Retrogressive shear band propagation and spreading failure criteria for submarine landslides

The paper proposes a mechanism of submarine spreading failures, which combines retrogressive shear band propagation (SBP) with a series of active block failures in the ‘stable’ zone of the slope, where the gravitational shear stress is smaller than the residual shear strength. Gradual downhill removal of the failed material causes a decrease in the supporting earth pressure, leading to the progressive uphill propagation of the shear band (termed here ‘retrogressive’ SBP) and uniquely determining its length, which can be linked to the dimensions of the spreading failure block mechanisms. Two conjugate energy balance based criteria have been formulated for the spreading failure, in terms of either the critical depth of the sliding surface or the critical drop of the failed seabed level. Both criteria have been validated against the observed phenomena (presence and absence of the evidence of spreading failure for different landslide depths and a step in the sliding surface) in palaeo-landslide examples from the Caspian Sea. The proposed framework provides closed-form criteria, which could be incorporated into a geographical information system based deterministic and probabilistic stability analysis framework for assessment of spreading failure risks in submerged slopes across offshore developments.