequate to adopt indicative VDF values discussed below for the pavement design. For calculating the VDF, the following categories of vehicles may be considered: 1) Laden Heavy Commercial vehicles (HCV) Fully loaded HCV (comprising heavy trucks, full-sized buses) have a Rear Axle Load of 10.2 tonnes and a Front Axle Load, about half the Rear Axle Load, i.e. 5 tonnes. The VDF works out to 2.60 (= 2.46 + 0.14). 2) Unladen/Partially Loaded Heavy Commercial vehicles (HCV) Since the extent of loading of commercial vehicles is difficult to determine, a Rear Axle Load of 6 tonnes and a Front Axle Load of 3 tonnes may be assumed for an Unladen/Partially Loaded HCV. The VDF works out to 0.31 (= 0.29 + 0.02). 3) Overloaded Heavy Commercial vehicles The extent of overloading may vary widely from one situation to the other. However, if an overload of 20% occurs, the VDF increases to 5.35 (= 5.06 + 0.29). However, if only 10% of the laden HCV are overloaded to the extent of 20%, the VDF works out to 2.86 (= 0.9 x 2.58 + 0.1 x 5.35). 4) Laden Medium-heavy Commercial vehicles (MCV) Fully loaded MCV (mostly comprising Tractor-Trailers) have a Rear Axle Load of 6 tonnes and a Front Axle Load of 3 tonnes. The VDF works out to 0.31 (= 0.29 + 0.02). 5) Unladen/Partially Loaded Medium-heavy Commercial Vehicles Since the extent of loading of commercial vehicles is difficult to determine, a rear Axle Load of 3 tonnes and a Front Axle Load of 1.5 tonnes may be assumed. The VDF works out to 0.019 (= 0.018 + 0.001). 6) Overloaded Medium-heavy Commercial Vehicles The extent of overloading may vary widely from one situation to the other. However, if an overload of 20% occurs, the VDF increases to 0.65 (= 0.61 + 0.04). However, if only 10% of the laden MCV are overloaded to the extent of 20%, the VDF works out to 0.344 (= 0.1 x 0.65 + 0.9 x 0.31). Towards the computation of ESAL applications, the indicative VDF values (i.e. Standard Axles per Commercial vehicle) are given in Table 3 below. For pavement design purposes, the number of: 1. HCV: Laden, unladen and overloaded. 2. MCV: Laden, unladen and overloaded. Table 3: Indicative VDF per vehicle type Vehicle Type Laden Unladen/Partially Laden HCV 2.86 0.31 MCV 0.34 0.02 3. SURVEYS AND INVESTIGATIONS  23  Must be obtained from actual traffic counts and, using appropriate VDF values, the number of Equivalent Standard Axles to be catered for over the design life can be determined. If, however, for some reason, it is not possible to carry out all the required traffic counts, recourse to local enquiries may be taken to estimate their proportions in as realistic a manner as possible. 3.2.3.7 Estimate Cumulative ESALs for all Vehicle Classes Over Design Life The estimated ESALs for all vehicle classes over the design life of the road may be calculated as follows: ESAL per day = number of commercial vehicles per day in the year of where T0 =  opening x VDF L = lane distribution factor (see Step 3.2.3.8) 3.2.3.8 Establish Traffic Lane Distribution The actual ESALs for all vehicle classes over the design life of the road need to be corrected for the distribution of heavy vehicles between the lanes in accordance with Table 4. Table 4: Lane width adjustment factors for design traffic loading [13] Cross Paved Corrected design traffic Explanatory notes Section width loading (ESA) Single lane 3 m, 3.75 m The sum of ESAs in both Traffic in both directions uses the carriageway directions same lane Intermediate lane 5.5 m 80% of the ESAs in both To allow for overlap in the centre carriageway directions section of the road 3.2.3.9 Select Traffic Category For pavement design using the DCP method, the traffic has been divided into 6 categories as shown in Table 5. Table 5: Traffic categories Traffic Category Cumulative ESALs (x 106) T1 < 0.010 T2 0.010 – 0.030 T3 0.030 – 0.100 T4 0.100 – 0.300 T5 0.300 – 0.700 T6 0.700 – 1.000 24  Guidelines for the Environmentally Optimized Design of Low Volume Roads 3.3 Route Survey  3.3.1 General The successful design of a road depends on ensuring that the pavement is appropriate for the characteristics of the subgrade or the embankment on which it is placed. A good subgrade is strong enough to resist shear failure and has adequate stiffness to minimize vertical deflection. The stronger the subgrade, the thinner the pavement layers above need to be and the lower the cost of the road will be. The designer usually has little choice about the subgrade except for when a raised formation or embankment is constructed. Therefore, it is essential that the characteristics of the subgrade along the alignment are carefully assessed and understood. In cases where the subgrade materials are unsuitable, cost-effective methods of improving these materials must be identified, e.g. stabilization or improving drainage (Sections 3.4 and 3.5). Initial reconnaissance surveys consisting of a desk study and walk-over survey should always be carried out prior to the actual survey. Issues that should be noted include: ™™ General soil types along the route. ™™ Moisture and drainage conditions. ™™ Variations in terrain and potential for effective side and cross drainage. ™™ Existing embankments heights, effectiveness against flooding and necessity, if any, for raising embankments. ™™ Road reserve status. ™™ Expected relevance of a DCP survey. When upgrading an existing track or road, it is equally important to determine the characteristics of the existing layers of material in the pavement structure because these should be utilized in the new pavement. The most cost-effective method of obtaining sub- surface information at small intervals along the entire route to a depth of approximately 800 mm is by using a Dynamic Cone Penetrometer (DCP). 3.3.2 DCP surveys The DCP is light and portable and tests are quick and simple. The advantage of the DCP is that information can be gathered with minimal disturbance to the in-situ material. Using this test, the strength characteristics and thickness of the in situ subsurface materials at their prevailing field moisture and density conditions are obtained directly. The DCP also has the advantage over the CBR in providing a continuous strength profile over a depth of 800 mm at much smaller intervals along the road. The required frequency of the DCP measurements depends on the variability in conditions along the route and the level of confidence required. Where obvious changes of surface conditions occur, the frequency of the tests should be modified to include the changes. Similarly, where surface conditions are uniform, the frequency of testing may be reduced. A guideline for the frequency of testing for upgrading an existing road to a paved standard is shown in Table 6. 3. SURVEYS AND INVESTIGATIONS  25  Table 6: Frequency of DCP testing Road condition Frequency of testing (number/km) Uniform, fairly flat, reasonable drainage - low risk 5 Non-uniform, rolling uneven terrain, variable drainage - medium risk 10 Distressed, uneven terrain, poor drainage – high risk 20 Several different correlations exist between the DCP penetration rate (mm/blow) and the more familiar CBR strength. These correlations are based on CBR values versus DCP penetration rates measured in different soil types and are generally material and test method dependent with correlation coefficients ranging from 0.67 – 0.79 [14]. It is thus recommended, however, that the actual DCP penetration rates (direct indications of the shear strength) are utilized in the pavement design instead of the more variable/less reliable CBR [15]. This approach is discussed further in Chapter 4 – Pavement Design. The general procedure for undertaking the DCP survey as part of the overall design process differs between that for new roads and for existing roads, and is discussed separately for these two situations. 3.3.2.1 Existing roads On existing roads and tracks, the new road is expected to be built directly on the material currently forming the road or track, and the DCP test will indicate the properties of the materials that will be a part of the new pavement structure. Even if the formation is raised slightly to facilitate drainage or pipe cover, the materials tested will usually influence the structural capacity of the new pavement structure. The DCP survey must thus be carried out along the full length of the road with each measurement being taken to a depth of at least 800 mm, in order to assess the balance of the final pavement. The DCP tests should be staggered across the road at left outer wheel-track, centre line, right outer wheel-track, centre line, etc. However, the variability of the road will only be fully apparent once the tests have been carried out. To ensure statistical reliability, at least 10 DCP tests should be carried out in each uniform section (see Chapter 4), hence additional tests may be required after analysing the first set of data obtained. Care must be exercised in carrying out the DCP survey to discard any measurements that could produce anomalous results. Such results could arise, for example, where large stones occur in the pavement layer (Figure 10). Where brick-soling or hard aggregate layers (e.g. water bound macadam) already comprise a part of the pavement, this layer should be removed and the DCP test carried out from the base of the layer, after measuring the thickness of this layer. This layer would then be included in the pavement design as an existing strong layer. The data should be captured in the form of the number of blows of the DCP hammer and the corresponding depth of penetration and can be entered directly into a spreadsheet for use in available software. Destructive testing (sampling) is not required at each DCP test site. 26  Guidelines for the Environmentally Optimized Design of Low Volume Roads Figure 10: DCP effects where large stones are present (C) (D) (A) (B) Cone (a)  Cone breaks stone. DCP (b)  Rod pushed aside and tilts at (c)  Normal (d)  cannot profile shows a plateau and an angle. Excessive friction result. penetrate. subsequent readings may on rod gives low reading. be low. 3.3.2.2 New roads The construction of new roads can result in two processes. In some cases, the new road will be at or slightly above natural ground level, in which case the DCP survey can be carried out along the alignment (as described above in Section 3.3.2.1). However, if the road is to be constructed on a new fill or embankment higher than about 500 or 600 mm, a DCP survey at the existing ground level will have little input into the pavement design or influence on the final pavement performance. The strength of the compacted embankment material needs to be determined as described above as this is what will influence the structural design of the pavement. In some cases, the DCP can be used at natural ground level to identify potential drainage and moisture problems beneath embankments that will be up to about 1 m or more in height, which may influence the later stability or settlement of the embankment. However, DCP testing of the in situ material at the prevailing moisture and density is generally meaningless for such embankments. Instead, testing of samples of the material in the laboratory, after compaction to the expected density and at the expected moisture content, can be used to determine whether the local material can be used for construction of the embankment. It is also possible, if the proposed embankment borrow source has been identified, to test this material in the laboratory and determine the expected embankment (subgrade) profile. 3. SURVEYS AND INVESTIGATIONS  27  3.3.3 Test pits and Sampling The structural assessment requires some test pits along the road to obtain samples for laboratory testing to assist with material characterization. These are also used to allow direct observations of the in-situ subgrade soil and potential fill material. The location, frequency and depth of pits and trenches for characterizing the subgrade depend on the general characteristics of the project area (the soil type and variability). In addition, the DCP testing carried out to assist delineation of uniform sections can be used to target areas for pitting and trenching. Spacing will decrease when the subsurface soils demonstrate more variability. In these areas, pits can also be staggered left and right of the centre line to cover full width of the road formation. It is necessary to ensure that sufficient test pits are excavated in each uniform section to provide sufficient data for the entire uniform section. This needs to be carried out in conjunction with analysis of uniform sections using the DCP data (Chapter 4). Requirements are different for existing roads and new alignments. 3.3.3.1 Existing roads Test pits should be excavated along existing roads at the required intervals to provide sufficient information per uniform section to a depth of at least the bottom of the imported selected layers in the pavement. Samples of the material in each layer should be collected for laboratory testing and an inspection and profiling of the pit carried out to identify the materials and the causes of any failures or problems. 3.3.3.2 New roads The depth of pits and trenches is determined by the nature of the subsurface. For sampling and soil description, pits should be dug to at least 0.5 m below the expected natural subgrade level. In cut sections, the depth can be reduced to 0.3 m but in potentially problematic materials (see following section), the depth may need to be extended to at least 1.0 m below the proposed subgrade. Greater depths may also be needed for high embankment design. A limited number of deeper pits may also be needed to ascertain groundwater influence and irregular bedrock, if data in this regard is unknown, although it can often be obtained from other sources. The location of each test pit should be determined precisely on the route alignment to cover the full range of in situ conditions and all layers, including topsoil, should be accurately described and their thicknesses measured. All horizons, below the topsoil should be sampled. This will promote a proper assessment of any materials excavated in cuts that could be used in embankments. The samples should be taken over the full depth of the layer by taking vertical slices of materials. 3.3.4 Moisture and Drainage 3.3.4.1 Assessment of moisture conditions along alignment It is essential that an estimate of the in situ moisture condition is made at the time of the DCP survey for comparison with the expected moisture regime in service. This moisture regime affecting the in situ materials must be accurately assessed in terms of whether 28  Guidelines for the Environmentally Optimized Design of Low Volume Roads the road will operate at that condition or in a wetter or drier state than at the time of assessment during its service life. This will be used later to statistically determine the percentile of the DCP penetration rates for pavement design purposes. In addition, at least 2 samples should be collected per kilometre of the proposed subgrade materials for moisture content and Optimum Moisture Content (OMC) determination from the outer wheel tracks of the road at depths of 0-150, 150-300 and 300-450 mm. This is best done during the test pitting and synchronisation of the identification of uniform sections and test pitting needs to be done as soon as possible after the DCP survey to ensure that samples are representative of all the uniform sections. 3.3.4.2 Local drainage problems and requirements During the survey, any drainage problems or constraints that would affect drainage locally (streams, marshy areas, flat poorly drained areas, etc.) need to be identified so that areas requiring specific side or cross drainage can be pin-pointed for the design. Cognisance should also be taken of the potential climate change effects in the long term, particularly in terms of drainage structures. Predictions indicate that the annual rainfall will increase over most of India in the long term, with more frequent extreme events, leading to potentially more flooding situations in certain areas. It is recommended that localised information is obtained from the relevant authorities (e.g. Indian Institute of Tropical Meteorology) regarding expected climate changes in areas being investigated. Areas that are visibly prone to possible flooding and water accumulation under extreme precipitation or flooding conditions must be noted, as soaked designs would be necessary in these areas. 