Concept of Lateral Confinement in Pavement Analysis and Design

Abstract

A flexible pavement is a three dimensional structure consisting of several layers of different materials, so stresses and strains due to wheel load are set up in layers in all directions and their magnitudes denend upon the mechanical properties of materials and details of construction. Measurements of both stresses and strains in actual road structures have been made by many researchers, thus showing that a layered pavement structure responds to applied loads in the same way as any other engineering structure and therefore the problem of design may be approached by the normal structure design procedure. The performance of flexible pavements depends to a large extent on the strength properties of pavement material and subgrade material in particular. When the vehicular load comes near the edge of pavement, due to lack of internal friction, cohesion in base structure and subgrade and lack of lateral confinement in the subgrade, shear failure takes place. This results in deformation of pavement under load and upheaval of shoulders, therefore reduces the lateral support to the pavement edge. The importance of studying the confinement effect in pavement structure including subgrade soil has been stressed by many researchers. Highway Research Board(IRC) stressing the need ii of studying the lateral confinement effect in pavement analysis anri design recommended to the Ministry of Shipping ana* Transport (Roads Wing) the research project entitled 'The effect of the extent of free board on design and performance of flexible pavements for different soils', lateral confinement effect is more pronounced in flexible pavements with sand subgrade material. Therefore in desert region where there is a scarcity of adequate road material at reasonable lead apart from other problems the concept of laterally confining the pavement would prove to be an effective and economical method for pavement stability. This would reduce the maintenance cost of such pavements and would in turn yield saving in use of bituminous binders for road construction and maintenance projects. The present research project is aimed to study the structural behaviour of flexible pavements on a typical subgrade under wheel load for suggesting a design methodology accounting for the advantages which can be derived from the lateral confinement mobilized beneath the pavement edges. The existing theories of designing thickness of flexible layers do not account for the lateral confinement aspect as a design parameter. The experimental study and analytical parametric study on two-layer pavement system using the finite prism model, provide sound basis for developing a design iii methodology incorporating lateral confinement under load as one of the design factors. CHAPTER I describes the objectives of present study. Various design approaches for flexible? pavements are highlighted. Th^ aspects supporting elastic theory for design have been discussed. The scope and outline of the present study are given. CHAPTER II reviews the literature on various aspects of flexible pavement Design. The concept of lateral confinement in pavement analysis is explained by discussing the variation in elastic properties of pavement materials with lateral confinement. The state of confinement changes due to vehicular load occupying different positions on the pavement surface, the pavement performance is affected accordingly. The gaps in the existing design methods have been identified. The application of finite element method solving pavement system problems is discussed. GHAPTT31 III deals with the experimental study on subsnrade soil, the compaction of which was done using slit fall method in order to simulate the field conditions of compacted sand. The plate load tests on sandy subgrade model were conducted for different position of vertical diaphragm with resnect to load positions. The diaphragms of varying flexibilitv were employed using metallic sheets of different iv thicknesses. But it was found that mild steel plate of particular thickness meets the flexibility requirement of the test and therefore this was used for further testing. The deformation characteristics of subgrade soil under different conditions of lateral confinement were studied. The variation in modulus of elasticity of subgrade soil at different lateral confinement conditions under load is quantified. An equation representing the correlation bet ween modulus of elasticity, E of soil and modulus of subgrade reaction, K for any lateral confinement condition has been suggested. CHAPTER IV presents the general procedure for the analysis of layered pavement system using two-dimen sional finite element model which was used for parametric study of pavement using strip load. A three dimensional analytical model using the finite prism method which is a combination of finite element and fourier series is developed and validated. Therefore with three-dimensional model, it is possible to determine the stresses and deflec tions in the layered pavement system for any type of loading condition including edge load conditions. CHAPTER V deals with the parametric study for layered pavement system using two-dimensional and. threedimensional models. The variations in stresses and deflections in two layered pavement system for different load positions on surface from the edge is studied. The above study is carried out for unconfined and laterally confined conditions of pavement system. The variations observed in the above design, parameters due to different load positions and pavement conditions helped in arriving at a conclusion to develop a design methodology for economical design of flexible pavement system. CHAPTER VI presents the design, methodology developed for two layer pavement system incorporating lateral confinement as one of the additional parameters. Design charts are developed for different thicknesses, wheel load, positions (from the pavement edge) and for unconfined and laterally confined pavement conditions. The design charts are based on deflection equations developed in the present study. Finally, CHAPTER VII presents various conclusions drawn from the present study.