Unsaturated shear strength of expansive soils from natural slopes in Queensland

Expansive soils exhibit significant changes in the swell and shrink behaviour upon variations of moisture content. Majority of soils in Queensland are expansive; consequently, failures in natural slopes with expansive characteristics are commonly reported under climate-induced soil moisture variations. In order to understand the failure mechanism of such soil slopes, it is vital to analyse slope stability under unsaturated conditions. The current study employed undisturbed soil samples from natural slopes in Queensland, and determined the basic soil properties, including the soil-water characteristic curve (SWCC), and unsaturated shear strength to facilitate the analysis of rainfall-induced slope failure mechanism. A series of conventional direct shear tests were conducted for different moisture contents under various vertical stresses. The test results were analysed to obtain the variation of the apparent cohesion and the effective friction angle of the soil with moisture content and suction. The study presents an empirical equation to predict the cohesive component in the shear strength of unsaturated expansive soils as an exponential function of suction. The formulation originated from multiple linear regression analysis for data sets obtained from shear tests using undisturbed soils with varying moisture contents. The empirical equations can realistically predict the reduction in soil cohesion due to wetting (R2 = 0.8791). The study provides an alternative to the quantitative estimation of unsaturated shear strength of expansive soils.