Submarine slope failure in sensitive clay due to buoyancy force of trapped gas

Abstract

Submarine landslides pose significant threats to coastal communities and offshore infrastructures. Free gas is regarded as a key triggering or pre-conditioning factor for slope failure. Slope instability caused by reduction of the shear strength due to the presence of gas has been investigated widely. An alternative trigger is the buoyancy force of trapped gas within the gas structure, as observed in both in-situ investigations and model tests. In this study, typical simplified initiation scenarios are investigated under plane strain conditions through finite element analyses, where the trapped gas within a gas structure is simplified as a pressure applied to the adjacent soils and increased to trigger slope failures in sensitive clay. Punching shear failures in the overlying layer and catastrophic landslides are identified under different soil conditions. The latter is attributed to the strength degradation induced by the tensile failure under buoyancy force. Empirical criteria for these two failure patterns are proposed. The proposed criteria are then validated to be applicable to three-dimensional conditions, providing a conservative assessment. A case study of the landslide offshore the northern Svalbard margin is conducted using the criteria proposed, finding that the safety factor of slope may be reduced significantly due to the buoyancy force within the gas structure.