3.3.5 Problem subgrades Many subgrades may be classified as problematic materials. These include a wide range of possible materials such as: ™™ Expansive/heaving clays (“cotton soils”) ™™ Wet/waterlogged areas ™™ Collapsible soils ™™ Dispersive soil ™™ Erodible soils ™™ Saline soils ™™ Soft clays Each of these potentially problematic soils requires unique investigation and test protocols. During the site investigation, it is important that such problem areas are identified and suitable advice be obtained regarding the implications and treatment from geotechnical specialists where it is felt necessary. The fact that most of these are affected by moisture fluctuations is also relevant to long-term climate changes. A summary of problem soil causes, recognition and treatment follows: 3. SURVEYS AND INVESTIGATIONS  29  3.3.5.1 Expansive Clays Causes Expansive clays are widespread and of major economic significance. Typical damage to roads includes longitudinal unevenness and bumpiness, differential movement near culverts and longitudinal cracking along the road. The presence of trees alongside the road often results in localized moisture extraction by their roots with the development of sporadic subsidence and arcuate cracking. Expansive clay damage to roads usually affects their serviceability more than their structural integrity, provided cracking and surface distress is timely and effectively repaired. Damage is generally restricted to areas that have significant seasonal rainfall or poor surface water drainage. Expansive soils are those containing smectite (montmorillonite) clays, which are mostly derived from the chemical weathering of basic rock forming minerals. Probably the worst expansive clay subgrades are in areas of deeply weathered gabbros, basalts and dolerites in tropical and sub-tropical zones. Expansive clays are also commonly found in transported soils derived locally or from some distance from weathered basic igneous rocks. Smectites can also form from the alteration under alkaline conditions of other silicate minerals low in potassium, as long as calcium and magnesium are present and leaching is impeded. Although the expansive potential of a soil can be related to many factors, it is primarily controlled by the quantity and type of clay minerals (e.g. smectites). Volume changes in expansive soils are confined to the upper few metres of a soil deposit where seasonal moisture content varies due to drying and wetting cycles. The zone within which volume changes are most likely to occur is defined as the active zone. The active zone can be evaluated by plotting the in situ moisture content with depth for samples taken during the wet and dry seasons. The depth at which the moisture content shows no seasonal variation is the limit of the active zone. This is also referred to as the depth of seasonal moisture change, but could change in the long-term due to climate changes. Recognition The simplest way of identifying the presence of expansive soils is through field observations where the surface expression of cracking in dark grey, black or sometimes red soils is evident as shown in Figure 11. However, the presence of a thick non-expansive transported or topsoil cover can sometimes mask these cracks and excavation of a test pit, in which cracking and slickensiding of the material will be observed is necessary. The identification of smectite in subgrade soils is best done using X-ray diffraction. By their nature, smectites will tend to be more plastic than other clay minerals and a measure of the Plasticity Index, or better still the activity (ratio of Plasticity Index to clay fraction) is a good indication of the presence of smectites. This is one of the earliest methods of indicating potentially expansive soils using Figure 12 based on the clay fraction of the soil (minus 2 μm) and the standard Plasticity Index (PI), which remains a simple but very useful means of identifying expansive soils. It should be noted that the estimates for the degree of swell using this technique do not consider the initial moisture content of the material, but assumes that they move from a state of dryness normally used in the laboratory to wet. It is known that an equilibrium moisture content develops 30  Guidelines for the Environmentally Optimized Design of Low Volume Roads Figure 11: Cracking on expansive clay under a road structure and the moisture fluctuation in this zone is minimal. However, from beneath the outer wheel track of roads with unsealed shoulders to the edge of the fill, significant and variable moisture fluctuations occur. It is unlikely that the initial moisture content in these zones is, however, particularly dry. An indication of potentially expansive soils can also be obtained from land type soil maps where materials identified as “vertic” soils will always have expansive characteristics, while soils with a high base status (or eutrophic) and clay content should be investigated more thoroughly, as they have the potential to be expansive. Countermeasures Although the estimation of potential heave is imperative for structures on expansive clay, it is not as critical for subgrades under roads, particularly LVRs. It is more important to identify the possible existence of the problem and the potential for differential heave along the road and take the necessary precautions. These will generally be based on the expected degree of swell determined from Figure 12. If the calculated potential heave exceeds 25 to 50  mm, countermeasures should be installed. If there is likely to be significant differential movement because of variable material properties or thicknesses, changing loading conditions or localized drainage differences, the countermeasures will need to take this into account to avoid localized sections of road with poor riding quality. 3. SURVEYS AND INVESTIGATIONS  31   dentification of expansive clay soils and estimate Figure 12: I of expansion [16] 70 60 Very High 8% Medium 2% 50 Low < 2% High 4% Pl of whole sample 40 30 20 10 0 0 10 20 30 40 50 60 70 80 Clay Fraction (Percent) Where culverts or small bridge structures are involved, it is usually necessary to quantify the potential movement more accurately. This is best done using oedometer testing of specimens cut from block samples. Correct orientation of the block samples is imperative as expansive clays tend to be highly anisotropic with significantly lower swells in the horizontal direction. This testing needs to be carried out in conjunction with good estimates of the potential changes in in-situ moisture content from season to season. Solutions that can be considered for LVRs over expansive clays include: 1. Flattening of side slopes (between 1V:4H and 1V:6H). 2. Remove expansive soil and replace with inert material (between 0.6 and 1 m depending on depth of clay). 3. Retain the road over the clay as an unpaved section. 4. Pre-wetting prior to construction of the fill or formation (to OMC). 5. Placing of un-compacted pioneer layers of sand, gravel or rockfill over the clay and wetting up, either naturally by precipitation or by irrigation (300 to 500 mm depending on clay thickness and potential swell). 6. Lime stabilization of the clay to change its properties (expensive as up to 6% lime may be required). 7. Blending of fine sand with the clay to change its activity (blend ratio to be determined by laboratory experimentation). 8. Sealing of shoulders (not less than 1 m wide). 32  Guidelines for the Environmentally Optimized Design of Low Volume Roads 9. Compaction of thin layers of lower plasticity clay over the expansive clay to isolate the underlying active clays from significant moisture changes. 10. Use of waterproofing membranes and/or vertical moisture barriers, which are generally geosynthetics (only limited success has been achieved using these methods. Figure 13 provides a preliminary indication of possible counter-measure options (numbered as above) as a function of potential expansiveness. It should be noted that usually a combination of these is most effective and all should go together with careful design and construction of side-drains, which should preferably be sealed. In many cases for roads with less than 20 vpd and swells higher than 4%, it may be better to retain the road as a gravel road over the expansive clay sections and apply the necessary maintenance. One of the most important considerations is to try and minimize the zone of seasonal moisture movement beneath the road (Figure 14) and increase the zone of moisture equilibrium. A combination of slope flattening, material replacement, sealed shoulders and lined side drains is usually the most cost-effective means of achieving this, but the design of counter-measures needs to be specific to any situation. This is a particular problem on narrow roads (e.g. 3.75 m, where only the central 1 or 1.5 m achieves equilibrium. Expansive clays are often thick and laterally widespread and this makes the implementation of countermeasures costly. The most successful technique for counteracting subgrades susceptible to high movement is to remove the expansive clay beneath the road structure and replace it with a raft of inert material. This would typically involve the excavation and removal of between 600 and 1 500 mm (or even Possible solutions for the construction of roads on Figure 13:  active clays Solutions for different road classes (vpd) Expansiveness > 200 50 - 200 20 - 50 < 20 Options Low (< 2%) 1, 4, 5, 8 Medium Options Options (2-4%) 1, 4, 5, 7, 8, 9 1, 4, 5, 8 Determine % age swell Options Options Options High (4-8%) 1, 2, 3, 4, 5, 1, 4, 5, 7, 8, 9 1, 4, 5, 8 7, 8, 9 Very High Options Options Options Options (> 8%) 1, 2, 3, 6, 8, 10 1, 2, 3, 4, 5, 1, 4, 5, 7, 8, 9 1, 4, 5, 8 7, 8, 9 3. SURVEYS AND INVESTIGATIONS  33  Typical moisture movement regime under roads on Figure 14:  expansive clays Progressive development of crack as moisture enters cracks and moisture content increases in zone of equilibrium Zone of seasonal material movement causing rotation about hinge x NGL x x Zone of moisture equilibrium Zone of seasonal moisture movement deeper in some cases) of material over the entire footprint of the road prism (or at least beneath shoulders and side slopes) combined with drainage structures that remove all water from adjacent to the fill slopes and culverts. Removal of material results in the reduction of the swell potential as well as slightly increasing the load on the expansive subgrade with a usually denser, better compacted material. Unfortunately, this is often impracticable or uneconomic for low volume roads, unless the problem is localized. More frequently, expansive materials cover a wide area and the importation of substitute material involves the haulage of large quantities of inert material over long distances. The recommended and probably most economical solution specifically for low volume roads showing high to very high potential swell is to partially remove the clay from the upper subgrade (250 mm) and replace it with a less active material, increase the fill height using inactive material to provide a greater load on the underlying clay, seal the shoulders of the road and flatten the fill slopes using the material removed from the subgrade and side drains. This has the effect of moving the zone of seasonal moisture fluctuation away from the pavement structure and inducing movements and cracking in the more flexible fill slopes rather than in the stiffer pavement structure. Particular attention should be paid to culverts. The clay beneath them must be replaced with an inert material, all joints must be carefully sealed to avoid leakage and inlets and outlets well graded to avoid ponding of water. It is essential, however, that a proper understanding of the potential moisture movements in and around the road is obtained and this is related to the swell potentials of the various pavement materials (fill, shoulders, subgrade, etc.). It is also good practice to remove and control the re-establishment of “water loving” trees. The roots of such trees seek water beneath the pavement and remove it from the clay, causing significant depressions in the road during the dry season, which may or may not recover in the wet season. This is usually associated with arcuate and/or longitudinal cracking. 34  Guidelines for the Environmentally Optimized Design of Low Volume Roads 3.3.5.2 Dispersive/erodible/slaking materials Causes Dispersive, erodible and slaking materials are similar in their field appearance (highly eroded, gullied and channelled exposures), but differ significantly in the mechanisms of their actions. Fortunately for road builders, only the (probably less common) dispersive soils present problems of any consequence. Figure 15 shows a typical dispersive soil in an embankment with definite evidence of piping. Figure 15: Dispersive soil showing formation of “pipes” Dispersive soils are those soils Dispersive soil (crumb) test Figure 16:  that, when placed in water, have showing suspension that does repulsive forces between the not settle out clay particles that exceed the attractive forces. This results in the colloidal fraction going into suspension and in still water staying in suspension (Figure 16). In moving water, the dispersed particles are carried away. This obviously has serious implications in earth dam engineering, but is of less consequence in road engineering except when used in fills. Dispersive soils often develop in low-lying areas with gently rolling topography and relatively flat slopes. Their environment of formation is also mostly characterized by an annual rainfall of less than 850 mm. Erodible soils will not necessarily disintegrate or go into dispersion in water. They tend to lose material because of the frictional drag of 3. SURVEYS AND INVESTIGATIONS  35  water flowing over the material that exceeds the cohesive forces holding the material together. Slaking soils disintegrate in water to silt, sand and gravel sized particles, without going into dispersion. The cause of this process is probably a combination of swelling of clay particles, generation of high pore air pressures as water is drawn into the voids in the material and softening of any incipient cementation. Slaking and erodible soils when occurring as subgrades or even when used in fills are unlikely to cause significant problems unless rapid flows of water through the fill or subgrade occur. Problems are thus mostly associated with poor culvert and drainage design. The inclusion of dispersive soils in the subgrade or fill on the other hand has been seen to lead to significant failures through piping, tunnelling and the formation of cavities in the structure. It is therefore important to identify dispersive soils timely. Recognition The testing and recognition of dispersive soils requires various soil engineering and pedological laboratory tests. These include: ™™ Determination of the Exchangeable Sodium Percentage (ESP) ™™ Pinhole test ™™ Cation Exchange Capacity (CEC) ™™ Crumb test ™™ Double hydrometer test ™™ Sodium Absorption Ratio (SAR) and the pH. The crumb test on undisturbed lumps of material is usually the best first indication, but is not always fool proof. Dispersive soils tend to produce a colloidal suspension or cloudiness over the crumb/lump during the test, without the material necessarily disintegrating fully. Disintegration of the crumb in slaking soils is very rapid and forms a heap of silt, sand and gravel. Erodible soils do not necessarily always disintegrate in the crumb test as they require a frictional force of moving water to loosen the surface material, without any of the loose material remaining in suspension. It is not very important (or even possible) to quantify the actual potential loss of dispersive material from subgrades and fills as the process is time related and given enough time, all the colloidal material could theoretically be dispersed and removed, leading to piping, internal erosion and eventually loss of material on a large scale. It is, however, important to identify the presence of dispersive soils, and their differentiation from erodible and slaking materials, so that the necessary precautions can be taken if they affect the constructed pavement. Countermeasures The countermeasures for avoiding dispersive soil damage in the road environment are relatively simple: 36  Guidelines for the Environmentally Optimized Design of Low Volume Roads ™™ Avoid its use in fills as far as possible. ™™ Remove and replace it in the subgrade. ™™ Manage water flows and drainage in the area well. As the presence of sodium as an exchange cation in the clays is the major problem, treatment with lime or gypsum will allow the calcium ions to replace the sodium ions and reduce the problem. The use of gypsum is recommended over lime as lime may lead to soil stabilization with its associated cracking, allowing water to move through the cracks. It is also important that the material is compacted at 2 to 3% above optimum moisture content to as high a density as possible. To avoid problems with slaking and erodible soils, the drainage must be well controlled. Covering of the soils with non-erodible materials and careful bio-engineering, assisted by geosynthetics where necessary, is usually effective. Once erosion has occurred, the channels and gullies should be back-filled with less erodible material and the water flows redirected. 3.3.5.3 Saline Soils Causes Unlike dispersive soils that are affected by the presence of excessive cations of sodium attached to clays, saline materials are affected by the combination of specific cations and anions in the form of soluble salts, independent of clays. These can be a major problem on road projects where migration of soluble salts to beneath bituminous surfacings (Figure 17) leads to weakening of the upper base and blistering and disintegration of the surfacings. Soluble salts, particularly sulphates, and their acids can also have a serious detrimental effect on the stability/durability of chemically stabilized materials and concrete. Soluble salt damage to roads has been reported primarily from arid, semi-arid and warm dry areas. Salts can originate from the in situ natural soils beneath the structures as well Mechanism and manifestation of soluble salt damage to Figure 17:  bituminous surfacings 3. SURVEYS AND INVESTIGATIONS  37  as from imported material for the pavement layers or from saline construction water. Only the presence of soluble salts in subgrade materials is considered in this report as the materials for other layers can be controlled provided the problem is identified timely. Subgrade materials in areas where the land surface shows some depression resulting in seasonal accumulation of water are particularly prone to the accumulation of salts leached from the surrounding areas. In other flat areas, capillary rise of groundwater and precipitation in saline soils can result in the upward migration of salts to or near the soil surface. Recognition In some cases, the visible presence of crystallized salt deposits at the soil surface is a certain indication of the need for additional investigation for possible salt problems. This is often associated with the presence of animals licking the soil surface. In most other cases, the presence of salt is best confirmed by using laboratory test methods. In the conventional road engineering context, the identification of possible soluble salt problems is based on the pH and conductivity of the materials. Most roads departments do not differentiate between the subgrade materials and the imported layer materials. It should be noted that the results of the electrical conductivity and pH tests can vary significantly depending on the pre-treatment, the moisture content at which the measurements are made and particularly on the material size fraction tested. Limits for the use of saline materials are generally based on work in specific countries and their applicability to other areas is unknown. In general, an electrical conductivity on the fraction passing 6.7 mm of more than 0.15 Sm-1 (or an electrical resistance of less than 200 Ω on the minus 2 mm fraction) should raise concern and indicate the need for further investigation. Similarly, soluble salt contents higher than 0.5% should be a cause for possible concern and need for additional investigations. Countermeasures The following measures should be considered: ™™ As soluble salt problems arise from the accumulation and crystallization of the salts under the road surfacing and in the upper base layer, minimization of salts in the pavement layers and subgrade should be attempted. ™™ If the surfacing is sufficiently impermeable (coefficient of permeability, k in nanometre/second)/surfacing thickness, T in mm or k/T < 30 (μsec)-1) to avoid water vapour passing through it, crystallization will not occur beneath the surfacing. ™™ Construction should proceed as fast as possible to minimize the migration of salts through the layers. Only impermeable primes should be used, e.g. bitumen emulsions. ™™ The addition of lime to increase the pH to more than 10.0 will also suppress the solubility of the more soluble salts. 38  Guidelines for the Environmentally Optimized Design of Low Volume Roads Even for the lowest classes of road (< 50 vpd), the effects of excessively saline materials can lead to a rapid and total loss of the bituminous seal and precautions should thus be taken for all road classes. The use of non-bituminous surfacings should be considered over saline materials. 