Pore pressure rebound and long-term instability in retrogressive submarine landslides
Presented by Prof. Derek SAWYER on 23 Sep 2013 from 17:15 to 19:15
Type: Poster presentation
Session: Poster session
Track: Poster presentations
Many large submarine landslide complexes on passive margins are retrogressive in which the head scarp back-steps over time. What controls the timescale of retrogression and are present-day slide complexes along the world’s continental margins stable or progressively weakening? Retrogressive failure is a coupled process. When an initial slope failure occurs, sediments behind the headwall experience short-term strengthening because unloading decreases the lateral stress and the pore pressure. However, over time, fluid flows toward the unloaded scarp resulting in a progressive increase in pore pressure (decrease in effective stress) and weakening of the slope. Specifically, once an initial scarp forms, the lateral stress is reduced in sediments near the scarp face. The reduction in lateral stress increases shear stress. However, since fluids in headwall sediments are also no longer laterally confined, fluid pressures are also reduced. At this point, the in situ shear stress is less than the failure strength and the system is stable. However, over time, lateral flow occurs toward the scarp and pore‐pressure begins to rise back towards its original values. On a p‐q (mean stress‐shear stress) plot, the stress path moves horizontally. As it does, sediments near the headwall further weaken until failure again occurs. This long-term pore pressure rebound may play a key role in driving cyclic, time-dependent retrogression in submarine landslide complexes. If the undrained unloading does not weaken the sediment below the gravitational driving stress, the subsequent pore pressure rise could progressively weaken the slope sediments until it does.