Experimental study of residual strength and the index of shear strength characteristics of clay soil

Using a ring shear apparatus, the variation in the residual strength characteristics of 21 groups of saturated clay soil samples were experimentally investigated, and the results are discussed in this paper. The clay soil samples had different plasticity indexes under 10 different consolidation conditions. The influences of shear rate, over-consolidation ratio, plasticity index, and multi-stage shear mode on the residual strength were investigated. New shear strength variation trend data and a corresponding equation for shear strength calculations as a function of plasticity index of soils under large deformation are presented. The experimental results indicate that 1) both the shear rate and over-consolidation ratio have a negligible influence on the residual strength; 2) the residual strength gradually decreases as the plasticity index increases; 3) the residual strength shows a significant nonlinearity with the normal stress when subjected to low- or high-levels of normal stress; 4) the internal friction angle of drained clay decreases as the plasticity index increases under large deformation. Finally, a formula is proposed to describe the relationship between the residual inner friction angle and plasticity index.