3.3.5.4 Soft Clays Causes Widespread problems, mostly in estuarine (lagoon) and marshy areas result from the presence of very soft alluvial clays in these areas. Deep soft clays in estuarine areas are formed mostly by periodic fluctuations in sea level. Inland soft clays tend to be much shallower having been deposited in marshy areas. Soft clays are generally, but not necessarily saturated and normally consolidated to lightly over-consolidated (because of fluctuating water tables). The materials thus have low shear strengths, are highly compressible and their low permeabilities result in time-related settlement problems. In addition, the frequent occurrence of organic material in the clays affects their behaviour and the determination of their properties. These materials are present predominantly in the coastal areas although they can also be associated with large mature river systems. The shear strength of these clays would normally be between 10 and 40 kPa, making them difficult to walk on. Soft clays are seldom uniform with depth and are usually interlayered with silts and sands, which provide more permeable drainage paths than would be determined from oedometer testing of undisturbed clay samples. However, the depths and strengths of the materials are such that inspection of the materials in test pits or auger holes is not recommended. Recognition The in situ condition of these materials is one of their most important properties that need to be considered – testing of disturbed samples will usually provide results that are meaningless. It is thus better to use in situ test methods such as Standard Penetration Testing (SPT), vane shear or Cone Penetration Testing (CPT) to determine the depths, presence of silt or sand layers, strengths and if possible, permeabilities. If these can be identified to a reasonable degree of confidence, estimates of the quantity and rate of settlement and the potential stability of embankments over the materials can be made. Countermeasures Road embankments built on soft clays thus need careful control during their construction to avoid stability failures as pore water pressures increase under the applied loads. It is recommended that embankments in these areas are constructed slowly, layer by layer, while monitoring pore water pressures and additional layers are only added once the pore water pressures have dissipated adequately. Despite even these measures, long- term settlement continues and problems are often encountered with large differential settlements between the approach fills founded on the clays and bridges founded on piles. These long-term differential settlements require ongoing maintenance to provide an adequate performance of the road. 3. SURVEYS AND INVESTIGATIONS  39  The use of the wide range of geosynthetic products as separation layers and to facilitate and accelerate drainage has contributed to improved construction over such areas in the past decade or two, and specialist advice in this respect should be obtained. 3.3.5.5 Wet and Flooded Areas/High Water Tables Causes It is possible that some non-clayey areas have a water table (permanent or temporary) close to the natural ground surface, which makes the placement of road structures difficult and can affect their structural integrity. Unlike the clay areas, the problem is not the low strength or settlement potential, but the effect of the water (and high pore-water pressures under traffic loading) on the pavement structure. Flooding has a similar effect when the road is open to traffic while still wet. High water tables result in a steady, high in situ moisture but it is also possible that fluctuating high moisture content conditions within the pavement sub-structure may occur because of seasonal precipitation. A good understanding of the moisture conditions and environment needs to be defined during any investigation involving subgrade materials. Various moisture indices such as Thornthwaite’s Moisture Index or water surplus maps can provide very useful information on potential problems in this regard. Many of the problems encountered in roads are common to specific moisture zones, and these have been highlighted under their respective headings in this document. Recognition It is usually easy to recognize potential wet conditions, which are characterized by areas of standing water, specific types of vegetation (reeds, papyrus grasses, etc.), localized muddy conditions and often the presence of crabs and frogs. Reports of seasonal flooding conditions will normally be available from local communities. Countermeasures The treatment of wet areas for roads can be costly if the aim is to reduce the water tables using sub-surface drainage systems. These would seldom be warranted for low volume roads, because of the cost and the ongoing need to maintain them diligently. However, in cases where they are essential, they should be designed by a drainage/ground-water specialist. The only cost-effective measures for low volume roads are to raise the level of the road to at least 750 mm above the natural ground or expected flood level, with a permeable gravel or rock fill layer (at least 100 to 150 mm thick) on the natural formation (after removal of the topsoil and vegetation). Properly designed and graded side drains should also be constructed to avoid the presence of standing water adjacent to the road. 3.3.5.6 Collapsible Soils Collapsible soils result from a unique condition in which “bridges” of fine materials (usually clays or iron oxides) within a framework of coarser and harder particles (mostly 40  Guidelines for the Environmentally Optimized Design of Low Volume Roads quartz) become weak when wet and collapse under load. The important condition is that the material must be in a partially saturated condition and then wetted up and loaded simultaneously, which is a common situation beneath road structures. Collapsible materials can occur on both residual and transported materials. Many granites and feldspathic sandstones when weathered result in the feldspar altering to kaolinite with the quartz particles staying intact. This forms a honeycomb type of structure, which, when wetted up and loaded, results in shearing or “collapse” of the clay bridges and a settlement or reduction in volume of the material. Certain basalts and dolerites with dry densities of 1200 to 1300 kg/m3 have also shown collapse potential. Indications of the possibility of collapsible materials are: ™™ A very low density, because of the large number of voids separating the quartz framework. ™™ Densities of less than about 1600 kg/m3 (mostly in the range 1000 to 1585 kg/m3). ™™ The presence of “pinholing” or voiding observed during the soil profiling. ™™ Usually more than 60% of the mass of the material lies in the 0.075 to 2 mm range and less than 20% is finer than 0.075 mm. ™™ When the material excavated from a pit is insufficient to fill the pit again (the collapse structure will be disturbed and the material will decrease in volume). If potentially collapsible soils are identified, specialist assistance should be used for roads carrying more than 200 vpd to avoid excessive rutting. The deformation that is likely to affect lower classes of roads will seldom have a major impact on their performance. The result of collapse of the subgrade is mostly manifested by the development of a deeply rutted and often uneven road surface and significant deterioration of the riding quality of the road (Figure 18). Figure 18: Typical manifestation of collapsible subgrade 3. SURVEYS AND INVESTIGATIONS  41  3.4 Construction Materials Survey  General 3.4.1 Part of the initial survey programme is also to identify potential sources of construction materials. Although the DCP design method attempts to optimise the use of in situ materials and minimise additional material usage, in many cases, the local materials may not be of suitable quality and other materials may be required. The availability of construction materials is becoming increasingly constrained as suitable materials are rapidly being depleted and environmental pressures limit possible material exploitation. This is leading to increased costs in obtaining and hauling material, long haulage distances that cause damage to the existing road infrastructure with associated increased vehicle emissions and possible construction delays, while alternative materials are sourced. The increasing need for environmental preservation and “Green” issues is also changing the road construction milieu. To minimize the cost of road projects, particularly those carrying light traffic, materials used in their structural layers should be sourced as close to the project as possible (haulage costs are frequently the highest component of material provision) and should be of the most appropriate standard for the respective layer. This means that the material should: ™™ provide the necessary strength and stiffness for the proposed layer in the road, without having excessively good properties. ™™ be able to retain those properties over the design life, and preferably longer, under the impacts of traffic and climate. It is essential that appropriate specification requirements are introduced for different categories of LVR, considering the main properties required, as well as the appropriate test methods (Section 3.5). This is particularly important for lightly trafficked rural access roads, where the traffic and loads imposed on most roads are minimal, and significant economies can be made by using material appropriate for the specific pavement characteristics. The design philosophy should be changed from “finding materials to suit the proposed pavement design” to “designing the pavement to suit the available materials”. The main requirements of materials in the structural layers of roads are to provide an adequate strength and stiffness to avoid shear failure or traffic induced compaction in the subgrade and to retain these properties over the expected life of the road, i.e. to be durable. It should be noted that traffic induced compaction (or rutting) can be almost eliminated by ensuring that all materials in the pavement are compacted to as high a density as possible (exclusion of as many voids as possible). Recent developments and research have indicated that the plasticity is in fact inherent in the strength of a material, together with other properties and in most cases, provided that the strength requirements are met, the plasticity is almost immaterial. It does, however, provide an indication of the potential moisture susceptibility of the material. One of the problems of prescribing various “interrelated” properties, in terms of specifications, is 42  Guidelines for the Environmentally Optimized Design of Low Volume Roads that if a material fails to satisfy one of these specifications, it will be rejected for use. In this way, materials that easily satisfy the “most important” strength criterion, but are marginally deficient on the plasticity or grading are often rejected for use. Despite the innumerable differences that exist among local materials, there are some dominant characteristics that affect pavement performance which should be appreciated to design and construct LVRs using such materials with confidence. These characteristics depend on whether the materials are used in an unbound or bound state, which affects the way they derive their strength in terms of the following intrinsic properties: ™™ Inter-particle friction. ™™ Cohesive effects from fine particles. ™™ Soil suction forces. ™™ Physico-chemical (stabilization) forces. The relative dependence of a material, and the influence of moisture, on each of the above components of shear strength will significantly influence the way they can be incorporated within a pavement. In this regard, Table 7 summarizes the typical relative characteristics of unbound and bound materials that critically affect the way in which they Table 7: Pavement material types and characteristics Parameter Pavement Type Unbound Bound Unprocessed Moderately Highly Very highly Processed processed processed Material types Category 1 Category 2 Category 3 Category 4 As-dug gravel Screened gravel Crushed rock Stabilized gravel Variability High Decreases Low Plastic Modulus High Decreases Low Development of Cohesion, suction Cohesion, suction Particle interlock. Particle interlock shear strength and particle & some particle & chemical friction/interlock interlock bonding Susceptibility to High Decreases Low moisture Design Material strength Selection criteria reduces volume of Material strength philosophy maintained only moisture sensitive, soft and poorly maintained even in a dry state graded gravels in wetter state Appropriate use Low traffic Traffic loading increases, High traffic loading in very environment becomes wetter loading in wetter dry environment environments Cost Low Increases High High Maintenance High Decreases Low requirement Of particular significance to LVRs 3. SURVEYS AND INVESTIGATIONS  43  can be incorporated into a pavement in relation to their properties and the prevailing conditions of traffic, climate, economics and risk. Unprocessed materials (Category 1), such as laterite, are highly dependent on suction and cohesion forces (gravel components will also add some interlock and friction, provided that the particles are strong enough to avoid excessive break-down) for development of shear resistance which will only be generated at relatively low moisture contents. Consequently, special measures must be taken to ensure that moisture ingress into the pavement is prevented, otherwise suction forces and shear strength will be reduced which could result in failures (Figure 19). Since most LVRs are constructed from unbound materials, a good knowledge of the performance characteristics of such materials is necessary for their successful use as discussed below: ™™ Category 1 materials: are highly dependent on soil suction and cohesive forces for development of shear resistance. The typical deficiency in hard, durable particles prevents reliance on inter-particle friction. Thus, even modest levels of moisture, typically approaching 60% saturation, may be enough to reduce confining forces sufficiently to cause distress and failure. ™™ Category 2 materials: have a moderate dependency on all forms of shear resistance – friction, suction forces and cohesion. Because these materials have rather limited strength potential, concentrations of moisture, typically 60-80% saturation may be enough to reduce the strength contribution from suction or cohesion sufficiently to cause distress and failure. This would occur at moisture contents lower than those necessary to generate pore pressures. Figure 19: Illustrative soil strength/suction relationship 75 Equilibrium moisture content Soil strength (CBR) 50 Optimum moisture content Soaked 25 0 pF 0 1 2 3 4 Soil suction 44  Guidelines for the Environmentally Optimized Design of Low Volume Roads ™™ Category 3 materials: have only minor dependency on suction and cohesion forces but have a much greater reliance on internal friction which is maximized when the aggregate is hard, durable and well graded. Very high levels of saturation, typically 80-100% will be necessary to cause distress and this will usually result from pore pressure effects. ™™ Category 4 materials: rely principally on physio-chemical forces which are not directly affected by water. However, the presence of water can lead to distress under repetitive load conditions through layer separation, erosion, pumping and breakdown. The management of moisture during the construction and operational phases of a pavement affects its performance, especially when unbound, unprocessed, generally relatively plastic materials are used. Emphasis should be placed on minimizing the entry of moisture into an LVR pavement to ensure that it operates as much as possible at an unsaturated moisture content. The beneficial effect of so doing is illustrated in Table 8, which shows the variation of a material strength (CBR) with moisture content. Table 8: Variation of CBR with moisture content Laboratory Soaked Approximate Laboratory Unsoaked CBR (%) at CBR (%) varying FMC/OMC Ratios 1.0 0.75 0.50 80 105 150 200 65 95 135 185 45 80 115 165 30 65 95 140 15 45 70 110 10 35 60 100 7 30 50 85 The FMC/OMC ratio is a significant contributory factor related to the performance of an LVR. If, through effective drainage, the materials in the road pavement can be maintained at a moisture content that does not rise above OMC in the rainy season, then more extensive use can be made of local, relatively plastic materials that might otherwise not be suitable if they were to become soaked in service. 3.4.2 In situ materials The in situ materials are those beneath the proposed road or in the immediate surroundings that may be utilised in the construction. They consist of those materials that are naturally in their current form (state) and position because of geological and geomorphological processes. There is no control over the quality of these materials and decisions must be made as to whether they are to be utilised or spoiled. These decisions are typically based on the results of the test pitting and testing. 3.4.3 Borrow pit materials On most projects, it will be necessary to make use of some imported materials. These would be natural gravels consisting of transported or residual materials that occur relatively close to the project, to minimise haul distances. 3. SURVEYS AND INVESTIGATIONS  45  During the initial surveys, it is important to identify existing borrow pits or use local indicators (excavations, plants, erosion channels, etc.) to identify other potential sources of construction materials. In most cases, these would be for use in base course construction. Materials of particular interest for construction of layer works include local moorums, kankar and laterite and particular note should be made of the presence of these. 