The importance of dilation strength in determining the dynamics of dilative slope failure

Breaching is a retrogressive slope failure process that is associated with the dilation of deposits. The dilation drops the pore pressure and increases the strength of the deposit, creating near vertical failure angles and slow erosion rates. We find the erosion rate of a retrogressive slope failure is in equilibrium with the rate of pore pressure dissipation during breaching. This equilibrium creates a stage of approximately steady state erosion rate, as well as a constant pore pressure when observed in a reference frame that moves with the failure surface. This steady state pore pressure is a function of the dilation strength. Smaller dilation strength for a deposit results in smaller magnitude of pore pressure drop in steady state and a smaller material strength; if this strength is too small it will lead to shear slope failure instead of breaching. Dilation strength can be measured in the lab with a drained shear test. We find dilation strength is a function of the effective stress and initial porosity. Dilation strength decreases with increasing effective stress and/or increasing initial porosity. As a result, breaching is limited by deposit thickness and grain packing. Breaching is most likely to occur in slowly deposited sediments. Deposits that are shaken by waves during slow accumulation are particularly susceptible to breaching failure.