Effects of bedrock anisotropy on hillslope failure in the Darjeeling-Sikkim Himalaya: an insight from physical and numerical models

Abstract

This study investigates the role of bed-material anisotropy in triggering landslides in metamorphic terrains of the Darjeeling-Sikkim Himalaya. The initial disposition of foliation (planar anisotropy) with respect to the hillslopes is found to be a crucial parameter in controlling the scale of landslides. Hillslopes with foliation dipping into the surface slope are mostly affected by deeper-seated larger-scale landslides, as compared with those occurring on hills with down slope-dipping foliations. To verify our field observations, we performed scaled slope–failure experiments in a tilted sandbox, simulating the foliation anisotropy in analogue models. Sand–mica beds with the anisotropy planes dipping into the surface slope developed shear localisation along deep-penetrating listric zones, leading to slope failure in the form of down-sliding blocks of large dimensions. In contrast, models with anisotropic planes dipping down slope produced failure zones restricted to the shallow level. The narrow failure zones in the latter case had little tendency to grow in depth but propagated up the hillslope direction. Using Drucker–Prager’s failure criterion, we also ran experiments with finite element models to substantiate the contrasting effects of bed anisotropy on hillslope failure processes.