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