3.4.4 Aggregate sources Fresh or unweathered rock is ultimately the source of all the above material groups and consists of igneous, sedimentary or metamorphic material that has undergone no (or very little) decomposition or alteration. It usually requires blasting, crushing and screening before its use in roads, but is often necessary for concrete aggregate for structures along the road as well as aggregate for surfacing seals and asphalt. These materials, depending on the rock type and origin, generally provide high quality, durable aggregates after excavation, crushing and screening. However, they are costly materials, require the development of a quarry and are difficult to locate in or close to urban areas where environmental and mining issues often result in severe technical constraints and cost implications. The use of fresh rock in low volume and rural access road structures should thus be limited to high quality surfacing aggregates where appropriate and concrete applications within the overall road system. There is seldom a need to use such high quality and expensive materials for pavement layers (or even many bituminous surfacings, such as sand and Otta seals) in LVRs and such materials should be conserved for use in higher standard roads in future. To promote “greenness” in road construction, the use of non-natural materials should be increased as far as possible. These include by-products from most industrial enterprises (e.g. mining, manufacturing, industrial, power generation, etc.) as well as wastes produced in the urban environment (small industrial, construction and demolition wastes and even domestic wastes). These materials have mostly been through a severe comminution and often heating process and generally incorporate a significant quantity of “embodied energy” through their primary processing as opposed to using a lot of additional energy in their production for use in roads. Their use also emits considerably less emissions than the processing of an equivalent quantity of natural material. There is a wide range of such materials available, many of which can be (and have been) used in structural layers for roads, while others can be used in lower support layers or as modifiers or stabilizers for improving materials. Common waste (or preferably termed “by-product” materials include, but are not limited to: ™™ Reclaimed bituminous material (RA) ™™ Crushed concrete ™™ Pulverized fuel ash (PFA or flyash) 46  Guidelines for the Environmentally Optimized Design of Low Volume Roads ™™ Blast-furnace slag ™™ Steel slag ™™ Other metallurgical slags (e.g. chrome, manganese, copper, zinc) ™™ Aluminium industry wastes ™™ Colliery spoil ™™ Construction and demolition waste ™™ Mine and stone processing waste (Marble, slate, granite, etc.) ™™ Phosphogypsum ™™ Tires ™™ China clay sand ™™ Foundry sand ™™ Crushed waste brick ™™ Used rail ballast ™™ Furnace bottom ash ™™ Cement kiln dust ™™ Glass ™™ Spent oil shale Some of the issues with the utilization of by-product materials are the variability often encountered in existing “waste” dumps due to uncontrolled disposal as well as the unique properties of each type of by-product material, which need research into their individual characteristics and usage requirements. Another issue is that many of these products are, usually incorrectly, classified as hazardous or toxic wastes by environmental authorities and require special permits for their use. It is thus important that all potential waste materials be tested for toxins and environmental acceptability before substantial work is carried out on them. Table 9 summarizes some of the uses and potential problems associate with by-product materials. 3.4.5 Typical Local Materials and Industry By-Products Typical local materials and Industry waste available in some states, which could be used in road construction for LVRs are presented below. It is recommended that other materials and industry waste, not mentioned here, should be investigated for use in road construction. 3.4.5.1 Kota Limestone There is considerable by-product material produced by the mining and production of limestone wall and floor tiles in the Kota area of Rajasthan that Kota Stone 3. SURVEYS AND INVESTIGATIONS  47  Classification and summary of uses and properties of Table 9:  by-product materials Classification Possible uses Potential problems Special requirements of by-product material Urban by-products Crushed concrete Base Can lead to over- Requires careful sorting Subbase stabilization at collection point Concrete aggregate Requires crushing and screening Aluminium Below subbase Very fine material Unknown at this stage industry wastes Stabilizer Often mixed with other wastes Construction and Base Requires careful None – should behave as demolition waste Subbase processing/sorting a normal aggregate Concrete aggregate May contain a lot of deleterious materials Tires Below subbase Variability of rubber Requires innovative Bitumen modifier types in tires (natural and research (e.g. highly Concrete aggregate synthetic) flexible pavements) Collection, sorting and processing must be formalized Foundry sand Subbase Fine grained. May Unknown at this stage Below subbase contain unusual Mechanical stabilizer chemicals (release agents) Cement kiln dust Stabilizer Very fine dusty material More rapid construction Pozzolanicity lost with (quicker reaction rates storage due to fineness). Difficult to place and mix Glass Surfacing aggregate Difficult to work with Special processing/ Mechanical stabilizer Fractures into flaky crushing technique particles needs to be developed Rural by-products Pulverized fuel ash Cemented layer Very fine/dusty. New innovative uses (PFA or flyash) Stabilizer Monopolies often control need to be assessed supply Blast-furnace slag Surfacing aggregate Contains unhydrated Must be conditioned Base oxides. Can be variable before use to eliminate Subbase Possible deleterious oxide Concrete aggregate components Variable density (voids) Steel slag Surfacing aggregate Contains unhydrated Must be conditioned Base oxides before use to eliminate Subbase Can be variable oxides. Otherwise Concrete aggregate Possible deleterious similar to a conventional components aggregate Variable density (voids) 48  Guidelines for the Environmentally Optimized Design of Low Volume Roads Other Base Contains unhydrated Must be conditioned metallurgical slags Subbase oxides. Can be variable before use to eliminate Possible deleterious oxides components Variable density (voids) Colliery spoil Subbase Highly variable materials Needs good sorting and Below subbase May contain sulfates processing. Otherwise Abrasive on crushing similar to a conventional plant aggregate Mine and stone Surfacing May contain deleterious Need to be assessed processing waste aggregate; materials individually Base; Subbase; Concrete aggregate Crushed waste Subbase Variable physical Needs investigation for brick Mechanical stabilizer properties each use Small individual deposits Furnace bottom Base Soft particles Needs careful ash Subbase Contains unburnt coal processing Below subbase May contain sulfates Spent oil shale Subbase Variable hardness and Little known Below subbase composition By-products from both rural and urban environments Reclaimed Asphalt Variable materials Each source needs bituminous Base evaluation and material (RA) Subbase investigation for each project Phosphogypsum Cemented layer Very fine powder Needs additional Changes moisture research for bulk use condition readily China clay sand Mechanical stabilizer Fine grained Little research done Below subbase Used rail ballast Base Collection of sufficient New disposal strategy Subbase quantities from railway authority Concrete aggregate required could be used in LVR construction. Quarries producing Kota limestone generate large quantities of solid waste. Every year nearly 23 million tonnes of solid waste is added to the existing quantities in the Kota area. This waste is in the form of overlying burden, inter bedded burden and production waste generation during cutting, sizing and splitting at quarry floor. The waste limestone has a high compressive strength of nearly 150 MPa. It can be used as a subbase and base material in road construction. Even though a large quantity of this material is available, its economic haulage will need to be ascertained. However, from an environmental perspective, it would be desirable to use this by-product material. 3. SURVEYS AND INVESTIGATIONS  49  3.4.5.2 Jarosite Jarosite is a by-product material produced during the extraction of zinc ore concentrate in the hydro-metallurgy operation. Technical specifications have been developed for the utilization of Jarosite material in the construction of embankment and subgrade layers of road. The findings indicate that Jarosite (100%), Jarosite-soil mixes (50-75%) and Jarosite-bottom ash mixes (50-75%) have the potential for the construction of road embankments while the Jarosite- Jarosite stockpile soil and Jarosite-bottom ash mixes (50- 75%) may be used for construction of subgrade layers of pavements [17]. The economic viability for use in roads by mixing with local soils and comparative increase in strength and its use in road subbase should be assessed. The high addition of lime and cement, however, may make it uneconomical except in certain areas. 3.4.5.3 Marble Dust The wastes generated from the marble industry have the potential to be utilised in various applications including road subbase material. A study in Turkey [18] indicated that fly ash, marble dust and waste sand are potentially useful additive materials in road subbases as they improve the CBR and reduce swelling in clayey soils. 3.4.5.4 Kankar Marble dust Kankar is available at the road-side in open borrow-pits in many areas. There may be several locations where similar material may be available for direct use in the roads especially for very low volume roads. 3.4.5.5 Copper Slag Copper slag is available as a waste product in Rajasthan and other states in India. It is a “waste” product from the copper smelting process. It is estimated that in the production of 1 tonne of blister copper, 2.2 tonnes Kankar 50  Guidelines for the Environmentally Optimized Design of Low Volume Roads of slag are generated. Copper slag can be classified as a non-hazardous material. However, it needs to be tested to verify that it meets the regulatory requirements. Granulated copper slag is more porous and, therefore, has similar particle sizes to coarse sand. Trials and past research [19] suggest that: ™™ Copper slag can be used as a construction material in combination with cement and fly Copper slag ash to improve the properties of expansive clay soils. ™™ From 30% to 50% copper slag can be mixed with soils to improve their characteristics. ™™ Fine sand with up to 40% copper slag can be used as fine aggregate in pavement quality concrete as well as in dry lean concrete. ™™ CBR values of cement mixed with soil and copper slag are 3 to 7 times higher than that of the soil with copper slag waste without the presence of cement. ™™ Expansive soils can be improved by utilizing 40% of the copper slag along with 2% Portland cement. ™™ The soaked CBR value of a 30% fly ash / 70% copper slag blend was reported to be 78% after 28 days of curing. ™™ To get good soil stabilization, combinations of 70% clay/30% copper slag ranging to 30% clay/70% copper slag were the most satisfactory combinations. ™™ In Texas, copper slag used as a coarse aggregate in a concrete pavement has performed well after over 15 years in service. 3.4.6 Material improvement There are many instances when the local materials, even if highly compacted and retained in a dry state do not meet the strength requirements for a specific layer. In these cases, the use of some form of mechanical or cementitious improvement may be necessary. There are several options available, many of them, however, being expensive. A laboratory investigation to assess the improvement in material properties with each of the techniques is essential, to optimize the cost and the material properties. This usually requires innovative thinking and appropriate laboratory testing on part of the design engineer. In this process, the following questions should be answered: ™™ What are the pavement requirements? ™™ What materials are available for investigation? ™™ How best are each of these materials treated? ™™ What laboratory test procedure is necessary to prove their “fitness for purpose”? 3. SURVEYS AND INVESTIGATIONS  51  3.4.6.1 Mechanical stabilization This involves the improvement of a material by mechanical means. Various techniques are available for this and must be seriously considered as they are far more cost-effective than chemical or other types of stabilization. Compaction Compaction is by far the most economic and simple, way of improving a material. All the properties of a material are enhanced by achieving as high a degree of compaction as possible. These include: ™™ Higher shear strength and stiffness, ™™ Lower permeability ™™ Less rutting potential ™™ A better support (anvil) for the compaction of overlying layers All attempts to improve the compaction of layers and achieve the highest possible compaction should be made. This requires good compaction equipment with operating vibration capacity, a uniform distribution of moisture (at close to or just below optimum moisture content for modified compaction) through the material and the necessary number of passes. Compaction is one of the cheapest construction activities and additional compaction generally involves only a small additional fuel cost – the equipment and operators are on site anyway, even when the compaction process is not taking place, and thus involve no additional costs. No compromise on compaction should be allowed: the use of well-compacted marginal materials and subgrades not only improves the pavement performance but also allows thinner pavement layers to be used, with significant material savings and the associated sustainability benefits. Compaction to refusal, which normally entails a maximum of 4 or 5 additional roller passes until no additional densification occurs, but avoiding breakdown of particles and de-densification, is generally considered a highly beneficial operation. Removal of oversize The removal of oversize material enhances the workability of the material as well as improving its properties. Material with excessive oversize material often acts as “plums in a pudding” resulting in no strength being contributed by the aggregate (no inter- particle friction/interlock), with the strength of the finer matrix determining the overall material properties. In addition, the impact of the oversize material on the properties of the bulk material cannot be determined as these aggregate particles are excluded from most testing. There are many ways of removing oversize, but this is done most economically at source where unnecessary haulage to the roads and usually back to the borrow pit can be avoided. Screening and/or crushing in the borrow pit (using small or mobile crushers) should be carried out when excessive oversize material is present, this usually being classified as material with more than about 5 or 10% larger than 37.5 mm. 52  Guidelines for the Environmentally Optimized Design of Low Volume Roads The presence of oversize materials results in a poor finish of the compacted layer surface and makes final cutting of levels difficult. This is not critical for subgrade and subbase layers (in fact a rough surface may enhance the bond between the layers) but is not permissible for base courses where a thin bituminous surfacing is to be applied. Large aggregate particles also interfere with the uniform compaction of the materials (material adjacent to large particles is not effectively compacted). Blending Blending of different materials can be used to improve the characteristics of many poor materials. Mixing of materials from different sources, usually one coarse and one finer can reduce (or increase when required) plasticity and improve the shear strength and performance of many materials. The optimum blend ratio is usually determined by laboratory testing, but can also be estimated mathematically using various processes. In India, the increased use of blending is perceived to be a major advantage in future construction of LVRs. Many of the natural subgrades are very weak fine materials (e.g. Ganga alluvial silts and clays) with no gravelly alternatives and their strengths can be significantly Figure 20:Change in strength (DN in mm/blow) as a result of blending 10 9 DN at 98% Heavy compection (mm/blow) 8 7 6 5 4 3 0 10 20 30 40 50 Proportion of aeolian sand by volume 3. SURVEYS AND INVESTIGATIONS  53  improved by the controlled addition of coarser materials. These coarse materials can often be obtained from the by-product materials discussed previously (a typical example being waste bricks), preferably materials in the range 5 to 25 mm in diameter. However, to optimize each possible blend, laboratory tests with different proportions of the two materials need to be carried out to identify the optimum blend ratio. The addition of a coarse fraction will usually raise the quality of the silty/clayey material to at least selected subgrade and often subbase quality. In such cases, laboratory investigations are necessary using various proportions of the two materials to determine the optimum blend ratio. This type of investigation is particularly relevant in parts of India, where there are often, for instance, large sources of reject bricks (brick-bat) that could be broken/crushed and added to the local silty materials to provide a better material. Representative samples of the reject bricks (even including the residual ash and soft bricks) should be crushed to a maximum size of about 20 or 25 mm and added in various proportions to local soils as indicated for the sands above, to determine whether the strength (in terms of DN value) can be improved to that required for the proposed layer. Each source of local material and by-product material needs to be tested individually to check for compatibility and effectiveness. This is usually best done by initially blending various ratios of the dominant material with smaller quantities of the “improving” material (e.g. 90:10, 80:20, 70:30, etc.) to identify the optimum blend ratio. 3.4.6.2 Chemical stabilization Chemical stabilization involves the addition of a chemical agent to the materials that affects the chemical properties of the material. This could be either chemical modification, in which only the actual minerals are affected without any chemical cementation (i.e. no tensile strength is developed) or cementation in which new cementitious bonds (hydrated calcium silicate minerals) are developed, imparting a significant tensile strength to the material. The quantity of stabilizer required to achieve the desired properties may be so high as to make the process totally non-cost-effective. For this reason, each material must be assessed using the most suitable stabilizers and following a standard test protocol. This protocol will vary depending on the types of stabilizer investigated. Conventional chemical stabilization makes use of cement, lime, lime/flyash blends and various other mostly by-product materials (ground granulated blast furnace slag (GGBS), cement kiln dust, burnt rice husk ash, etc.). Cement is typically used for materials with a Plasticity Index (PI) below about 10%, lime for materials with a PI greater than 15% and combinations of the two with intermediate PI values. It should also be noted that cement can have a range of compositions, depending on the type and quantity of extender (i.e. PFA, GGBS, pozzolan, ground limestone, etc.) added and the rate of reaction depends significantly on the fineness to which the cement is ground. Flyash or GGBS alone cannot be used to stabilise soils. They must be activated by the addition of lime or cement (preferably lime) to allow their pozzolanic or latent hydraulic 54  Guidelines for the Environmentally Optimized Design of Low Volume Roads reactions to occur. Thus, it is always necessary to carry out laboratory testing with various combinations of the flyash or GGBS with and lime or cement for each soil type being investigated. Once the best stabilizer type has been identified (in terms of both availability and reactivity, based on initial testing with a nominal stabilise content), it is necessary to determine the optimum stabilizer content. This requires that the material is tested with various contents of the selected stabilizer to determine the optimum content at which the design strength (Unconfined (UCS) and tensile (ITS)) is obtained. It should be noted that, to ensure long- term durability of the cemented layer, this percentage of stabilizer should exceed the Initial Stabilizer Demand (ISD) by at least 1%. It is also often useful to treat blended materials (coarse and fine blends (see 3.4.5.1)) with a chemical stabiliser to improve the material even further. This is done on the pre-blended material as described above. Chemical stabilization must not be a purely mechanical process in which a fixed quantity of stabilizer is added to any material, as every material has unique mineralogy and properties resulting in different stabilizer requirements – both in terms of quantity and type of stabilizer. Testing of the ISD is essential to determine which stabilizer is best and what quantity should be added to produce the necessary properties. Suitable guidelines in this respect do not currently exist in India and need to be developed as a separate Guidance Note. Chemical stabilization is an art and champions in this art need to be developed in India. Many problems with chemical stabilization occur during construction and it is essential that a pool of experts be developed in India, who can respond to problems and quickly identify the mechanisms for avoiding them during projects. It is also important that the behaviour and performance characteristics of stabilised materials are fully understood by users. 