Volume 25, Issue 2 (June 2002)
Predicting the Shear Strength Envelope of Unsaturated Soils
Determining the shear strength envelope of an unsaturated soil involves a sophisticated and time-consuming testing program. Hence, it is advantageous to minimize the number of tests required to establish the envelope. In this paper, a power additive function is adopted to predict the shear strength envelope of unsaturated soils from knowledge of the unsaturated shear strength at residual suction, the effective stress friction angle for saturated conditions, and the soil-water characteristic curve. Two boundary conditions are required to quantify the two fitting parameters of the function. The first is φb = 0 at residual suction, and the second is the contribution of matric suction to the shear strength at residual suction, which should be experimentally evaluated. The validity of the method is tested for a number of unsaturated shear strength data covering a wide range of suctions. Good agreement was found to exist between the predicted envelopes and the experimental data. The method requires only one experimental evaluation of the unsaturated shear strength and is most suitable for coarse- to medium-grained soils where the residual suction may be achieved reliably in the laboratory.