Effects of initial density on soil instability at high pressures

A high-pressure triaxial compression and extension testing apparatus was utilized to investigate the effect of initial relative density on soil instability. Stress paths from a series of undrained tests, at confining pressures from 8 to 60 MPa, were evaluated to determine the location of the potential instability region. Experiments were performed on specimens with relative densities of 30% and 60%, and compared with previously published results for 90%. The results indicate that initial relative density has a small effect on the slope of the instability line, with the slope of the line decreasing as initial density is reduced. In addition, the conditions under which a specimen exhibits unstable behavior were confirmed by performing series of specialized instability tests. For tests with undrained conditions imposed inside the region of potential instability, instability was observed immediately. For tests with undrained conditions imposed outside, but relatively close to this region, creep effects would cause pore pressures to increase resulting eventually in instability. In general, it was found that initiation of unstable behavior depends on the relationship between the shape of the yield surface and the effective stress path followed.