Effects of Earthquakes on the Mechanics of Active Landslides

Abstract

Landslides are a global threat in mountainous regions, causing thousands of fatalities every year. Less attention is paid to active, slow-moving landslides because they do not usually manifest themselves in catastrophic events, but can cause severe damage to buildings and infrastructures. They are characterised by a mobile state that is highly sensitive to environmental influences. As a result, they are often thought to be particularly susceptible to seismic acceleration, which is even supported by conventional simplified models. Unfortunately, corresponding field observations are very rare, but they tend to show only small co-seismic landslide displacements. The potential consequences of a landslide collapse, for instance on the flank of a water reservoir, underlines the importance of reliable mechanical models and a better understanding of the behaviour. This thesis presents a framework based on the material point method for the large-deformation seismic response analysis and in particular for the simulation of active landslides. This approach is applied to the slow-moving La Sorbella landslide in Italy, where co-seismic displacements were recorded during three moderate earthquakes. The results not only validates the landslide model, but also shows that geometry and rate effects in the shear zone are responsible for such small displacements. Furthermore, the subsequent simulation for strong motions shows that a catastrophic collapse can only be provoked by softening mechanisms. A phenomenon often observed in active landslides is an increased post-seismic activity for days to several months. It has been suggested that excess pore water pressure is generated outside of the shear zone during seismic shaking and later migrates into this zone by seepage, causing an acceleration of the landslide. A numerical model, using the hydro-mechanically coupled finite element method, confirms the plausibility of this hypothesis. A detailed parametric study reveals the underlying mechanisms and identifies the most controlling factors as ground motion intensity, rate-dependency, pre-seismic velocity and consolidation time. While rate effects in clay-dominated soils has been well investigated, less is known about silts and sands with various clay content. Shear zones from alpine landslides are often characterised by this type of material, making their understanding crucial for a reliable risk assessment. Therefore, an improved ring shear apparatus is presented and applied to investigate rate, pore water pressure and temperature effects in the shear zone from two alpine landslides. For both a moderate rate-hardening for slow to rapid shearing was observed. Together with extensive field investigations, this leads to a comprehensive case study of the slow-moving Marsc landslide at the Luzzone reservoir in Switzerland. The whole process of formulating an appropriate geotechnical landslide model and assessing its behaviour under earthquake loading is presented. This provides a general approach that can be applied to other active landslides.