Study on seismic behavior of nonplastic silt by means of ring-shear apparatus

Abstract

The influence of static shear stress on undrained cyclic behavior of nonplastic and low-plasticity silts has been studied by means of undrained cyclic torque-controlled ring-shear tests. The cyclic and post-cyclic behavior of silty soils assumed on sliding surface were investigated to assess the liquefaction potential and cyclically induced deformation of silty slopes. Six different initial static shear stresses corresponding to slope angles from 0° to 25° were examined. To better understand undrained cyclic behavior of silt governed by a change in clay content, three different mixtures were achieved by mixing of nonplastic silt with 0%, 10%, and 20% of commercially available clay. These tests were conducted to simulate field conditions prior to earthquake with initial static shear stresses corresponding to slopes and those with no initial static shear stresses of level grounds. The gradual loss of mobilized undrained cyclic shear resistance after failure and pore water buildup in relation to a number of cycles was observed. The undrained response of the soil to cyclic shear stress loading with the constant amplitude revealed the significant effect of the initial static shear stress on the excess pore water pressure generation and post-failure shear resistance. Test results showed that an increase in the initial static shear stress at the given initial effective normal stress is associated with an increase of mobilized shear resistance at its peak state; however, the actual resistance to liquefaction diminished for both nonplastic and low-plasticity silts. During both cyclic and post-cyclic stages of loading, distinctly different types of shear deformation were identified. In order to evaluate mobility of landslides, a modified conventional brittleness index for seismic loading, \(I_{{\text{BS}}} = {{\left( {S_{{\text{peak}}} - S_{\text{u}} } \right)} \mathord{\left/ {\vphantom {{\left( {S_{{\text{peak}}} - S_{\text{u}} } \right)} {\left( {S_{{\text{peak}}} - S_{{\text{initial}}} } \right)}}} \right. \kern-\nulldelimiterspace} {\left( {S_{{\text{peak}}} - S_{{\text{initial}}} } \right)}}\), was proposed and used to characterize unlimited deformation of silts.