P-Constant Condition Applied to Undrained Cyclic Triaxial Test of Unsaturated Soils

Abstract

To evaluate the liquefaction resistance of unsaturated soils by a triaxial system, it is suggested to keep a p-constant condition rather than the σh-constant condition (i.e., cell pressure constant) that is normally applied to the same test for saturated specimen. The main concern is that the pore pressure coefficient (B value) of unsaturated specimens may be far less than unity, which causes undesired effects on the behaviors of unsaturated soils. A modified triaxial system that can maintain the p-constant condition has been recently developed. By using the system, the behaviors of an unsaturated soil subjected to undrained cyclic triaxial loading are compared between the p-constant condition and the σh-constant condition, which implies that the testing conditions affect behaviors of pore pressures generation, stress strain relationship, etc. Results suggest that tests under the σh-constant condition underestimate the liquefaction resistance of soils. In addition, the modified triaxial system could effectively reduce the variation of p by more than 60 % in the full tested range, or by 75 % in a range with relatively small magnitude of the vertical loading based on test results of three sandy soils.