Effect of Groundwater on Landslide Triggering

Landslides belong to the most important natural hazards in all mountainous regions. It is wellknown that water is one of the major triggers of landslides. Numerous landslide studies discuss different effects that water may have on slope stability: decreasing suction, rising groundwater table and subsequent increasing pore water pressure, groundwater exfiltration from the bedrock, seepage erosion, hydraulic uplift pressure from below the landslide, and influence of water on the plasticity of the landslide. Geological and hydrogeological characteristics such as permeability and saturation of the landslide and the substratum determine to a large extent, which of those processes are dominant in causing triggering. The first objective of this study is to investigate the origin, flow paths and effect of groundwater in three different slopes and landslides in Switzerland. These are 1) an artificially triggered shallow landslide in Rüdlingen located in the distal Molasse in the North of Switzerland; 2) the active Pont Bourquin landslide located in a tectonically complex zone in the Western Swiss Prealps; and 3) the Rufiberg, a slope prone to shallow landslides located in the Subalpine Molasse in Central Switzerland. Hydrochemical analyses, groundwater monitoring in observation wells and geophysical profiling are found to be promising methods to trace the origin and flow paths of groundwater in these three slopes and landslides. Based on hydrogeological, hydrochemical and geophysical data, three conceptual hydrogeological models are constructed which show the different effects groundwater may have on landslide triggering: Large joints in the bedrock are draining the loose soil cover in Rüdlingen, which has a stabilizing effect on the slope. At Pont Bourquin, groundwater flow through the fractured bedrock and the formation of perched groundwater in the heterogeneous highly-plastic landslide mass is found to be crucial for the triggering. And at Rufiberg, artesian or uplift pressure that builds up in the bedrock below the potential landslide may influence the triggering. These different case studies show how diverse and complex the hydrogeological pattern in landslides may be. So far, no classification system exists that is based on the dominant hydrogeological characteristics of landslides and slopes prone to landslides. Therefore, the second objective of this study is to develop a classification for the hydrogeological categorisation of landslides and slopes prone to landslides. This classification is based on the parameter "permeability contrast" between the landslide and the substratum (hydrogeological predisposition) and the time dependent parameter "saturation" of the different geological layers. The classification provides a tool to describe the hydrogeology of landslides and to evaluate the potential hydrogeological triggering mechanisms. It can be applied to illustrate the evolution of the saturation of the different geological layers during rainfall and snowmelt. This classification may be applied to estimate, whether a slope is in a critical equilibrium stage depending on the saturation. In the present study, the classification is applied to the three landslides and slopes investigated (Pont Bourquin, Rüdlingen and Rufiberg) and on seven other well-known landslides in the Alps. The classification supports the understanding of complex hydrogeological processes occurring in landslides. It is a framework for the construction of conceptual hydrogeological models of landslides and can be applied as a tool for planning further investigations, numerical modelling and mitigation measures.