Effective Stress and Compressibility of Unsaturated Clayey Soil under Drying and Wetting Cycles

Naturally, soil moisture reduces during dry seasons when the soil is in drying state; while it increases during wet seasons when the soil is in wetting state. Previous studies have shown that for an unsaturated soil sample, soil-water characteristic curves (SWCCs) do not match in wetting and drying paths. The differences between wetting and drying paths are called the hydraulic hysteresis. The hydraulic hysteresis plays an important role in mechanical properties of soil such as shear strength, volume change, and settlement. The objective of this research is to study the effects of drying and wetting on the effective stress and compressibility of unsaturated clayey fine-grained soils. To this end, saturated and unsaturated triaxial tests were performed on the soil samples under various normal mean stresses, and matrix suctions in drying and wetting paths. It was found that soil samples bear higher levels of effective stress in the drying path than the wetting path under a same level of suction. The attained values of effective stress parameter (χ) showed that the Bishop’s effective stress parameter (χ = Sr) is not properly applicable for the clayey soil. Moreover, the resulting loading-collapse curve (LC) revealed that the effective pre-consolidation pressure in the drying and wetting stages changed even with same degrees of matrix suction.

A u-p Formulation for Fully Coupled Dynamic Analysis of Flow and Deformation in Unsaturated Soils

A u-p formulation based on the mixture theory is presented for describing the dynamic flow and deformation behaviour of unsaturated soils. In the formulation proposed, the solid displacement, pore water pressure, and pore gas pressure are considered as primary variables. The spatial discretization of the governing equations is achieved using finite element method, whereas the time integration is conducted using the Newmark technique. The coupling between solid and fluid phases is enforced according to the effective stress principle taking suction dependency of the effective stress parameter into account. Numerical examples and comparisons with known analytical solutions are presented, demonstrating the performance of the proposed approach.

Capillary-based effective stress formulation for predicting shear strength of unsaturated soils

A number of investigations have shown that the shear strength of unsaturated soils can be defined in terms of effective stress. The difficulty in this approach lies in quantifying the effective stress parameter, or Bishop’s parameter. Although often set equal to the degree of saturation, it has recently been suggested that the effective stress parameter should be related to an effective degree of saturation, which defines the fraction of water that contributes to soil strength. A problematic element in this approach resides in differentiating the water that contributes to soil strength from that which does not contribute to soil strength. To address this difficulty, the paper uses theoretical considerations and experimental observations to partition the water retention function into capillary and adsorptive components. Given that the thin liquid films of adsorbed water should not contribute to effective stress, the effective stress parameter is solely related to the capillary component of water retention. In sample calculations, this alternative effective stress parameter provided very good agreement with experimental data of shear strength for a variety of soil types.