Liquefaction flow of submarine slopes under partially undrained conditions: an effective stress approach

Abstract

The stability of submarine slopes is commonly assessed based on undrained soil response. Liquefaction flow may occur when pore-pressure rise in soil elements under constant shear stress causes a reduction in effective stresses. Recent experimental studies have shown that a small net flow of water into an element results in additional pore-pressure generation and further reduces its strength. As a result, soil elements can liquefy due to expansive volumetric strains that cannot be predicted by undrained analysis. Submarine deposits of the Fraser River delta are known to contain high concentrations of methane gas. Tidal variations can cause unequal pore-pressure generation with depth and time in these soils. Such changes reduce the effective stresses during low tides and may induce liquefaction flow of slopes due to partial drainage conditions. An effective stress approach based on an elastic–plastic stress–strain relationship is presented to model liquefaction flow of sand. A fully coupled analysis is carried out to evaluate the behaviour of both saturated and gassy deposits of loose Fraser River sand and Ottawa sand. Triggering of liquefaction resulting in retrogressive flow slides is predicted for an unsaturated underwater slope similar to those observed near Sand Heads at the front of the Fraser delta.Key words: sand liquefaction, Fraser River delta, submarine slopes, flow slide, gassy sand, partial drainage.