Post-liquefaction deformation and strength characteristics of sand in torsional shear tests

Abstract

The estimation of strength deterioration with the increase in the undrained cyclic shear-induced strain in liquefiable soils is an important topic in geotechnical earthquake engineering that is still poorly understood and needs to be properly addressed. In this paper, in an attempt to provide new insights into this topic, the post-liquefaction undrained strength and deformation of Toyoura sand were investigated through a series of laboratory tests performed by using a large-strain torsional shear apparatus. In the tests, simple shear conditions were reproduced on a medium-size hollow cylindrical specimen to apply stress conditions that soil experiences in the field during earthquakes. Specimens prepared by the air-pluviation method were first liquefied by applying a constant amplitude undrained cyclic shearing to induce a target cyclic shear-induced damage strain (γΔ). The tests were terminated at different amplitudes of γΔ and subsequently loaded monotonically in undrained conditions to obtain stress-strain and excess pore water generation responses. The test results indicate that the post-liquefaction monotonic loading (ML) stress-strain behavior of sand can be divided into three regions, namely Region 1, 2 and 3. The sand behavior changes to strain-hardening from essentially zero strength and stiffness from Region 1 to 2. While Region 3 marks the beginning of strain-softening state in the sand. It is found that the strain localization is associated with the transition of sand behavior from strain-hardening to strain-softening state. It is proposed that the sand undrained strength in monotonic shearing is the true representation of specimen response until the state of uniform deformation is maintained, based on the sudden drop of the differential stress (σd = σ v′ - σh′). The test results also revealed that there is a progressive degradation in the shear strength with the increase in the amplitude of γΔ. A correlation depicting the degradation ratio (τd) with the increase in the γΔ is proposed. This correlation can be used to estimate the degree of degradation with the increase in the applied γΔ, and it is valid for different density states, confining stresses and cyclic stress ratios.