Behaviour of an Unsaturated Silty Sand under Monotonic and Cyclic Loading

Abstract

The objective of this thesis is to experimentally study the behaviour of unsaturated soil in terms of shear-dilatancy relationship under constant matric suction (CS) and constant water content (CW) during monotonic or cyclic loading. The influence of matric suction and its history (wetting/drying) on volume behaviour of unsaturated soil was traditionally discussed under isotropic or one-dimensional condition. This thesis studied the influence of matric suction and its history (with/without one cycle of wetting/drying) on shear-dilatancy response of unsaturated soil under drained (CS) or undrained (CW) shear loading. Two groups of tests with/without a wetting/drying cycle during matric suction initialization manifested significant different shear-dilatancy response. The influence of matric suction history on the soil structure appeared to be significant. After wetting/drying cycle, unsaturated soil achieved more stable structure (less shear-dilatancy). However, the influence of matric suction initialization became less significant under higher net confining pressure. In this study, the behaviour of unsaturated soil was also studied under two stress level of cyclic loading with limited number of load cycles. Stiffening, overall volume compression and matric suction decrease with loading cycles were observed in both stages of cyclic loading. Matric suction change during cyclic loading appeared to correlate with the volume strain response. The change rates of deformation response and matric suction decreased with the number of cycles, as their magnitudes became stable after limited loading cycles. Significant jumps in dilatancy were detected when loading direction reverses at both the peak and the trough of deviator stress, but continuous shear-dilatancy relationship was detected after the jumps during both stages of cyclic reloading or unloading. The shear-dilatancy response of unsaturated soil during both stages of cyclic reloading is different from that of virgin loading or the first loading. This study provides significant evidences that equivalent effective stress gave better interpretation than two independent stress variables in terms of strength and shear-dilatancy response of unsaturated soil.