LIMITING EQUILIBRIUM AND LIQUEFACTION POTENTIAL IN INFINITE SUBMARINE SLOPES

Stability evaluation of submarine slopes is hampered by the difficulty of making field measurements. Owing to the scarcity of detailed field data, stability is commonly assessed by assuming homogenous infinite slopes with steady seepage. For these conditions, it is necessary to measure only the slope angle, friction angle, cohesion, and pore pressure at some distance into the sediment to evaluate stability. Depth of submergence is irrelevant since it is the gradient of pore pressure in excess of hydrostatic that affects stability. The boundary conditions at the slope surface differ for submarine and subaerial slopes, and lack of appreciation of this difference has resulted in some confusion in the marine slope-stability literature. Submarine slopes have a constant head rather than constant water pressure along the slope surface. For a condition of steady seepage in infinite submarine slopes, constant head is also required on all subsurface planes, including potential slip planes, that parallel the slope surface. As a consequence, submarine slopes with steady seepage always undergo Coulomb failure prior to liquefaction. However, if slope angles are low, submarine slopes are close to a liquefied state when failure occurs. Examination of available data shows that conditions close to those required for liquefaction are necessary for Coulomb failure in many continental shelf areas. This favors long landslide runouts and flow of sediment subsequent to failure.