Instability of Silty Sands Under Partially Drained Conditions

Instability in the soil element is its inability to sustain the alterations to the current stress-state. The instability line (IL) or critical stress ratio line (CSR) is the maximum stress-state achieved under undrained conditions, and the drained failure envelope (DFE) is the line joining the origin and failure point under drained conditions. The present experimental study involves the effect of effective stress ratio (ESR) on the instability behavior of silty sands under partially drained conditions. Isotropically consolidated compression (CIUC) triaxial tests were conducted on the soil specimens prepared at in-situ dry density and moisture content. Four predefined ESR (T1, T2, T3, T4) were selected such that two of them could depict the stress-state corresponding to a region below IL (T1 and T2) and two of them could depict the region between IL and DFE (T3 and T4). ESR of instability line (η<inf>IS</inf>) for the silty sand soil specimens was found to be 0.64. During shearing, the partially drained conditions were imposed on the stress-state corresponding to the chosen ESR. It was observed that the maximum effective stress-state in T1 and T2 was achieved after the imposition of partially drained conditions followed by static liquefaction, whereas T3 and T4 showed runaway instability, i.e., the deviatoric stress rapidly reduced indicating the steep fall in stress–strain and q−p’ curves. Soil specimens with stress-states corresponding to regions of potential instability showed runaway instability under partially drained conditions, which were otherwise stable under completely undrained conditions.