Experiments on seepage-triggered cliff landslides using cohesive wet sand

Abstract

Unsaturated wet sand possesses capillary cohesion that is lost when it becomes saturated. Thus, it can form a cliff, but a slide may be triggered upon saturation. Here we conduct cliff landslide experiments using cohesive wet sand where the groundwater seeps in from the hydraulic head hw located at the rear of a cliff (height H) and vary these parameters. Importantly, we measure both the total stress σ and pore water pressure u to obtain the effective stress σ′= σ- u. The experiments show that for a fixed H (≃ 20 cm), a slide is triggered when the hw exceeds a critical level. The slide occurs nearly simultaneous or after the groundwater seeps out from the cliff toe and the vertical velocity increases approximately exponentially during the slide. As hw rises, 2 slides are triggered that progress downslope, and for the highest hw, the whole cliff is pushed forward after the first slide. On the other hand, when the H is high, the slide becomes deep seated. The time needed for the water to seep out from the cliff toe decreases with the hw and increases with the H, as modeled by a permeable flow with a permeability that decreases with the σ′. The σz (vertical) is initially uneven and deviates from the lithostatic value by arching. For tall cliffs, the σz near the cliff toe falls precipitously soon after the seepage starts prior to the rise in u, indicating that a stress redistribution occurred as the wet sand loses cohesion and slip plane develops. This also indicates the efficacy of σ measurement because the changes are detected before the groundwater arrives. A stability analysis that models the drop in cohesion and a rise in u explains the cliff becoming unstable with hw and the slide becoming deep seated with H. However, it overestimates the factor of safety Fs because it does not include the capillary rise and the fall in σz. [Figure not available: see fulltext.]