Effect of hydraulic hysteresis on shear strength of unsaturated sand / Wael Mahmood Jaafar Albadri

Soil in the field is subjected to variations in the water content due to water infiltration and evaporation processes that put the soil under either drying or wetting conditions. The shear strength of soil is affected by hydraulic hysteresis (the difference between drying and wetting). Defining the shear strength of unsaturated soil based on the two independent stress state variables approach was under argument by other researchers since the mentioned approach is not incorporating the variation of degree of saturation under given suction caused by the hydraulic hysteresis. In this research, the suction controlled triaxial test was used to conduct experiments on sand under different hydraulic phases (first drying, first wetting and second wetting). On top of that, the pressure plate extractor was modified to measure the wetting portion of SWCC. According to the test results, it was found that hydraulic hysteresis reduces with increasing the confining pressure and with increasing the drying-wetting cycles. Moreover, Although the maximum shear strength was obtained from the specimens under drying condition, the difference between the drying and wetting shear strengths was not significant for the tested soil. Also, the shear strength of unsaturated sand at any suction below the air entry value is equivalent to the saturated shear strength. The SWCC was found to be capable of not only estimating unsaturated soil property functions but also anticipating the shear strength behaviour of soil under different hydraulic phases. Based on the strong correlation between the SWCC and the apparent shear strength, this research has developed a shear strength model that incorporate the effect of hydraulic hysteresis and the alternate D-W cycles. The model was successfully validated by using experimental results from previous studies.