COLLAPSE OF SATURATED SOIL DUE TO REDUCTION IN CONFINEMENT

The potential for collapse due to a reduction in confinement is evaluated for two very different soils: a uniformly graded sand and an undisturbed clayey colluvial soil. Soil specimens were consolidated anisotropically and subjected to constant-shear-drained (CSD) tests. During the CSD lest, the effective confining pressure is gradually reduced while the shear stress is held constant. Only for very loose specimens is collapse observed before the failure envelope or steady state is reached. In general, specimens subjected to the CSD stress path behave differently than specimens of similar initial density and confining pressure subjected to typical compression stress paths. This observation indicates that knowledge of the stress path is necessary to accurately predict the collapse potential, and thereby the potential for flow failure, of loose saturated soils.