Considering the influence of the volumetric behavior on the soil state

a void-ratio-dependent shear strength criterion for unsaturated soils is proposed by introducing the state surface expression into the Vanapalli’s shear strength equation. The saturated shear strength parameters and two series of soil water characteristic curve（SWCC） corresponding to different void ratios are employed in the proposed criterion. A series of SWCC tests

isotropic compression tests and suction-controlled triaxial tests are performed on a soil mixture of tailing sand and speswhite China kaolin. The testing results indicate that the proposed criterion can provide a more accurate prediction of shear strength for unsaturated soils than that of convention prediction using a unique SWCC. It is proved that the principal source of errors in the conventional estimation of shear strength is related to the evolution of soil state caused by the volume change. Moreover

a method for accurately identifying the failure envelopes in different stress spaces is developed. Several mechanical characteristics of unsaturated soils

including an increase in the slope of failure envelope with suction in net stress versus shear strength plane and the influence of the net stress on the shape of the failure envelope in the suction-strength plane

are reasonably interpreted by this method.