MPM analysis of wetting-induced failure in collapsible loess slopes

Abstract

In-situ loess is typically characterised by a meta-stable, highly porous structure that remains stable under unsaturated conditions. Upon wetting, loess undergoes structural degradation, significant volumetric contraction (i.e., wetting collapse) and even failure, accompanied by positive pore-water pressure and strain-softening behaviour. These features have rarely been incorporated into previous numerical studies of loess slopes, especially in post-failure simulations such as runout analysis. This study developed an advanced material point method (MPM) framework to analyse the pre- and post-failure behaviour of unsaturated loess slopes. This framework incorporates established equations to model comprehensive hydro-mechanical coupling, specifically: (i) the moisture effects on yield stress and strength for simulating wetting-induced collapse and instability; and (ii) the influence of soil density on the hydraulic properties for simulating the impact of wetting collapse on water flow. Moreover, strain-softening during undrained shearing is modelled by considering contractive behaviour. The model was validated using laboratory element tests and physical model tests. Then, it was applied to back-analyse the 2015 Jiaojiayan South landslide in the Heifangtai terrace. The computed results closely match field observations. Results show that irrigation-induced groundwater rise can trigger wetting collapse, generating positive excess pore-water pressure and reducing Bishop's stress, a primary factor in loess slope failure. The landslide was divided into three stages, transitioning from block-like failure to flow-like failure. Parametric studies show that the slope remains relatively stable when strain-softening is neglected. Neglecting wetting-induced yielding leads to an underestimation of the mobilised volume by 25% and the failure depth by 37%.