Exponential Equation for Predicting Shear Strength Envelope of Unsaturated Soils

Journal article, 2019

An exponential equation is introduced to predict the nonlinear variation of the shear strength with matric suction of the unsaturated soils. The proposed equation involves three constant parameters, two of which are effective shear strength parameters (i.e. ϕ´ and c´). The third parameter is the maximum capillary cohesion,c´´max, that is the maximum possible increase in shear strength due to matric suction. A procedure for determination of c´´max  from the soil-water characteristic curve (SWCC) is devised. The proposed equation is validated through a series of constant-suction consolidated drained triaxial tests conducted on specimens reconstituted by isotropic consolidation from slurry state. In addition, validity of the equation is investigated by applying it to test results of five other soils that were available in the literature for low suction range (i.e. up to 1500 kPa). A comparative study on prediction of shear strength was carried out between the proposed equation and six other shear strength equations found in the literature. The results show that the proposed equation provides reliable predictions of shear strength of unsaturated soils when shear strength converges to an asymptotic value at the residual water content.