Stiffness and shear strength of unsaturated soils in relation to soil-water characteristic curve

This paper introduces a methodology to predict the non-linear stiffness–suction and shear strength–suction relationships for unsaturated soils within the lower suction range from the non-linearity of the soil-water characteristic curve (SWCC), using a normalised function formulated with ‘suction times exponential degree of saturation’. The information required in this methodology includes (a) measurements of the shear strength or stiffness properties at saturation condition and one unsaturated condition and (b) the SWCC. Published experimental data on the stiffness and shear strength properties and the SWCC obtained from 25 different soils varying from coarse-grained sands to expansive clays are used to validate the proposed normalised function and to calibrate the exponent value. It is found that the normalised function, using exponent values of 1·0 and 2·0, respectively, for cohesionless and cohesive soils, provides reasonable predictions of the stiffness–suction and shear strength–suction relationships for all the soils used in this study, taking account of various influencing factors including external stress, soil structure, anisotropy, hydraulic hysteresis and testing technique.