[H-DS05] Landslides and related phenomena

[HDS05-07] Experiments on seepage-triggered cliff landslides using cohesive wet sand

*Ikuro Sumita¹, Fumi Shibuya^1,2 (1.Earth and Planetary Science Course, Division of Geosciences and Civil Engineering, Graduate School of Natural Science and Technology, Kanazawa University, 2.National Institute of Advanced Industrial Science and Technology)

Keywords:Granular matter, Capillary cohesion, Arching, Total stress, Permeable flow, Stability analysis

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_w 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. Such measurement of σ has been limited in previous works. The experiments show that for a fixed H (≃ 20 cm), a slide is triggered when the 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_w rises, 2 slides are triggered that progress downslope, and for the highest 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_w and increases with the H, as modeled by a permeable flow with a permeability that decreases with the σ′. The σ_z (vertical) is initially uneven and deviates from the lithostatic value by arching. For tall cliffs, the 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 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_w and the slide becoming deep seated with H. However, it overestimates the factor of safety F_s because it does not include the capillary rise and the fall in σ_z. The stability analysis also assumed a circular slide that occurs once. Our experiments which show that 2 slides and a translational movement can occur at high h_w caution such assumption.

Reference
https://progearthplanetsci.springeropen.com/articles/10.1186/s40645-022-00501-7