3.4.6.3 Proprietary soil stabilizers Internationally, the road construction market is currently flooded with various proprietary chemical soil stabilizers and improvers. These chemicals should be used with caution, only after rigorous laboratory testing, careful economic analysis and preferably carrying out some properly controlled and monitored field trials. The use of chemically treated layers near the top of the pavement structure, as proposed by many of the chemical vendors to use local materials, should ensure that the pavement balance is not disturbed too much. Road performance is seldom satisfactory when a strong treated layer is constructed on a poor support. Proprietary soil stabilizers make use of various physical and chemical additives, each with its own mechanisms, actions and results. Many of these are not communicated fully by the suppliers and it is often difficult to carry out tests to assess their performance, without knowing the fundamental principles related to the products. 3.4.7 Construction water Road construction requires significant sources of water to ensure proper compaction, stabilisation and finishing of the structural layers. During the initial investigations and 3. SURVEYS AND INVESTIGATIONS  55  surveys, it is thus also important to identify potential sources of construction water. Normal river water is usually adequate for construction, but groundwater resources may be slow and expensive to extract and may also be excessively saline for road construction in many cases. Typically, water that is potable to humans should be used for construction, to minimise potential soluble salt problems. 3.5 Materials Testing  3.5.1 General The samples collected during the route survey must be tested in the laboratory to assess their properties and suitability for possible use in the pavement. All testing should comply with local standards and the methods must be meticulously followed, using the correct and recently calibrated equipment. These standard material testing techniques (and their accompanying material specifications) have been developed to assess the quality and durability of road construction materials over the past century or so. Unfortunately, many of these have not been reviewed for the past 70 or 80 years and are still applied in the standard manner, despite numerous changes to road design philosophies over this period. An example of this is the almost international use of a minimum soaked California Bearing Ratio (CBR) of 80% and a maximum Plasticity Index of 6% for all base course materials irrespective of traffic and environment. It is well known that both tests have different protocols in different countries, and thus produce significantly different results, and yet the same limits are used in various countries. It should also be noted that a CBR of 80% for base course materials is used in many design guides, irrespective of the need of the road, i.e., the specification is applied to roads carrying 1,000,000 standard axles as well as those designed to carry only 10,000 standard axles. This results in high construction costs as well as a wastage of material that should be conserved for roads with higher volume of traffic. 3.5.2 Test methods Samples of the local in situ materials as well as the possible construction materials collected during the soil survey should be tested in an approved laboratory for routine properties that allow the material to be classified. Classification testing will include as particle size distribution, plasticity and compaction characteristics (maximum dry density and optimum moisture content) and material strength. Two important deviations from conventional Indian practice are, however, required: 1. Compaction testing must include the use of IS 2720 Part 8 – (heavy compaction) as the use of higher compactions alone can result in significant improvements in material properties in the field as well as leading to savings in material quantity necessary for any single traffic class. 2. Instead of conventional CBR testing, the materials are compacted into the CBR moulds (150 mm diameter) and testing using a DCP directly in the mould (Figure 21). 56  Guidelines for the Environmentally Optimized Design of Low Volume Roads This testing should Figure 21: Determination of laboratory DN value be done at various moisture and density combinations for the typical soils to get a good understanding of the moisture/density/ strength relationships of the materials as shown in Figure 22. When blending or improving materials, the standard test protocols should be followed on the individual materials S75 as well as the different blends. If blending alone is insufficient to improve the materials to the required quality, controlled addition of different stabilisation agents, (e.g. cement, cement/flyash mixtures, lime, etc.) to the blended materials should be attempted. It is unlikely that such mixtures, without prohibitively high cement contents, First reading at “zero blows” when will lead to any top of shoulder of cone is level with Ruler with significant cementation top of sample in mould mm scale development, and conventional CBR testing (at heavy compaction effort) should be carried out Annular on the blends. weight Empty upside down The data obtained CBR mould or siliar through this testing will be applied directly to the design of the pavement structure using the DCP method (Chapter 4). In addition, the results of the laboratory testing at the specified density and compaction moisture content can be used later for quality control purposes during construction. 3. SURVEYS AND INVESTIGATIONS  57  Figure 22: DN/density/moisture relationship 12 10 Soaked DN (mm/blow) 8 6 OMC 4 0.75 OMC 2 0 92 93 94 95 96 97 98 99 100 Compaction level (% IS Heavy Compaction) Soaked OMC 0.75 OMC 58  Guidelines for the Environmentally Optimized Design of Low Volume Roads 4. PAVEMENT DESIGN 4.1 Introduction  4.1.1 Background The objective of pavement design is to produce an economic, well balanced pavement structure, in terms of material types and thicknesses, that can withstand the expected traffic loading over a specified period (the chosen design life of the pavement), without deteriorating below a pre-determined level of service. To achieve this goal, sufficient knowledge of the subgrade strength, pavement materials, traffic loading, local environment factors (particularly climate and drainage) and their interactions is required to be able to predict reasonably the performance of any pavement configuration. In addition, there should be a clear view as to the level of performance and pavement condition that is considered appropriate in the circumstances for which the pavement structure is being designed. Pavement design for LVRs presents a particular challenge to designers. This is largely because, until relatively recently, such roads were not specifically catered for and the step from a gravel road to a paved road one was a large one. However, considerable research has been carried out internationally that has led to the development of simplified pavement design methods that enable unpaved roads to be upgraded economically to a paved standard by making optimal use of local materials that do not meet the standard specifications found in most design manuals. It is these design methods that are described in this chapter. One of the biggest changes in the design process discussed in this chapter is that it is based on extending design procedures for unpaved roads upwards for low volume paved roads. The more conventional practice of trying to reduce conventional paved road designs for low volume roads has been found to have numerous pitfalls. Probably the most significant of these is the use of the normal assumptions of isotropic, elastic, uniform material concepts, that are less appropriate for natural gravels and local materials used in LVRs. 4.1.2 Purpose and Scope The purpose of this chapter is to provide details of the manner of determining the structural requirements of an LVR pavement in terms of the required layer thicknesses and material quality for different traffic categories. The design method is based entirely on the use of the DCP in contrast to the more traditional methods that are based on the California Bearing Ratio (CBR). 4. PAVEMENT DESIGN  59  The scope of the chapter is to provide an overview of the DCP design method and includes a step-by-step procedure to be followed in producing a pavement design for an LVR based on the key characteristics of subgrade strength, the expected traffic loading and the properties of the available materials for use as the pavement layers. 4.2 Design Principles  4.2.1 Approach to design The general approach to design of LVRs differs in several respects from that for HVRs. For example, conventional pavement designs are generally directed at relatively high levels of service requiring numerous layers of selected materials. However, significant reductions in pavement costs for LVRs can be effected by reducing the number of pavement layers and/or thickness, by using local materials and by using lower cost, more appropriate surfacing options. An important aspect of the design of HVRs is the minimization of pavement deflections. However, many of the lighter LVR pavement structures can tolerate relatively higher deflections (more than 1.0 mm). This is not necessarily a problem, but the choice of surfacing would certainly be influenced, with more flexible types of bituminous seal being necessary. Ultimately, the challenge of good pavement design for LVRs is to provide a pavement that is appropriate to the road environment in which it operates and fulfils its function at minimum life cycle cost at an optimal level of service. However, positive action in the form of timely and appropriate maintenance, as well as adequate control of vehicle overloading will be necessary to ensure that the assumptions of the design phase hold true over the design life of the road. 4.2.2 Pavement structure and function When the natural subgrade of a road is not strong enough to support the repeated application of axle loads without deforming, it will be necessary to protect it from overstressing by traffic loads. This can be achieved by introducing stronger materials above the subgrade (the pavement layers) to provide a chosen level of service as cost effectively as possible. The materials comprising these pavement layers must possess the following attributes if the pavement is to perform satisfactorily within the dictates of the prevailing road environment: 1. Sufficient stiffness (load-spreading ability) which is achieved essentially through inter-particle friction and shear strength (as measured with the DCP), which depend on the presence of horizontal confining stresses. 2. Sufficient bearing capacity which is the ability to withstand repeated cycles of vertical stress without excessive deformation. Figure 23 illustrates conceptually the way in which a pavement functions under loading. In essence, the wheel load, W, is transmitted to the pavement surface through the tire. The pavement then spreads the wheel load to the subgrade so that the maximum pressure 60  Guidelines for the Environmentally Optimized Design of Low Volume Roads Dispersion of surface load through a granular pavement Figure 23:  structure W Vertical stress Wearing course Lateral Load dispersion Base and subbase Conflining Stresses Subgrade or selected fill Subgrade stresses Depth on the subgrade is reduced sufficiently to avoid overstressing it to an unacceptable level. This can be achieved by proper selection of pavement materials of appropriate thickness and quality. 4.3 Design Methods  4.3.1 General 4.3.1.1 Design methods There are a number of methods that may be used for the design of LVRs which may be categorised as follows: 1. CBR cover method 2. The AASHTO design method 3. Catalogue/chart methods: Examples a. UK Overseas Road Note 31 b. South African TRH4 c. Austroads d. SADC/TRL e. IRC : SP : 72 The above methods all require four main design activities namely: ™™ Assessing the strength of the subgrade or of the layers of an existing old road prior to improvement or upgrading. ™™ Assessing the design traffic loading. ™™ Selecting materials for the pavement layers. ™™ Determining the thicknesses of the pavement layers. 4. PAVEMENT DESIGN  61  4.3.1.2 CBR cover method This is one of the original design methods that was developed in the 1950s from empirical data. The method is based on the approach of protecting the subgrade by providing enough cover of sufficient strength to protect the subgrade from the traffic loading. Various design charts have been prepared from which the depth of construction required to protect a subgrade of any defined strength (in terms of CBR) is defined for various traffic categories and equivalent wheel loads. Due to its empirical nature, and the fact that it was developed under locally prevailing conditions in the USA, the CBR Cover Curve Method should be evaluated critically before it is applied to local environmental and traffic conditions. This method is currently not used extensively in many countries due largely to the development of a number of “Catalogue” design methods based on mechanistic-empirical and empirical design, as discussed below. 4.3.1.3 AASHTO design method The AASHTO design method, which was developed from the results of the AASHO Road Test that was conducted in Chicago, United States, during 1959 and 1960, is still used widely in many tropical countries. However, it suffers from a number of drawbacks which cast serious doubts on its applicability to the design of LVRs in tropical countries, including: ™™ The Road Test was located in an area where winter temperatures cause up to 1.5 m of frost penetration for three months of the year. ™™ Almost all the pavement deterioration took place during the spring thaw when the pavements were saturated with water. ™™ It is very difficult to estimate how the roads would have performed in the absence of the freeze/thaw cycle. However, it is very likely that the long term deterioration would not have been the same as was observed during the later months of summer at the Road test. ™™ The Road Test was built on one type of subgrade of very low strength. Estimating the performance of roads built on other subgrades from the results of the Road test is very difficult. ™™ The Road Test was an accelerated test. Environmental effects that play an important role in the deterioration of roads in tropical countries was not evaluated. ™™ heel load and contact pressures remained constant. If mixed traffic had been W used, the conclusions regarding the relative damage effect of each wheel load would have been somewhat different. ™™ he axle loads used in the Road Test were 13 tonnes on a single axle and 21.5 tonnes T on a tandem axle. Such loads are not the norm on LVRs. 4.3.1.4 Design catalogues Design catalogues/charts are the easiest design process to use as all the practical and theoretical work has been carried out and different structures are presented in catalogue form for various combinations of traffic, environmental effects, pavement materials and design options. Three main types of catalogues have been developed as follows: 62  Guidelines for the Environmentally Optimized Design of Low Volume Roads 1. Those based on accelerated testing, theoretical analyses, in situ testing and evaluation of pavements in service. Examples include: a. South African TRH4 design catalogues b. DCP design catalogues 2. Those based on the results of full-scale experiments where all factors affecting performance have been accurately measured and the variability quantified. Examples include: a. UK Overseas Road Note 31 design catalogues 3. Those based on back-analysis of existing pavements involving the monitoring, collection and analysis of performance data. Examples include: a. The SADC/TRL design catalogues b. Austroads design charts The design approach that has been adopted in these Guidelines is based on the DCP-DN method which offers the following advantages: ™™ The fundamental principle of the method is based on moving technologies up from unpaved roads instead of attempting to downscale from conventional roads based on several non-valid assumptions. ™™ It moves away from the more traditional, empirically developed, CBR design approach, which provides an indirect measure of the strength of a material, to a more direct method of measuring in situ shear strength based on the use of the Dynamic Cone Penetrometer (DCP). ™™ It focuses on the use of the DCP for evaluating in situ road conditions and, by integrating the design strength profile optimally with the in situ strength profile, for designing LVR pavement structures in a highly cost-effective manner that minimizes the use of imported materials. ™™ It facilitates the greater use of local, more abundant, and therefore less expensive, locally available and by-product materials in the road pavement by a variety of techniques for improving these materials and for evaluating their properties in the laboratory using the DCP. ™™ Under circumstances where unsoaked conditions can reliably be assumed based on good drainage conditions and preferably sealed shoulders (ref. Section 4.4.4.3) and adequate maintenance. The DCP method of design is described below and an example is provided in Annex II. 4.3.2 DCP Design Method 4.3.2.1 General The philosophy behind the DCP design approach is to achieve a balanced pavement design whilst also optimizing the utilization of the in situ material strength at the expected in-service moisture content as far as possible. This is achieved by: ™™ Determining the design strength profile needed, which is related to the design traffic loading. ™™ Integrating the design strength profile with the in situ strength profile. 