Estimation of the unsaturated shear strength of expansive soils in relation to capillary water-retention curve

Abstract

The hydromechanical behavior of expansive soils is one of the challenging topics in geotechnical engineering. Direct measurements of unsaturated shear strength in the laboratory are complicated and time-consuming. This work introduces a novel model to estimate the contribution of suction to the shear strength of expansive soils. Non-linear relationships in the space of suction and shear strength can be clearly observed, which indicates that the friction angle (ϕb) changes with the evolution of suction. Capillary saturation has been used to upscale suction from pore-scale stress to macroscopic stress using the capillary part of soil-water retention curve. Experimental results indicate that the relationships between capillary suction stress and shear strength show linear properties. However, the friction angle (ϕs) contributed by capillary suction stress to shear strength is not equal to the internal friction angle (ϕ) of saturated soils. The proposed model uses the shear strength value measured at any specific unsaturated state to obtain the evolution of friction angle (ϕs). Results show that the effect of different reference points on computed strength can be ignored. The proposed model is successfully validated with historical data of various kinds of expansive soils from literature.