Influence of combined incision and fluid overpressure on slope stability: Experimental modelling and natural applications

Onshore slides are driven by gravitational forces that are either related to a basal or a surface slope. Resisting these driving forces are the friction at the base of the slide, and the strength to compressional failure at the downslope edge of the slide. Two distinct processes can reduce these resisting forces and thereby promote slides. On one hand, fluid overpressure at the base of low-permeability layers decreases the effective stress, shifting the Mohr circle closer to the failure envelope. On the other hand, river incision removes the downslope buttresses. We undertook analogue experiments to investigate the combined influence of both processes on promoting landsliding. We applied air pressure at the base of horizontal or tilted models made of high and low-permeability layers to induce basal overpressure, combined with local incision similar to river incision in nature. We also tested the differences in deformation as a function of whether the incision was continuous throughout the models’ evolution. ormation occurred in the regions not subjected to overpressure. In models subjected to continuous incision, normal faults formed first along the valley flanks, then propagated upslope retrogressively. Where incision was not continuous through time, a downslope buttress progressively formed with the sliding mass comprised of an extensional domain upslope, a long, translated but non-deformed slab at mid-slope, and a shortened domain downslope. In our models, the size of the deformed area increased with incision depth and/or increasing basal slope. These results show that river incision, combined with fluid overpressure is a potential landslide-triggering factor, as suggested by field data gathered at the Waitawhiti landslide complex, North Island, New Zealand.