4. PAVEMENT DESIGN  63  To utilize the existing gravel road strength, the materials in the pavement structure need to be tested for their actual in situ strength, using a DCP. This instrument has been designed to provide a rapid, low-cost, non-destructive method of estimating the in situ strength of fine-grained and granular subgrades, granular base and sub-base materials and weakly cemented materials. All of the previous Indian design manuals (IRC: SP20-2002; IRC: SP 72-2007 and IRC: SP72-2015) permit the use of unsoaked CBR strengths in selected locations: although this is one of the fundamentals of the DCP design concept, it thus does not differ fundamentally from the understanding in India over the past 15 years. 4.3.2.2 DCP Design Procedure Existing tracks and roads A flow diagram of the DCP design process is shown in Figure 24. The process entails carrying out a series of activities that are aimed ultimately at determining a suitable pavement structure from a design catalogue (this is related to the design traffic loading and corresponding traffic design category) and comparing that with the existing pavement structure determined from the DCP survey. Steps 1–5 are addressed in Chapter 3. The remaining design steps, Steps 6-12 are described in this chapter. Figure 24: Flow diagram of DCP design procedure [16] Define Design Period Determine uniform sections 01 07 (CUSUM analysis) Chapter 3 Chapter 4 Adjust DN values for design 02 Determine Design Traffic 08 moisture content Chapter 3 Chapter 4 Determine in situ LSP for Determine Traffic Class 03 09 each uniform section Chapter 3 Chapter 4 Undertake DCP Survey Determine required LSP for 04 10 each uniform section Chapter 3 Chapter 4 Determine moisture Compare in situ LSP 05 content along road pavement 11 with required LSP for each uniform section Chapter 3 Chapter 4 Obtain DN values in pavement layers of entire road Determine upgrading 06 (from DCP programme 12 requirements Chapter 4 Chapter 4 64  Guidelines for the Environmentally Optimized Design of Low Volume Roads Step 6: Obtain DCP penetration rates in pavement layers and input into a DCP analysis programme: The DCP results obtained at each measurement point can be analysed manually using conventional spread-sheets but can also be analysed using software such as the AfCAP DCP program (http://www.research4cap.org/SitePages/ LVRDCPSoftware.aspx) and the data processed to obtain weighted average DN values (penetration rate in mm/blow), for each test point and 150 mm layer of the pavement structure and the DSN800 value (total number of blows required to reach a depth of 800 mm. For many existing pavements, the underlying structure may be adequate for the design traffic (already constructed and compacted by traffic) and only the upper layers may need reconstruction or addition. This will be confirmed by the DCP profile. Step 7: Determine uniform sections: The DN values for each 150 mm layer as well as the DSN800 should be plotted against the chainage of the road, using a simple cumulative sum (CUSUM) technique to identify uniform sections along the road (described in Annex II). This will typically identify changes in underlying material types, transitions from cut to fill or variable soil moisture conditions. The manner of determining uniform sections, based on consideration of the upper three,150 mm layers, as well as the DSN800 of an existing gravel road is illustrated in Figure 25. This method has the advantage of looking at the entire pavement structure including material types, moisture conditions, compaction, etc. and not just the limited areas from where CBR samples were collected. When defining uniform sections, short sections (less than about 500-1000 m) should be included in longer sections, after considering their conditions. This may entail using a slightly heavier design for the entire section to avoid localized weak spots or may entail the strengthening of one or more layers over these sections to bring them up to the Figure 25: Example of uniform sections obtained from CUSUM analysis of DCP results 10 800 0 Cusum of DSN800 Values 600 Cusum of DN Values -10 400 -20 200 -30 -40 0 0.000 0.500 1.000 1.500 2.000 2.500 Chainage km 0-150 mm 150-300 mm 300-450 mm DSN800 4. PAVEMENT DESIGN  65  same standard as the rest of the section. Short sections should be combined as far as possible to minimize the number of different pavement structures along the road. In areas where submergence is likely, soaked values should be used and it is suggested that the DCP testing is carried out immediately as the monsoon retreats and while the pavement is still soaked. Step 8: Adjust DN results for design moisture: The DCP results must be adjusted for moisture conditions. Based on the estimate of the in situ moisture condition at the time of the DCP testing (Step 4) adjust the DCP results in accordance with the percentile values shown in Table 10. The DCP data collected during the dry season will be stronger (lower DN) than that collected during the wet season. The use of the respective 80th and 20th percentiles (for design traffic < 0.5 MESA) or the 90th and 30th percentiles (for design traffic 0.5–1.0 MESA) effectively results in an estimate of the expected in service conditions. The manner of determining percentiles for adjusting DN results for the chosen design moisture content is described in Annex III.  ecommended percentiles of minimum in situ strength Table 10: R profile to be used Percentile of minimum strength profile (maximum Anticipated long-term penetration rate – DN mm/blow) in-service moisture content in pavement Design traffic Design traffic < 0.5 MESA 0.5–1.0 MESA Drier than at time of DCP survey 20 30 Same as at time of DCP survey 50 65 Wetter than at time of DCP survey 80 90 The in situ moisture content tends to be a function of the height of the pavement layer(s) above natural ground level, adjacent cuttings, material properties and the depth of the water table below natural ground level. Given a conducive moisture regime, after surfacing the moisture content in the base tends to stabilize at typically 70–90% of OMC. Nonetheless, since the performance of the road will be sensitive to the in-service moisture content of the pavement layers, it is of paramount importance that the long-term moisture condition is realistically assessed and, in cases of uncertainty regarding the adequacy of the drainage system, it would be prudent to assume a worst case, soaked condition. Step 9: Determine in situ layer strength profile for each uniform section: By analyzing each uniform section on its own the spread of the data, and hence the risk level, is reduced. As illustrated in Figure 26, the in situ layer strength profile is determined by an average analysis of the DN values within each uniform section by using the DCP programme and applying the correct percentile based on the moisture condition in the pavement at the time of the DCP survey. Step 10: Determine required Layer Strength Profile (LSP) for each uniform section: For a particular design traffic class, the required layer strength profile for each uniform section is determined from the DCP design catalogue (Table 11) for different traffic classes. Approximate CBR values are included in this design catalogue for comparison purposes with other catalogues. The design catalogue is based on the anticipated, long 66  Guidelines for the Environmentally Optimized Design of Low Volume Roads Figure 26: Average maximum and minimum strength profiles for a uniform section LAYER-STRENGTH DIAGRAM LAYER-STRENGTH DIAGRAM DN (mm/blow) DN (mm/blow) 1 3 10 30 1 3 10 30 0 0 200 200 PENETRATION DEPTH (mm) PENETRATION DEPTH (mm) 400 400 Maximum Average 600 600 Minimum 800 800 300 100 30 10 3 300 100 30 10 3 CBR (%) CBR (%) Table 11: DCP design catalogue for different traffic classes Traffic Class DN in mm/blow (approx. CBR%) for Traffic Class (MESA) 0.003-0.01 0.01-0.03 0.03-0.10 0.10-0.30 0.30-0.70 0.70-1.00 0 - 150 mm Base DN ≤ 8 DN ≤ 5.9 DN ≤ 4 DN ≤ 3.2 DN ≤ 2.6 DN ≤ 2.5 ≥ 98% Mod. AASHTO (30) (45) (70) (90) (120) (130) 150-300 mm Sub-base DN ≤ 19 DN ≤ 14 DN ≤ 9 DN ≤ 6 DN ≤ 4.6 DN ≤ 4.0 ≥ 95% Mod. AASHTO (10) (15) (25) (45) (60) (70) 300-450 mm Subgrade DN ≤ 33 DN ≤ 25 DN ≤ 19 DN ≤ 12 DN ≤ 8 DN ≤ 6 ≥ 95% Mod. AASHTO (5) (7) (10) (17) (20) (45) 450-600 mm in situ DN ≤ 40 DN ≤ 33 DN ≤ 25 DN ≤ 19 DN ≤ 14 DN ≤ 14 material (4) (5) (7) (10) (15) (15) 600-800 mm DN ≤ 50 DN ≤ 40 DN ≤ 40 DN ≤ 25 DN ≤ 25 DN ≤ 25 in situ material (3) (4) (4) (7) (7) (7) DSN 800 ≥ 39 ≥ 52 ≥ 73 ≥ 100 ≥ 128 ≥ 143 (Figures in Brackets indicate approx CBR %) term, in-service moisture condition. If there is a risk of prolonged moisture ingress into the road pavement, then the pavement design should be based on the soaked condition. Figure 27 shows the same catalogue in a “Layer strength diagram” format as used for analysis. 4. PAVEMENT DESIGN  67  Figure 27: DCP layer-strength diagram for different traffic classes 0 100 200 300 Depth (mm) 400 500 600 700 800 1 10 100 DN (mm/blow) Traffic classes 0.003-0.010 0.010-0.030 0.030-0.100 0.100-0.300 0.300-0.700 0.700-1.000  omparison of DCP design and Figure 28: C In the AfCAP software the above in situ strength profiles DCP catalogue is illustrated as 0 shown in Figure 27. Inadequate in-situ Step 11: Compare in situ strength 100 Layer Strength Profile (LSP) with required LSP for each Adequate uniform section: The required in-situ 200 strength profile is plotted on the strength same layer-strength diagram on which the uniform section layer 300 strength profiles were plotted (Figure 28). The comparison Depth (mm) Required strength 400 profile (From DCP between the in situ strength Design Catalogue) profile (blue and brown lines) and the required design strength 500 profile (red broken line) allows the adequacy of the various pavement layers with depth to 600 be assessed for carrying the In-situ strength profile 20th, 50th or 80th percentile expected future traffic loading. as appropriate 700 Step 12: Determine upgrading requirements for each uniform 800 section: The options for 1 3 10 30 upgrading the pavement must DN (mm/blow) then be considered as follows. 68  Guidelines for the Environmentally Optimized Design of Low Volume Roads Option 1: If the in situ strength profile of the existing gravel road complies with the required strength profile indicated by the DCP catalogue for the particular traffic class, the road would need to be only re-shaped, compacted and surfaced (assuming that the drainage requirements are adequate). Option 2: If the in situ strength profile of the existing gravel road does not comply with the required strength profile indicated by the DCP catalogue for the particular traffic class, then the upper pavement layer(s) would need to be: ™™ Reworked- if only the density is inadequate and the required DN value can be obtained at the specified construction density and anticipated in-service moisture content. ™™ Overlaid – if the material quality (DN value at the specified construction density and anticipated in-service moisture content) is inadequate, then appropriate quality material will need to be imported to serve as the new upper pavement layer(s). ™™ Mechanically stabilized – as above, but new, better quality material is blended with the existing material to improve the overall quality of the layer. ™™ Augmented – if the material quality (DN value) is adequate but the layer thickness is inadequate, then imported material of appropriate quality will need to be imported to make up the required thickness prior to compaction. If none of the above options produces the required quality of material, recourse may be made to more expensive options, such as soil stabilization (Section 3.4.6.2). In cases where the existing road consists of a water-bound macadam or brick soling, this can be reworked, blended with additional material or broken down (e.g. by heavy vibrating or impact rolling) and used as a new base course where it is suitable, or where underlying layers do not need to be improved. New roads The approach to be adopted in the DCP design of new roads is similar, in principle, to that adopted for existing roads. However, two scenarios need to be considered, viz.: a. When the road pavement is to be founded at, or just above existing ground level. b. When the road pavement is to be founded on a new fill or embankment, higher than about 500 or 600 mm above existing ground level. In the case of Option (a) the DCP survey can be undertaken along the alignment (as described in Section 3.3.3.1) as the outcome would provide a meaningful input into the design process. In addition, the density and moisture content are not likely to be equivalent to that of the material in the designed and compacted embankment. However, in the case of Option (b) undertaking a DCP survey at existing ground level would be pointless as, at that depth, it would have little input into the pavement design process. As a result, the strength of the compacted embankment material would need to be determined in the laboratory as this is what will influence the structural design of the pavement. 4. PAVEMENT DESIGN  69  4.3.3 CBR Design Method For designing LVRs using the CBR, rather than DCP, method of pavement design, two options are available: 1. Base the design on CBR values obtained from the DCP/CBR correlation using the Kleyn equation [20]. The CBR values so derived (Table 11) are less conservative than those presented in the IRC: SP: 72–2015. 2. CBR values can be determined directly and the pavement designed using IRC:SP:72- 2015 (Guidelines for the Design of Flexible Pavements for Low Volume Rural Roads). 4.3.4 Comparison of DCP-DN and IRC DCP Catalogues A comparison of the structural capacities provided by the DCP-DN and IRC: SP: 72–2015 CBR catalogues is presented in Annex IV from which it can be seen that they are almost identical in terms of traffic carrying capacity. However, if the strength distribution of the two structures are plotted with depth and compared with the theoretical balance curves, the DCP DN design shows a well-balanced structure compared with the relatively shallow pavement structure exhibited by the IRC design which makes the latter very sensitive to overloading with a number of stress concentrations in the upper 100 mm. Although the standard catalogue for the DCP design method uses layers of 150 mm (primarily to overcome problems often seen on site with layer thickness control), the method can be adapted for any selected layer thicknesses or materials available, as illustrated in Annex IV. 4.4 Practical Considerations  4.4.1 General There are some practical considerations associated with the performance of a road that are of particular significance to LVRs. These include: ™™ Compaction ™™ Drainage ™™ Shoulders and cross section 4.4.2 Compaction Effective compaction of the existing running surface of the gravel road which is to be upgraded is one of the most cost-effective means of improving the structural capacity of the LVR pavement. A well compacted running surface possesses enhanced strength, stiffness and bearing capacity, is more resistant to moisture penetration and less susceptible to differential settlement. The higher the density, the stronger the layer support, the less the required thickness of the overlying pavement layers and the more economical the pavement structure. Thus, there is every benefit to achieving as high a density and related strength as economically possible in the subgrade 70  Guidelines for the Environmentally Optimized Design of Low Volume Roads and pavement layers, and by assessing the density values obtained in relation to a heavy (e.g. Modified Proctor) rather than light (e.g. Standard Proctor) compaction standard. Maximizing the strength potential of a subgrade soil can be achieved, not necessarily by compacting to a pre-determined relative compaction level, as is traditionally done but, rather, by compacting with the heaviest equipment available to attain the highest uniform level of density possible (“compaction to near refusal”) without significant strength degradation of the particles. In so doing, there is a significant reduction in permeability as well as a beneficial gain in density, strength and stiffness, with the latter correlating directly with longer pavement life, as illustrated in Figure 29. For these compelling reasons, where the higher densities can be realistically attained in the field from field measurements on similar materials or other established information, they should be specified in the tender documents. Figure 29: Benefits of compaction to refusal Elasto- Plastic plastic Elastic S2 S2 Increase in Point of compaction stiffness Pavement stiffness Density/stiffness S1 to near refusal S1 (near-zero air voids) Increase in life N1 N2 E2 E1 No. of roller passes Structural capacity (Cum. ESAs) 4.4.3 Drainage Moisture is the single most important factor affecting pavement road performance and long-term maintenance costs. This factor is even more important for LVRs which are generally constructed from natural, often unprocessed, materials which tend to be more moisture sensitive than traditional materials. It is therefore necessary to provide a pavement structure in which the weakening effects of moisture are contained to acceptable limits in relation to the traffic loading, nature of the materials being used, construction/maintenance provisions and degree of acceptable risk. The crown height of an LVR, i.e. the vertical distance from the bottom of the side drain to the finished road level at the center line (hmin), is a critical parameter that correlates well with the in-service performance of pavements constructed from naturally occurring materials[1]. This height must be sufficiently great to prevent moisture ingress into the potentially vulnerable outer wheel track of the carriageway for which a minimum value of 0.75 m is recommended (Figure 30). 4. PAVEMENT DESIGN  71  Figure 30: Minimum drainage requirements Centre Line Base Side Crown height, h min slope Subbase Natural ground level d Improved subgrade min In situ subgrade Drain The recommended minimum crown height of 0.75 m applies to unlined drains in relatively flat ground (longitudinal gradient, g, less than 1%). The recommended values for sloping ground (g > 1%) or where lined drains are used, for example, in village areas, are shown in Table 12.  ecommended crown height in relation to drain type and Table 12: R longitudinal gradient Crown height, hmin (meters) Unlined drains Lined drains g < 1% g > 1% g < 1% g > 1% 0.75 0.65 0.65 0.50 In water logged areas, where the subgrade is within the zone of capillary saturation, consideration should be given to the installation of suitable capillary cut-off as per IRC: 34 at an appropriate level underneath the pavement. The height between the natural ground level and the bottom of the subbase (d min) shall be not less than 150 mm (Figure 30). 