IMPACT OF PERMEABILITY AND COHESION IN SUBMARINE SLOPE FAILURE TYPE

Submarine slope failures pose hazards to coastal communities through their ability to damage seafloor infrastructure and generate tsunamis. Sediment properties and overpressure play vital roles in determining such slope failure characteristics, yet are poorly understood. Furthermore, models of submarine slide-induced tsunami make several assumptions about failure evolution (e.g., failure type, velocity) directly related to sediment composition (e.g., cohesion and permeability).

To better understand failure dynamics, we explored the impacts of permeability and cohesion on slope failures induced via overpressure in physical experiments. We developed a 2D cross section representative of a marine delta prograding over a basin-floor fan. We investigated failures for different mixtures of fine-grained quartz sand homogeneously mixed with smectite or clay-sized quartz. Thus, we isolated the effects of cohesion and permeability on slope stability and failure evolution in the presence of overpressure creating flow through the delta.

Our experiments provide first-order observations on failure dynamics. In high-permeability, sand-rich systems, overpressure required to induce failure increased with smectite content from 0-5 wt% . At smectite concentrations of ≥25%, overpressure required to induce slope failure remained relatively constant. Two separate regimes of failure evolutions were also identified. In sand-rich systems, a single failure event occurred even with increasing overpressure. With smectite concentrations of ≥25%, a series of slope failures occurred with progressive increases in overpressure. Comparison of sand mixed with clay-sized quartz and sand mixed with smectite revealed that lower permeability systems have the potential for larger failures and a necessity for cohesion in producing brittle failure features (e.g., fractures, rafted blocks). While producing multiple types of failure and winnowing of fines due to fluid flow, our experiments did not mobilize intact blocks downslope. Thus, our experiments provide new insights on slope failure as a function of clay and sand content, stressing the importance of low permeability and cohesion for large failure blocks, but we also note that additional forces are needed to mobilize large blocks important for tsunami genesis.