Influence of weak layers on seismic stability of submarine slopes

Abstract

Instability of submarine slopes and the risk posed by submarine slides are a concern in offshore activities related to energy, communications, food production and tourism. In particular, much of the offshore infrastructure needed for the extraction and transportation of hydrocarbons in deep waters are located on the continental slope, with significant exposure to the risk posed by a submarine landslide. Recent studies of the triggering mechanisms of a submarine landslide show that the combination of a predisposition factor like a weak layer within the soil profile, together with a “short duration trigger” such as an earthquake, is one of the most likely scenarios for slide initiation. In this paper, the influence of a weak layer on the stability of a submarine slope before, during and after a strong earthquake event is examined. The slope under study is located in the southern part of the Gulf of Mexico, on the continental slope in a water depth of 500 m.The slope stability assessments in the study include static slope stability using the limit equilibrium method and the finite element method, pseudo-static analyses using the limit equilibrium method, as well as 1-D and 2-D non-linear dynamic analyses using the finite element method. The earthquake-induced shear strains within the soil deposit are a key parameter in the slope stability assessment. Therefore, the dynamic analyses focused on estimating the maximum and permanent shear strains within the soil profile.Given the complexity of the problem, the decrease of the soil strength during and after the earthquake loading was estimated through a decoupled approach based on the calculated shear strains and results of advanced laboratory tests.The results of the study show quantitatively the important role of a weak layer in the initiation of a submarine landslide under a strong earthquake.