4.4.4 Shoulders 4.4.4.1 General Shoulders fulfil several important functions which are enhanced when they are sealed – a feature which is strongly recommended, where possible. The advantages of sealing shoulders include: ™™ Provision of better support and moisture protection for the pavement layers and also reduces erosion of the shoulders (especially on steep gradients). ™™ Improved pavement performance by ensuring that the zone of seasonal moisture variation does not penetrate to under the outer wheel track. ™™ Reduced maintenance costs by avoiding the need for regravelling at regular intervals. 72  Guidelines for the Environmentally Optimized Design of Low Volume Roads ™™ Reduced risk of road accidents, especially where the edge-drop between the shoulder and the pavement is significant or the shoulders are relatively soft. 4.4.4.2 Width of Shoulder Sealing This should be such as to ensure that any moisture variation is contained within the shoulders and does not penetrate the outer wheel track of the pavement. The recommended minimum paved shoulder width will depend on the extent of lateral infiltration into the shoulder at the wettest time of the year. The recommended minimum widths for a 2-lane road would be of the order of 1.0  m. However, narrower widths would also be advantageous on single-lane carriageways where vehicles tend to use the central portion of the road. 4.4.4.3 Type of Shoulder Surfacing Ideally, the same type of surfacing used on the carriageway should be extended over the shoulder as well, particularly where it is likely to be heavily trafficked as in towns and built-up areas. Where a two-layer surface treatment is used on the carriageway, at least the first layer should be extended to the shoulder with an appropriate second cover seal which may be of a different type to that used on the carriageway. The objective is to achieve a durable, close-textured surfacing on the shoulder. This will generally require an increased binder application rate to cater for the lesser trafficking of shoulder compared with the carriageway. 4. PAVEMENT DESIGN  73  5. SURFACINGS 5.1 Introduction  5.1.1 Background The surfacing of any road plays a critical role in its long-term performance. It: ™™ Prevents gravel loss ™™ Eliminates dust from unpaved roads ™™ Improves skid resistance, and ™™ Reduces water ingress into the pavement once surfaced. The latter attribute is especially important for LVRs where moisture sensitive materials are often used. There are many surfacing options, both bituminous and non-bituminous, that are available for use on LVRs. They offer a range of attributes which need to be matched to such factors as expected traffic levels and loading, locally available materials and skills, construction and maintenance regimes, and the environment. Careful consideration should therefore be given to all these factors in order to make a judicious choice of the most suitable type of surfacing to provide satisfactory performance and minimize life cycle costs. 5.1.2 Purpose and Scope The main purpose of this chapter is to provide a broad overview of: ™™ he various types of bituminous and non-bituminous surfacings available for use T on LVRs. ™™ The factors that affect the selection of these surfacings. 5.2 Bituminous Surfacings  5.2.1 General Bituminous surfacings for purposes of these Guidelines are divided into surface treatments (often referred to as surface dressings) and asphalt surfacings, as shown in Figure 31. 5. SURFACINGS  75  5.2.2 Surfacing Types As shown in Figure 31, various options for bituminous surfacings are available for low volume roads. These can basically be classified as follows with various configurations within each of these groups. ™™ Asphalt surfacings ™™ Surface treatments •• Sprayed Seals •• Slurry (conventional slurries, microsurfacings) •• Combination seals and are mainly used on low volume roads. The design of surface treatments (surface dressings) is addressed in a companion IRC document on the Design, Construction and Maintenance of Surface treatments. Figure 31: Bituminous Surfacings for LVR Bituminous Surfacings Surface Treatments Asphalt Surfacing Thin Premix Thick Asphalt Combination Sprayed seals Slurry seals Carpets overlays seals (< 30 mm) > 30 mm Dense Graded Conventional Sand seal Cape Seal Mix (Mixed seal Slurry surfacing) Single/Multiple Open Graded Microsurfacing Slurry-bound Surface Mix with Liquid (MAC) MacAdam Dressing seal Coat Dense Graded Graded Cold Mix Aggregate Seal (Graded Gravel) 5.3 Non-Bituminous Surfacings  5.3.1 General Surfacings constructed from materials such as stone, clay and concrete can also be considered for use on LVRs instead of conventional bituminous surfacings. The current practice of utilizing concrete pavements through villages, at very steep grades or where there is risk of water overtopping the road is an example of an environmentally judicious choice of surfacing in circumstances where bituminous surfacings often do not perform well. 76  Guidelines for the Environmentally Optimized Design of Low Volume Roads Figure 32: Examples of typical non-bituminous surfacings for LVRs Non-Bituminous Surfacing Stone Clay Concrete Cobble/ Clay Concrete Dressed Stone Bricks Blocks Concrete Slab Hand-Packed Un-reinforced/ Stone reinforced 5.3.2 Surfacing Types The main types of non-bituminous surfacings can be divided into three distinct groups as shown in Figure 32. The non-bituminous surfacings listed above all act simultaneously as a surfacing and base layer and provide a structural component to the pavement because of their thickness and stiffness. They all require the use of a sand bedding layer which also acts as a load transfer layer for the overlying construction. In some cases they act additionally as a drainage medium. In some circumstances (e.g. on steep slopes in high rainfall areas and in areas with weak subgrades and/or expansive soils) it may be advantageous to use mortared options. This can be done with Hand-packed Stone, Cobblestone (or Dressed Stone), and Fired Clay Brick pavements. The construction procedure is largely the same as for the un-mortared options except that cement mortar is used instead of sand for bedding and joint filling. 5.4 Surfacing type selection  5.4.1 Selection criteria The selection of the best option depends on a wide range of factors with the main influencing factors affecting the choice as follows: ™™ Material quality and availability. ™™ Traffic volume and vehicle type distribution. ™™ Cost effectiveness (cost and expected service life). 5. SURFACINGS  77  ™™ Purpose and life-cycle strategy. ™™ Safety and contractual requirements. ™™ Social and environmental requirements and impacts. ™™ Institutional capacity and maintenance capability. ™™ Urban or rural drainage systems. ™™ Construction method and risks. ™™ Road gradients (see Table 13). ™™ Turning actions/external stresses (see Table 14). ™™ Base or existing substrate quality/macro texture (see Table 15). The key physical factors that exert a strong influence on the choice of appropriate surfacing for an LVR are discussed briefly below. 5.4.2 Gradient Table 13 indicates the categories of gradient that affect the choice of surfacing, as illustrated in Table 16. Table 13: Construction gradient Very steep >10% Steep 6–10% Flat -Gentle 0–6% 5.4.3 Turning actions/external stresses Table 14 indicates the severity of turning actions/external stresses that affect the choice of surfacing, as illustrated in Table 16. Table 14: External stresses Very high High risk of water overflowing, high risk of farm or industrial equipment damage, high risk of landslides and subsequent material removal High High occurrence of heavy vehicles turning/braking, high probability of loose material on the road surface and slight probability of the risks mentioned under “Very high” Medium Occasional turning/braking of heavy vehicles, possibility of loose material and very low probability of risks mentioned under “Very high” Low Very low risk of damage due to external stresses mentioned above 5.4.4 Macrotexture The macrotexture of a pavement refers to the visible roughness of the pavement surface and is defined as texture (“bumps and dips”) in a pavement with a wavelength (distance from “bump” to “bump”) ranging from 0.5 mm to 50 mm. 78  Guidelines for the Environmentally Optimized Design of Low Volume Roads Single and double surface dressings are particularly sensitive to coarse textures due to the applied binder running into the voids of the existing surface and not properly adhering to the surfacing aggregate. Fine slurry is often applied as a pre-treatment (Void fill) to obtain a uniform fine texture before the final surface treatment is applied. Table 15 categorises macrotexture in terms of texture depth as a basis for selecting an appropriate type of surfacing, as illustrated in Table 16. Table 15: Texture depth by type of macrotexture Macrotexture Texture Depth (mm) Typical Base Type Very coarse >5 WBM or excessively brushed crushed stone Coarse 2-5 Well brushed crushed stone Medium-Fine <2 Well brushed natural gravel 5.4.5 Preliminary selection guideline Given the current practice in India, the drive towards surface treatments (surface dressings), the scarcity and high transport cost of crushed aggregate in some states and availability of clay, natural gravels and sand, Table 16 has been developed as a guide for the selection of appropriate initial construction surfacings. The selection of potential surfacing types is based on: ™™ Availability of materials\ ™™ Gradients at which construction will take place (Table 13) ™™ External stresses expected on the road (Table14) ™™ The macro texture of the base before surfacing (Table 15) Based on risks of poor performance, only the following bituminous surfacings are considered for adoption as initial construction seals on LVRs: ™™ Asphalt premix •• 20 mm Premix Carpets •• 30–40 mm Continuous Graded Asphalt (Cold or hot mix) •• 25–40 mm Graded Gravel Mix (Cold mix) ™™ Surface treatments •• Cape seals •• Otta seals (Graded aggregate seals) •• lurry-bound Macadam (A 20 – 30 mm layer of 13.2 mm aggregate with a fine S slurry vibrated into the voids and covered with a final fine slurry layer) •• ow-risk double seals or Stone plus sand seals (where suitable coarse sand is L available). A low-risk double seal is defined as a double seal, where the second layer aggregate is a third or less the size of the first aggregate layer size •• Thick slurry seals or Microsurfacing •• Double “graded sand” seals 5. SURFACINGS  79  The choice of one or combination of these seal types is mainly dependent on the existing macro texture of the base and availability of suitable aggregate. If the base is coarse or very coarse, only double Otta seals, thick coarse slurry/ Microsurfacing or a void filling slurry seal in combination with a Cape seal or double seal should be considered. The type and grade of coarse slurry (Slurry Type II or Type III) appropriate for use as a void filling seal depends on the depth and volume of voids to fill. The direct application of a Cape seal or double seal should only be considered if the base macro texture could be described as medium to fine (Volumetric texture depth < 2 mm). 80  Guidelines for the Environmentally Optimized Design of Low Volume Roads TABLE 16: Guide for selection of LVR bituminous surfacings External Min. Gradient Macro Texture Appropriate Bituminous Surfacings Stresses Thickness Very High No Conventional Bituminous Surfacings recommended Very Steep High Steep AC No Conventional Bituminous Surfacings < 40 mm recommended 40 mm Flat-Gentle AC Very Coarse AC CMGG 30PMC Otta VF+CS VF+DS SL/M SBMac CS DS DGSS Very Steep Coarse AC CMGG 30PMC Otta VF+CS VF+DS SL/M SBMac CS DS DGSS 30 mm Medium-Fine AC CMGG 30PMC Otta VF+CS VF+DS SL/M SBMac CS DS DGSS Very Coarse AC CMGG 20PMC Otta VF+CS VF+DS SL/M SBMac CS DS DGSS Medium Steep Coarse AC CMGG 20PMC Otta VF+CS VF+DS SL/M SBMac CS DS DGSS Medium-Fine AC CMGG 20PMC Otta SL/M SBMac CS DS DGSS 20 mm Very Coarse AC CMGG 20PMC Otta VF+CS VF+DS SL/M SBMac CS DS DGSS Flat-Gentle Coarse AC CMGG 20PMC Otta VF+CS VF+DS SL/M SBMac CS DS DGSS Medium-Fine AC CMGG 20PMC Otta SL/M SBMac CS DS DGSS Very Coarse AC CMGG 30PMC Otta VF+CS VF+DS SL/M SBMac CS DS DGSS Very Steep Coarse AC CMGG 30PMC Otta VF+CS VF+DS SL/M SBMac CS DS DGSS 30 mm Medium-Fine AC CMGG 30PMC Otta VF+CS VF+DS SL/M SBMac CS DS DGSS Very Coarse AC CMGG 20PMC Otta VF+CS VF+DS SL/M SBMac CS DS DGSS Low Steep Coarse AC CMGG 20PMC Otta VF+CS VF+DS SL/M SBMac CS DS DGSS 20 mm Medium-Fine AC CMGG 20PMC Otta SL/M SBMac CS DS DGSS Very Coarse AC CMGG 20PMC Otta VF+CS VF+DS SL/M SBMac CS DS DGSS Flat-Gentle Coarse AC CMGG 20PMC Otta VF+CS VF+DS SL/M SBMac CS DS DGSS 15 mm Medium-Fine AC CMGG 20PMC Otta SL/M SBMac CS DS DGSS AC = 30–40 mm Asphalt (Premix) Continuous Graded VF = Void filling slurry seal SBMac = Slurry-bound Macadam CMGG = 25–30 mm Cold Mix Graded Gravel CS = Cape Seal DS = Double seal PMC = 20–30 mm Premix Carpet SL/M = Thick slurry or Microsurfacing DGSS = Double graded sand seal Otta = Double Otta seal 5. SURFACINGS  81  REFERENCES 1. Gourley CS and P. A. K. Greening. (1999). Performance of low volume sealed roads: Results and recommendations from studies in Southern Africa, TRLProject Report PR/OSC/167/99. Transport Research Laboratory, Crowthorne, UK. 2. Pinard M I. (2011).  Performance review of design standards and technical specifications for low volume sealed roads in Malawi, African Community Access Programme Report MAL/016. 3. Rolt J, Mukura K, Dangare F and A. Otto. (2013). Back analysis of previously constructed rural roads in Mozambique. African Community Access Programme Project MOZ/001/G.CPR1612. DFID,UK. 4. Paige-Green P. (2003). Strength and behaviour of materials for low volume roads as affected by moisture and density. Transportation Research Record,1106, TRB, Washington, DC, pp 208-214. 5. C. Overby. (1990). Monitoring of sealed low volume roads in Botswana,1980-1989, NRRL Report 1478, Norwegian Public Roads Administration, Oslo. 6. IRC:SP:72-2015. (2015). Guidelines for the Design of Flexible Pavements for Low Volume Rural Roads. Indian Roads Congress, New Delhi. 7. NRRDA. (2015). New Technology Initiatives under PMGSY: Report of the Expert Group on Measures for Achieving Economy in Construction of Rural Roads Under PMGSY. Delhi. 8. IRC:SP:42-2014. (2014). Guidelines of Road Drainage”. Indian Roads Congress, New Delhi. 9. MORD. (2014). Specifications for Rural Roads. Indian Roads Congress, New Delhi. (First Revision). 10. MORD-2016 (2016). Quality Assurance Handbook for Rural Roads. 11. IRC:SP: 34-2011. (2011). Recommendations for road construction in areas affected by water-logging, flooding and/or salt infestation. Indian Roads Congress, New Delhi. 12. Howe J D G F. (1972). A Review of Rural Traffic Counting Methods in Developing Countries. RRL Report LR 427. Road Research Laboratory, Crowthorne, UK. 13. Ministry of Transport and Public Works. (2013). Malawi. Design Manual for Low Volume Sealed Roads Using the DCP Design Method. Lilongwe. 82  Guidelines for the Environmentally Optimized Design of Low Volume Roads 14. Sampson, LR and F Netterberg. (1990). Effect of material quality on the relationship between DCP-DN value and CBR. 1990 Annual Transportation Convention, Pretoria. 15. South African National Standard (SANS). 2013. Determination of the California Bearing Ratio (SANS 3001-GR40). South African Bureau of Standards, Pretoria. 16. Van der Merwe D.H. (1976). Plasticity Index and percentage clay fraction of soils. Proc. 6th Regional Conference for Africa on Soil Mechanics and Foundation Engineering (2) 166-167. 17. Sinha K et al. (2011). Feasibility study of jarofix waste material for road construction. Indian geotechnical conference, Kochi, India, 1, 685-688. 18. Gurbuz, A. (2015). Marble powder to stabilize clayey soils in subbases for road construction, Gazi University, Ankara. 19. Lavanya C, Sreerama Rao A and N Darga Kumar. (2011).  A Review on Utilization of Copper Slag in Geotechnical Applications; Proceedings of Indian Geotechnical Conference December 15-17, 2011, Kochi (Paper No.H-212). 20. Kleyn EG. (1982). Aspects of pavement evaluation and design as determined with the aid of the Dynamic Cone Penetrometer (DCP). M. Eng. Thesis, University of Pretoria, Pretoria, 1982. REFERENCES  83  ANNEX-I: Example of Design Traffic Determination Basic data  1. Design life = 10 years 2. Annual growth rate of traffic for all categories of commercial traffic = 6% 3. Two harvesting seasons in the area 4. Traffic data collected 1 year before opening the road to traffic 5. Traffic counting conducted over a period of 3 days during the lean season 6. Duration of each harvesting season = 75 days 7. Results of traffic survey data presented in Table 1. Table 1: Results of traffic survey (Refer Figure 7 in Chapter 3) (a) Average Daily Traffic as per last count No. (Lean Harvesting Season) (i) Cars, Jeeps, Vans, Three Wheelers = 29 (ii) Motorised Two Wheelers = 25 (iii) Light Commercial Vehicles = 29 (iv) Heavy Commercial Vehicles (HCV) (A) Trucks Loaded = 2 Unloaded = 2 Overloaded = 0 (B) Buses Loaded = 2 Unloaded = 2 Overloaded = 0 (v) Agricultural Tractor Trailers (MCV) Loaded = 14 Unloaded = 14 Overloaded = 0 ANNEX-I: Example of Design Traffic Determination  85  (vi) Cycles = 0 (vii) Cycle Rickshaws = 0 (viii) Animal Drawn Vehicles = 5 Total T=124 b. Computation of the Design Traffic (i) Harvesting Seasons in the area = 2 No. (ii) Duration of each Harvesting Season (t) = 75 Days (iii) Average Daily Traffic during the Lean Season (T) = 124 No. (iv) Annual Average Daily Traffic (AADT) AADT = T + (1.2nTt)/365 = 124 + (1.2 x 1 x 124 x 75)/365 = 124 + (11160)/365 = 124 + 30.6 = 154.6 say 155 (v) AADT before opening to traffic = 155 x (1.06)1 = 164.3 say 164 (vi) Proportion of HCV (a) Laden = (164 x 4)/124 = 5.30 say 6 (b) Unladen = (164 x 4)/124 = 5.30 say 6 (c) Overloaded = 0 (vii) Proportion of MCV (a) Laden = (164 x 14)/124 = 18.52 say 19 (b) Unladen = (164 x 14)/124 = 18.52 say 19 (c) Overloaded = 0 (viii) Animal drawn carts have not been considered for design purposes (ix) T0 for HCV = 6 x 2.60 + 6 x 0.31 = 17.46 say 18 (x) T0 for MCV = 19 x 0.31 + 19 x 0.02 = 6.27 say 7 (xi) Total for HCV + MCV = 25 (xii) Cumulative ESAL applications (N) over the design life (10 years) @ 6% growth rate N = T0 x 4811 x L = 25 x 4811 x 2 = 240,550 ESA = 0.241 MESA (xiii) Traffic category = T4 (0.100 – 0.300) MESA 86  Guidelines for the Environmentally Optimized Design of Low Volume Roads ANNEX-II: Determination of Uniform Sections from Cusum Analysis 1. CUSUM Analysis  B C CUSUM Chainage (km) Measured DCP (DN Difference from (Accumulated Value-mm/blow) average (A-B) Values of C) 1 14 -1.2 -1.2 2 13 -0.2 -1.4 3 15 -2.2 -3.6 4 14 -1.2 -4.8 5 13 -0.2 -5.0 6 14 -1.2 -6.2 7 7 5.8 -0.2 8 9 3.8 3.4 9 8 4.8 8.2 10 13 -0.2 8.0 11 15 -2.2 5.8 12 18 -5.2 0.6 13 14 -1.2 -0.6 14 16 -3.2 -3.8 15 14 -1.2 -5.0 16 14 -1.2 -6.2 17 15 -2.2 -8.4 18 18 -5.2 -13.6 19 14 -1.2 -14.8 20 15 -2.2 -17.0 21 9 3.8 -13.2 22 10 2.8 -10.4 23 9 3.8 -6.6 24 12 0.8 -5.8 25 9 3.8 -2.0 26 11 1.8 -0.2 Average: A = 12.8 ANNEX-II: Determination of Uniform Sections from Cusum Analysis  87  Homogenous Sections 10 5 CUSUM 0 -5 -10 -15 -20 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Chainage 88  Guidelines for the Environmentally Optimized Design of Low Volume Roads ANNEX-III: Determination and Choice of Dn Percentile Values DCP Survey results–Uniform section derived from CUSUM analysis of DN 0-150 (Base) 1.  (N.B. DN 0-150 = DN in first 150 mm of pavement. Percentile of minimum strength Chainage Point DN 0-150 Profile (max. penetration rate – DN) (km) No (Base) 20th 50th (Mean) 80th 0.00 1 2.29 0.25 2 4.44 0.50 3 2.00 0.75 4 8.67 1.00 5 3.75 3.46** 5.24 8.19 1.25 6 8.07 1.50 7 5.11 1.75 8 5.37 2.00 9 6.60 2.25 10 10.12 Anticipated long-term in-service moisture content in pavement* Drier than at time of DCP survey 3.46 N/A N/A Same as at time of DCP survey N/A 5.24 N/A Wetter than at time of DCP survey N/A N/A 8.19 * This is one of the most carefully considered decisions the designer will have to make to ensure that a reliable DCP design is achieved. See Section 2.5.5 for the basis on which the decision should be made. ** The percentile value in an Excel spreadsheet may be obtained from the expression: = PERCENTILE (N$1:N$10,0.2) where N is the column containing the DN values. ANNEX-III: Determination and Choice of Dn Percentile Values  89  2. Definition of Percentile A percentile of a range of values is the point in the range at or below which a given percentage of values is found. For example, the 80th percentile of the distribution of DN values given in the above example is the point at or below which 80% of the values fall, i.e. 8.19, as illustrated below. Anticipated long-term in-service DN value % ile* DN %ile moisture content in pavement 2.00 2.29 3.75 20th 3.36 Drier than at time of DCP survey 4.44 5.11 5.37 50th 5.24 Same as at time of DCP survey 6.60 8.07 8.67 80th 8.19 Wetter than at time of DCP survey 10.12 * Percentile value may need to be adjusted for moisture sensitivity of material. 90  Guidelines for the Environmentally Optimized Design of Low Volume Roads ANNEX-IV: Comparison of Dcp-Dn and Irc Cbr Catalogues It is informative to compare the structural capacities, materials requirements and pavement balance of some of the existing Indian catalogue designs and the DCP-DN designs. The current catalogue in IRC SP72: 2015 is shown below. T1 T2 T3 T4 T5 T6 T7 T8 T9 TRAFFIC CATEGORY (10,000 to (30,000 to (60,000 to (1,00,000 to (2,00,000 to (3,00,000 to (6,00,000 to (1,000,000 to (1,500,000 to 30,000) 60,000) 1,00,000) 2,00,000) 3,00,000) 6,00,000) 1,000,000) 1,500,000) 2,000,000) 50 SUBGRADE STRENGTH 75 75 CBR 225 75 150 150 75 75 75 75 75 75 200 225 250 S1 75 75 175 250 200 125 VERY POOR 150 125 225 200 200 CBR = 2 100 100 100 150 150 150 75 50 75 75 150 75 225 150 75 75 150 75 S2 75 75 125 150 150 275 100 200 POOR 200 125 175 150 150 150 200 CBR = 3 to 4 100 100 75 50 75 75 150 225 75 75 75 150 S3 75 75 125 100 200 FAIR 175 250 275 200 150 175 CBR = 5 to 6 100 100 100 75 50 75 75 75 75 150 225 S4 75 75 75 150 GOOD 150 175 225 200 150 125 150 175 150 CBR = 7 to 9 75 50 75 75 75 150 225 75 75 150 S5 75 VERY GOOD 125 150 175 150 125 150 125 175 125 CBR = 10 to 15 Modified Soil/Improved Subgrade (CBR not < 10) Granular Subbase (CBR not < 20) in exceptional care can be 15 Gravel Base (CBR not < 80). in Lower base course shall not be less than 50 Clause 2.3.5 (in exceptional case may be relaxed suitably) Base of Gravel/CRMB/WBM (CBR not < 100 Where 100mm thickness is recommended it be modified to 75 mm for WBM with corresp. Increase of 25 mm in Subbase WBM Grade-3 Bituminous Macadam Surface Dressing OGPC ANNEX-IV: Comparison of Dcp-Dn and Irc Cbr Catalogues  91  For less than 1,00,000 standard axles There are only two designs for paved roads carrying up to 1,00,000 standard axles and they are for weak subgrades of 2-4% soaked CBR. In the example below, the IRC SP72 catalogue for 1,00,000 axles and a subgrade CBR of 3% is compared with the DCP-DN design for similar traffic. The subgrade CBR is automatically included in the DCP design profile. CBR values according to the IRC catalogue have been allocated and converted to DCP DN values. These have then been distributed through the pavement profile according to the layer thicknesses as shown in the table below. Thickness CBR DN Blows/layer Blows/layer Blows/layer (mm) (%) (mm/bl) (no AC) DCP DN catalogue IRC SP72 Catalogue for < 100 000 esa and SG CBR = 3% < 100, 000 esa 20 200 1.5 13 75 100 3 25 25 37.5 75 100 3 25 25 16.5 175 20 14 12.5 12.5 8 455 3 60 7.6 7.6 6 5 DSN800 83 70 73 The DSN800 of the IRC design is 83 (i.e. 83 DCP blows would be required to penetrate 800 mm into the pavement) including the 20 mm AC layer. However, this thin layer is used primarily to protect the pavement structure and generally contributes little to the structural capacity of the road. If the theoretical strength contribution of the AC is ignored, the DSN800 then becomes 70 which is very close to the 73 of the DCP design method. In other words, the two structures are almost identical in terms of theoretical structural capacity (the IRC design is very slightly weaker). Percentage of Pavement Structure Number DSN% 0 20 40 60 80 100 0 10 20 30 Pavement Depth (D in %) 30 40 40 50 IRC 100 000 50 DCP 100 000 60 IRC with AC 70 80 90 100 92  Guidelines for the Environmentally Optimized Design of Low Volume Roads However, if the strength distribution of the two structures are plotted with depth and compared with the theoretical balance curves as shown below, the DCP DN design shows a well-balanced structure with the ideal balance number for a granular pavement of between 30 and 40. The IRC design shows a shallow pavement with a balance number of higher than 50 (i.e. a pavement structure that is extremely sensitive to overloading) with a number of stress concentrations in the upper 100 mm. The structural effect of the asphalt can also be seen to be negligible, slightly worsening the balance. Example of pavement design for different layer thickness Although the standard catalogue for the DCP design method uses layers of 150 mm (primarily to overcome problems often seen on site with layer thickness control), the method can be adapted for any selected layer thicknesses or materials available. Essentially, the DSN800 values for the different road categories relate the required structural capacity of the road to the design traffic. This DSN800 is distributed through the layer thicknesses selected and the materials available as shown below. One of the disadvantages of moving away from the standard designs, however, is that the pavement balance could move away from the optimum. Theoretical Example Granular base material with an unsoaked CBR value of 95% (soaked CBR of 45%) is available for a road being designed to carry 300 000 standard axles. However, no suitable sub-base material is available and it is planned to use the same base material with a reduced thickness for the subbase. What thickness is appropriate using an environmentally optimised design where the base and subbase material will operate at optimum moisture content and what will happen to the pavement balance? The existing layers in the road conform to the lower layer requirements for this design, but the capping layer has a CBR of 25%. The pavement structure and DSN800 for a 3,00,000 esa road are shown below. Required DN value Layer Revised design (mm/blow) Base – 150 mm 3.2 3.1 Subbase – 150 mm 6 3.1 (thickness reduced) Selected/capping layer – 150 mm 12 9 Subgrade – 150 mm 19 19 Subgrade – 200 mm 25 25 Layer Thickness (mm) DN (mm/bl) Blows/layer Base 150 3.1 48 Subbase ? ? (19.4) Capping 150 9 16.6 Subgrade 1 150 19 8 Subgrade 1 200 25 8 DSN800 100 ANNEX-IV: Comparison of Dcp-Dn and Irc Cbr Catalogues  93  By dividing the layer thicknesses by the DN values the number of blows per layer can be determined. The sum of these should be 100 for the required design (i.e. the DSN800) but without the subbase make up only 80.2% of the requirement. The subbase thus needs to make up the difference, which is the 9.4 deficit multiplied by the DN value (3.1 In this case) giving a thickness of 60 mm. As this layer is too thin to work effectively, the road should be constructed with two 105 mm layers making up the base and subbase. The impact of this design on the pavement balance is shown below: Percentage of Pavement Structure Number DSN% 0 20 40 60 80 100 0 10 20 30 0.030 - 0.100 Pavement Depth (D in %) 30 40 40 50 50 Revised Design 60 70 80 90 100 It is evident that by increasing the strength of the upper portion, the balance number increases slightly but the pavement is still a well-balanced relatively deep structure. The main advantages of the DCP DN design method illustrated above are that: ™™ The pavement balance is significantly improved and less overloading sensitive. ™™ uch weaker materials can be used for the upper layers, i.e. natural gravels instead M of crushed aggregate (water-bound or wet-mix macadam). ™™ t is much easier to design for unsoaked conditions, but soaked conditions can be I equally simply handled. ™™ O ffers improved design reliability due to the much larger data set of DCP-DN measurements for statistical analysis and pavement design based on discrete uniform sections rather than general blanket designs. 94  Guidelines for the Environmentally Optimized Design of Low Volume Roads ANNEX-V: DCP Design Example 1. Project Details ™™ Project name: Alpha-Beta Road ™™ Road length: 4.4 km ™™ DCP survey carried out in dry season 2. Design Procedure ™™ As per section 4.3.2.1 3. Step 1 – Design Period ™™ Design period = 15 years (only for illustrative purposes) 4. Step 2 – Design Traffic ™™ 0.241 MESA (Annex-I – Determination of Design Traffic Loading) 5. Step 3 – Design Traffic ™™ Traffic Class = T4 (0.10 – 0.30 MESA) (see Annex I) After entering all the DCP data, the AfCAP LVR DCP programme calculates all the DSN and DN values for each test point as shown below. DSN450/800 is the number of blows to penetrate to a depth of 450mm and 800mm respectively. The DN values are the weighted averages (or 20th/80th percentiles) of the penetration/blow through each 150mm layer. Averages Weighted Average Percentiles Normal Distribution Weighted Average DCP Number, DN in mm/blow DCP DCP Survey Chain- Road Distance DSN450 DSN800 Radio 0-150 mm 151-300 mm 301-450 mm 451-600 mm 601-800 mm Test Test date age Side from CL (Blows) (Blows) (%) Point nr Point (km) (m) 20P Mean 80P 20P Mean 80P 20P Mean 80P 20P Mean 80P 20P Mean 80P Name 1 Test 1 11/07/2016 4,4 LHS 0,00 95 116 82 1,540 2,690 3,830 7,460 9,370 11,290 16,600 17,260 17,920 18,570 20,350 22,130 12,440 16,390 20,330 2 Test 2 11/07/2016 4,5 RHS 2,00 106 142 75 2,760 4,420 6,080 3,170 3,880 4,590 5,580 6,140 6,690 8,190 8,850 9,510 10,230 10,790 11,360 3 Test 3 11/07/2016 4,6 CL 2,00 78 124 63 4,970 6,170 7,380 4,530 4,110 7,690 5,250 5,990 6,740 6,720 6,850 6,980 7,410 8,220 9,020 4 Test 4 11/07/2016 4,7 LHS 0,00 111 172 65 3,380 3,810 4,250 3,900 4,410 4,290 3,980 4,250 4,520 4,890 5,230 5,560 5,530 6,480 7,430 5 Test 5 11/07/2016 4,8 RHS 2,00 87 137 64 2,680 3,390 5,160 5,340 6,810 8,270 5,660 6,640 7,620 6,090 6,920 7,740 5,540 7.940 10,330 6 Test 6 11/07/2016 4,9 CL 2,00 97 143 68 2,880 4,110 5,350 4,050 4,750 5,460 5,760 5,930 6,100 6,660 7,520 8,370 6,720 8,190 9,670 7 Test 7 11/07/2016 5 LHS 0,00 128 183 70 2,090 2,840 3,590 2,820 3,780 4,740 5,060 5,180 5,290 4,880 5,570 6,260 6,450 7,620 8,800 8 Test 8 11/07/2016 5,1 RHS 3,00 57 97 59 6,860 7,830 8,800 6,680 7,250 7,810 8,280 8,800 9,320 8,180 8,970 9,950 9,750 8,870 10,180 9 Test 9 11/07/2016 5,2 CL 2,00 124 248 50 2,390 3,100 3,810 2,870 3,800 4,740 5,000 5,540 6,080 3,940 4,860 5,770 1,900 2,220 2,550 10 Test 10 11/07/2016 5,3 LHS 0,00 131 180 73 2,070 2,870 3,660 2,690 3,350 4,010 5,170 5,830 6,480 6,120 6,530 6,950 -0,0540 11,730 24,000 11 Test 11 11/07/2016 5,4 RHS 2,00 100 155 65 3,920 4,610 5,290 3,720 4,250 4,780 4,600 4,910 5,230 5,860 6,090 6,320 6,050 6,670 7,280 12 Test 12 11/07/2016 5,5 CL 2,00 92 138 67 2,850 3,780 4,720 5,150 7,150 7,150 9,150 5,480 5,890 6,300 6,060 6,820 7,590 8,720 9,820 13 Test 13 11/07/2016 5,6 LHS 0,00 92 170 54 -0,400 8,550 17,510 2,910 11,220 19,530 4,140 4,500 4,860 4,520 4,660 4,790 4,060 4,450 4,840 14 Test 14 11/07/2016 5,7 RHS 2,00 119 171 70 2,590 3,620 4,650 3,050 3,910 4,770 4,280 4,860 5,450 5,730 6,100 6,460 6,430 7,720 9,010 15 Test 15 11/07/2016 5,8 CL 2,00 151 209 72 2,070 2,420 2,760 2,440 2,870 3,310 4,140 4,620 5,100 5,200 5,370 5,550 6,090 6,590 7,090 16 Test 16 11/07/2016 5,9 LHS 0,00 183 229 80 1,890 3,080 4,270 1,670 1,980 2,3000 2,650 3,380 4,110 8,610 13,000 17,390 6,010 6,570 7,140 17 Test 17 11/07/2016 6 CL 2,00 112 151 74 1,950 2,510 3,070 3,620 4,560 5,510 7,600 15,630 23,670 8,270 9,050 9,830 8,400 9,190 9,970 18 Test 18 11/07/2016 6,1 RHS 2,00 143 194 74 2,000 2,680 3,370 2,580 3,050 3,520 4,390 4,900 5,410 4,560 6,770 8,970 7,030 8,410 9,790 19 Test 19 11/07/2016 6,2 CL 2,00 166 213 78 2,070 3,030 3,990 1,840 2,290 2,750 3,540 3,830 4,130 5,100 6,040 6,970 7,870 10,320 12,770 20 Test 20 01/07/2016 6,3 LHS 0,00 195 258 75 1,160 1,850 2,540 2,240 2,710 3,170 4,080 4,450 4,810 4,850 5,220 5,600 5,460 5,830 6,200 21 Test 21 18/07/2016 6,4 CL 2,00 143 187 76 1,630 2,180 2,730 2,960 3,580 4,210 5,250 5,760 6,270 6,860 7,170 7,480 8,110 8,660 9,210 22 Test 22 18/07/2016 6,5 LHS 0,00 122 168 72 2,110 3,320 4,520 3,000 3,790 4,590 5,490 6,550 7,610 6,320 7,560 8,800 6,580 8,190 9,800 23 Test 23 18/07/2016 6,6 RHS 2,00 184 221 84 1,240 1,680 2,120 2,690 3,260 3,820 5,020 5,820 6,610 6,440 7,830 9,230 11,480 12,640 13,800 24 Test 24 11/07/2016 6,7 CL 2,00 163 195 84 1,290 1,940 2,590 2,150 3,360 4,580 6,190 7,420 8,650 7,940 9,340 10,740 11,760 14,740 17,730 25 Test 25 11/07/2016 6,8 LHS 0,00 196 263 75 1,390 2,080 2,760 2,030 4,830 7,630 1,960 2,550 3,140 3.390 3,750 4,110 5,920 8,490 11,060 26 Test 26 11/07/2016 6,9 RHS 3,00 67 83 80 3,520 3,940 4,360 7,330 8,860 10,390 13,620 14,210 14,800 17,480 19,770 22,060 21,120 23,190 25,260 ANNEX-V: DCP Design Example  95  In this example, the DCP tests were carried out in the dry season, hence 80th percentile DN values were used for the pavement design assuming that the existing pavement may be wetter (i.e. weaker) in service than when the DCP tests were carried out. As shown in the following screenshots, the software then allows the user to determine uniform sections based on a Cumulative Sum analysis of the data, both for the DSN values and each layer separately. Property DSN450 197 4.5 DSN450 5.70 177 0.7 Cumulative sum of difference (Z) 158 -3.1 DSN450 138 -6.9 118 -10.7 DSN450 >=84 DSN450 DSN450 < 84 99 -14.5 Cusum 79 -18.3 Avg 59 -22.1 39 -25.9 20 -29.7 102 (avg) 152 (avg) 0 -33.5 4.4 4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 Chainage (km) Display chainage (km) 2.5 Help Copy Finish Property DN Layer 2 80P 20.00 2.0 5.20 5.50 5.70 6.70 DN 80P (mm/blow) 18.00 1.1 Cumulative sum of difference (Z) 16.00 1.4 14.00 1.1 DN 80P (mm/blow) 12.00 0.8 DN <= 6 DN > 6 10.00 0.5 Cusum 8.00 0.2 Avg 6.00 -0.1 4.00 -0.4 2.00 5.67 19.53 7.53 -0.7 6.77 (avg) (avg) (avg) 3.80 (avg) (avg) 0.00 -1.0 4.4 4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 Chainage (km) Display chainage (km) 2.5 Help Copy Finish 96  Guidelines for the Environmentally Optimized Design of Low Volume Roads Following the separate analysis of the DSN and DN values, the user can then determine uniform sections on an overall assessment of all properties as shown in the next screenshot. In this case four sections have been determined. Property Display all properties CUSUMS 5.50km 5.70km 5.90km 101.81 (evg) 152.47 (evg) DSN450 DSN800 155 (avg) 196 (avg) Layer 1 DN 80P 5.16 (avg) 17.51 (avg) 3.36 (avg) Layer 2 DN 80P 6.77 (avg) 5.67 (avg) 19.53 (avg) 3.80 (avg) 7.53 (avg) Layer 3 DN 80P 31 (avg) 5.94 (avg) 23.67 (avg) 4.78 (avg) 7.85 (avg) 4.4 4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 Chainage (km) Display chainage (km) 2.5 Help Copy Finish On this basis, the AfCAP LVR DCP software produces the design report as shown below. The design report can be exported to Excel for incorporation in reports. ANNEX-V: DCP Design Example  97  Averages from Weighted Average Percentiles from Normal Distribution Pavement Layer Required DN value Section no. (mm) for TLC 0.3 1 2 3 4 4.4 to 5.5 km 5.5 to 5.7 km 5.7 to 5.9 km 5.9 to 6.9 km 0-150 <=3.2 (3.5) 4.9 (80P) 10 (80P) 3.4 (80P) 3.0 (80P) 150-300 <=6 (6.9) 5.8 (80P) 11 (80P) 2.6 (80P) 4.5 (80P) 300-450 <=12 (14) 7.0 (80P) 5.1 (80P) 4.6 (80P) 8.1 (80P) 450-600 <=19 8.3 (80P) 5.6 (80P) 11 (80P) 9.0 (80P) 600-800 <=25 10.0 (80P) 6.8 (80P) 7.0 (80P) 12 (80P) Inadequate (non-compliance) in situ layer Adequate (margin al compliance) in situ layer(s) that need to be impro Adequate (full compliance) in situ layer(s) Pavement Layer Required DN value Section no. (mm) for TLC 0.3 1 2 3 4 4.4 to 5.5 km 5.5 to 5.7 km 5.7 to 5.9 km 5.9 to 6.9 km 0-150 <=3.2 (3.5) 3.2 3.2 3.4 (80P) 3.0 (80P) 150-300 <=6 (6.9) 4.9 (80P) 6.0 2.6 (80P) 4.5 (80P) 300-450 <=12 (14) 5.8 (80P) 10 (80P) 4.6 (80P) 8.1 (80P) 450-600 <=19 7.0 (80P) 11 (80P) 11 (80P) 9.0 (80P) 600-800 <=25 8.5 (80P) 5.3 (80P) 7.0 (80P) 12 (80P) New base added with DN values <=3.2 New sub base added with DN values <=6 Inadequate (non-compliance) in situ layer Adequate (margin al compliance) in situ layer(s) that need to be improve Adequate (full compliance) in situ layer(s) The Design Report shows four uniform sections identified within the 2.5km, each analysed separately for the upgrading requirements. The top part of the Design Report shows the situation for each uniform section before upgrading where each layer has been assessed against the specifications to the left for the appropriate Traffic Load Class, in this case TLC 0.3 for traffic loading between 100,000 and 300,000 Equivalent Standard Axles over the design life of the road. 98  Guidelines for the Environmentally Optimized Design of Low Volume Roads It is shown that Sections 1 and 2 needs a new base layer with a DN value ≤ 3.2 mm (from the DCP-DN catalogue). Section 2 also needs a new subbase with a DN ≤ 6 mm. In Section 3 it is assumed that the existing base layer can probably be improved by scarifying, reshaping and compaction to refusal, hence the amber colour. In Section 4 the existing base is already strong enough and needs only reshaping before surfacing. The bottom part of the Design Report shows the situation after upgrading. Each section can then be analysed separately using the “DCP Section Analysis per Section” feature as shown below. Below is shown the analysis for Section 1 after upgrading by adding a new 150 mm base layer. Section 1 will then have a “Well balanced deep structure”, which one should always try to achieve (but which is not always possible). It can also be seen that the DSN 800 value is above the minimum requirement of 100 blow for this design class. 0 DSN800 (90<=B<=90) TLC 0.3 Ave. B=13, A=684 20/80P Inadequate 100 Catagory IV: Well-Balanced Adequate Deep Structure (WBD) 200 0 0 Pavement depth (mm) 100 100 300 Pavement depth (mm) Pavement depth (mm) 200 200 400 300 300 500 400 400 600 500 500 600 600 700 700 700 800 800 800 10 20 30 40 50 60 70 80 90 100 100 0.1 10 1 10 20 30 40 50 60 70 80 90 100 ANNEX-V: DCP Design Example  99