Failing Just Right: Unraveling the Seabed Sediment Collapse Phenomenon

The dynamics of seabed granular avalanches underwater is a consequence of granular mixing with ambient fluid. Generally, underwater avalanches are characterized by the transition of granular packing from a solid to liquid phase, by a phenomenon called “slumping”, similar to a melting phase transition. However, under specific conditions, the granular packing can instead transition from a solid to a gas phase, in a process called “breaching.” This granular sublimation phenomenon produces turbidity currents, similar to those observed on the ocean floor. Previous studies have demonstrated the importance of packing fraction and pore pressure on dictating the failure mode of underwater avalanches. However, the effect of particle size on the nature of the failure transition remains elusive. Here, we perform controlled experiments that delineate the transition from slumping to breaching in a two-parameter phase space: grain size and packing fraction. In our experiments, we use silica sand, classified by size, in a custom-built chamber with two-fold capabilities: an in-built fluidization chamber that adjusts the initial packing fraction of the granular material, and a pneumatic valve that mimics a controlled “dam break” releasing the material from the fluidized pressure chamber to ambient water. As grain size decreases and packing fraction increases, we observe a transition from slumping to breaching. We attribute the physical mechanism as a competition between particle and pore pressures; we predict that breaching occurs where pore pressure is greater than particle pressure. Our work provides a general framework to predict the failure of seabed sediments, which can aid in protecting underwater infrastructure.