Characterisation of cross-anisotropic pavement materials

The prime objective of the research reported in this thesis was to develop a procedureto characterise unbound, elastic, cross-anisotropic subgrade soils. The work comprisedthe design and manufacture of a new form of rigid-boundary, repeated-loading, true-triaxialtest equipment, the development of a laboratory procedures and techniques foranalysing the experimental data. In order to verify the validity of the procedure, theselected technique was implemented on a typical, unbound, fine-grained subgrade soil.The new repeated-loading, true-triaxial equipment consisted of an advanced pneumaticloading system which was controlled through a personal computer. The systemprovided a versatile control of forces in three perpendicular directions, enabling agood, if incomplete, simulation of the in-situ stress conditions in pavements.The test specimens consisted of an A-6 silty clay, and were prepared using a cubicalcompaction mould. Two compaction methods were used. The first was the floatingmould static compaction method and the second was a dynamic compaction methodsimilar to the standard and modified compaction testing procedures.Two mathematical techniques were considered. The first was the simplified methodproposed by Graham and Houlsby (1983) and the second was a least square solutionintroduced by Yang and Shackel (1995). For comparison, a least square solution forisotropic soils was proposed.The experiment embraced a study of the effects of stress conditions and the physicalstate conditions on the cross-anisotropic elastic behaviour. The stress factors consistedof the number of stress applications, the octahedral shear stress, the octahedral shear-to-normal stress ratio and the intermediate principal stress. The physical conditionsconsisted of the density, the degree of saturation and the method of compaction.The main conclusions drawn from this experiment were as follows. Firstly, the newrepeated-loading cubical true-triaxial equipment enabled the measurement of theprincipal stresses and strains required to determine the elastic cross-anisotropicparameters with reasonably accuracy. Secondly, the experiment verified that, of thesolutions considered, the use of Graham and Houlsby approach provided a bettersolution in interpreting the results of the cubical true-triaxial tests. This solutionenabled the determination of the elastic parameters required to describe cross-anisotropicmaterials. Thirdly, the experiment justified the assumption of materialanisotropy. Finally, the procedure introduced in this experiment has been proven tofacilitate the study of material cross-anisotropy in material subjected to repeatedloading similar to those commonly encountered in pavements