Transient Shear Strength of Saturated Sand under Cyclic Loading Considering Strain-Rate Effect

Abstract

The shear strength of saturated sands under undrained cyclic loading is defined as transient shear strength in order to characterize its time dependency and to distinguish it from the undrained cyclic strength based on the equivalent cycle concept. It has been found from the undrained cyclic triaxial tests on two saturated Fujian sands that: (a) the transient shear strength of saturated sand is strain-rate dependent when the shear strain rate is more than 10% per second. In general, the transient shear strength of saturated sand consists of frictional resistance and viscous resistance, which can be described as τdt=τft+τηt=σt′tanϕd′+ηγ.t; (b) the angle of dynamic internal friction ϕd′ is not affected by the number of cycles, consolidation stress ratio and frequency of cyclic stress (1-20 Hz) and the apparent viscosity coefficient η almost remains constant within the range of the tested shear strain rates (10-60% per sec) for a given density; and (c) the angle θ0, formed by the phase transformation line in τ – σ plane, is nearly constant for a given sand. Furthermore, undrained cyclic torsional shear tests on hollow cylindrical specimens show that the frictional strength and the stress states at the phase transformation define two three-dimensional curved surfaces for the effective principal stress; and in addition, both of them can be described by the Matsuoka-Nakai criterion. Methods are proposed to determine the dynamic strength parameters and to correct the damping of the test apparatus system, the effect of inertia force of the sand specimen and the time lag of measured excess pore water pressure in cyclic triaxial tests with a higher frequency.