Vane Shear Data on Undrained Residual Strength

Abstract

The ratio of ultimate undrained residual strength to consolidation pressure for San Francisco Bay mud is a constant in the manner that the ratio of peak undrained strength to consolidation pressure is a constant. Laboratory and field vane shear tests were used to study the variation of peak and ultimate residual undrained shear strength with water content for normally consolidated Bay mud. The average ratio of ultimate undrained residual strength to peak undrained strength determined from the laboratory vane tests was 0.42, and did not vary with water content. Based on a constant ratio of peak undrained strength to consolidation pressure equal to 0.32, the ratio of ultimate undrained residual strength to consolidation pressure is equal to 0.13. The values from the field vane tests were somewhat different, but the relationship was similar. A constant ratio of ultimate undrained residual strength to consolidation pressure for a cohesive soil allows a simple prediction of the ultimate undrained residual strength over a wide range in consolidation pressure from only a few tests. The ultimate undrained residual strength of a cohesive soil can provide an important lower bound in assessing the strength loss that accompanies landslide movement.