Modelling the spatio-temporal evolution of a rainfall-induced retrogressive landslide in an unsaturated slope

Numerical modelling, particularly fully-coupled hydro-mechanical large-deformation models, greatly helps in properly simulating the complex failure and post-failure mechanisms of rainfall-induced landslides. The affected soils, in fact, evolve from none or small deformation rates to large deformation rates during the initiation stage and vice-versa during deposition, with relevant interactions between the solid skeleton and interstitial water. The Material Point Method (MPM) has the potential to reproduce entirely those complex processes. However, a comparison with standard tools (e.g. FEM: Finite Element Method, LEM: Limit Equilibrium Method) may guide in the optimal choice (or in the combined use) of the various modelling approaches. A framework is here proposed based on a multi-tool approach consisting in the combination of: a) no-deformation LEM, b) small-deformation FEM, c) large-deformation MPM. The LEM slope stability analyses are performed for a realistic assessment of the major slip surface(s) and to back-analyse uncertain slope parameters. The FEM stress-strain analyses assess the progressive failure, the onset of initial velocity and the later acceleration of the landslide body, until large deformations occur in the slope and numerical convergence of FEM is lost. The MPM analyses are used to reproduce the whole landslide process, from the initiation to propagation and final deposition. Such an integrated framework is tested for an international landslide benchmark (the 1995 Fei Tsui Road landslide in Hong Kong). The results achieved through the different approaches are discussed in relation to the wide scientific literature available for the general topics and the specific case study. The paper highlights that the fully-coupled hydro-mechanical large-deformation model properly reproduces the complex failure and post-failure mechanisms of rainfall-induced landslides. However, no-deformation LEM analyses and small-deformation FEM analyses allow a reasonable understanding of both the pre-failure stage and the failure mechanism. These more traditional tools are confirmed as indispensable tools in the engineering practice and research.