Step-changed strain rate effect on the mechanical properties of undisturbed expansive clay

Abstract

Up to now, most of the research on rate effects has investigated the behavior of soft soils, much less is known for the stiff clays. In particular, the method of stepwise changing strain rate has rarely been used in studies of soil rate effects, but many studies have well demonstrated that this method is beneficial to overcome the problems of variability between samples and the time requirements associated with strain rates. Isotropic consolidated-undrained triaxial tests were conducted on specimens with the same void ratio at step-changed axial strain rates under four different confining pressures in this study. We carried out the effect of axial strain rates from 0.09 to 300%/hour on the mechanical properties of naturally expansive soils. Experimental results showed that the higher the strain rate, the higher the effective stress for a given strain. The shear strength increases from 5.9% to 11.4% per log cycle of axial strain rate for saturated undisturbed expansive soil specimens by introducing the strain-rate parameter ρ 0.09, which is similar to that of soft soils. The excess pore water pressure gradually decreases after reaching a peak at lower cell pressures and increasing at higher ones. In addition, the failure mode of the specimens is different from soft clay with both the main shear band and multi-fissures band. The new findings are helpful for better understanding the time-dependent mechanical properties of stiff clays.