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 h_mathrm{w} located at the rear of a cliff (height H) and vary these parameters. Importantly, we measure both the total stress sigma and pore water pressure u to obtain the effective stress sigma ' = sigma - u. The experiments show that for a fixed H (simeq 20 cm), a slide is triggered when the h_mathrm{w} 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 h_mathrm{w} rises, 2 slides are triggered that progress downslope, and for the highest h_mathrm{w}, 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 h_mathrm{w} and increases with the H, as modeled by a permeable flow with a permeability that decreases with the sigma '. The sigma _mathrm{z} (vertical) is initially uneven and deviates from the lithostatic value by arching. For tall cliffs, the sigma _mathrm{z} 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 sigma 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 h_mathrm{w} and the slide becoming deep seated with H. However, it overestimates the factor of safety F_mathrm{s} because it does not include the capillary rise and the fall in sigma _mathrm{z}.