Impact of Static Preshearing on Undrained Anisotropy and Shear Characteristics of Sand

Abstract

The preshearing history of soil is a prominent factor governing its mechanical behavior at various loading conditions. An experimental program was developed to study the effects of preshearing at drained compression and extension conditions on the undrained behavior of saturated sand. Triaxial compression and extension tests on both loose and medium-dense sand showed distinctly different behaviors: the response in compression was dominated by stable and dilative behavior, whereas instability occurred in extension with strain softening. The initial inclination of the effective stress path varying with the preloading history is related to the fabric anisotropy of the soils, characterized by their stiffness values at very small strains, whereas at intermediate to large strains, the undrained stiffness tends to decrease in triaxial compression but grows in triaxial extension with the preshearing stress ratio. Moreover, the instability behavior of sand characterized by the reduction of shear strength is governed by the magnitude and direction of the preshearing stress. The stress ratio at which the instability occurs is not unique and can be correlated with the proposed state index accounting for the combined influences of the preshearing stress and relative density of the sand. In contrast, a unique phase-transformation line exists that is independent of the preshearing stress conditions. This is probably due to the gradual diminishing of the stress-induced anisotropy that is created by the static preshearing during the subsequent undrained loading.