Co-supervisors: Prof. Michel Jaboyedoff, Prof. François Marillier
In Switzerland, landslides cover about 7% of the national territory. Affected areas are represented by the Alpes, the Prealps between the lakes of Geneva and Constance and a part of the Jura mountains. In order to undertake effective measures against instabilities, all types of hazards must be considered.
The study area is located in the “Glaive” forest above Ollon, southwest of Switzerland. It concerns the problematic of a large-scale landslide in gypsum (600 x 400 meters, average of 45°) combined with several mud flows originated within the landslide area. The last event happened in 1998 and stopped just above a road, 400 meters away from the starting zone. The soil nature is suitable to mud flows due to several forest fires between 1960 and 1997.
Two scales are considered in this study:
- A large scale study that provides an overall view of the area activity and more precisely on the SSW mountainside landslide
- A smaller scale that focuses on mud flow processes that affect the SSW mountainside
First, a cartography of all phenomena based on geomorphological observations in the Glaive forest provide an overview. Then, spontaneous polarisation, electromagnetic (very low frequency) and seismic refraction-reflexion surveys were carried out to test the existence of failure surface. Seismic survey is useful to determine the failure surface depth and lithologies using P wave’s analysis. The results were compared to a numeric method based on the sloping local base level (SLBL).
The second part of this work is focused on mud flow processes and the predisposition of the south west watershed. It is based on field measures and observations, vegetation analyses and samples collected in the mobilizable material. Samples were analysed in laboratory to determine several mineralogical and geotechnical parameters that characterized the starting zone of the mud flows.
The analysis of the seismic data provides a four layers model. The first layer is very thin and shows a velocity varying from 400 to 1000 m/s. The second layer (25 m deep) shows velocities of 1500 m/s, its base corresponds to a probable failure surface. The third layer (40 m deep) shows velocities of 3000 m/s and the fourth exhibits velocities of 4000 to 4600 m/s that correspond to anhydrite. The interface between the third and fourth layer is interpreted as the maximum depth of a deep failure surface.
Processing of the reflexion data encountered many problems because of the low data qualities. However, after some simplifications, two reflectors that may cincide with failure surfaces were revealed. But the depth ot these surfaces are uncertain. Globally, the seismic results are comparable to the SLBL method and show that the south western mountainside could be affected by several landslides.
Vegetation analyses show that the distribution of the vegetation on the landslide is driven by complex interactions between topography, soil and fire dynamics. Species growing on the mud flow starting zone is represented by the molinia caerulea that shows the highest degree of xericity. This species is not affected by high concentrations of carbonates and sulphates, but it is a bad slope stabiliser. In addition, the soil characteristic favours erosion processes: low compacity, low clay content, low cohesion, powdery texture, low Atterberg indexes and low root densities in the deeper horizons.
Given the high erodibility of the soil, flow events are expected be regular with low intensities, but the vegetation cover plays a significant role, too. During rainfalls, it is possible that the deep soil horizons become liquid before the upper horizons. This phenomena combined with a water level elevation reduces the effective stress and causes ruptures. Grain size distribution indicates that the rheology of the mud flow would be close to a viscoplastic fluid.
Local climatic conditions induce a creation of the mobilisable material evaluated at about 2 mm/year, which is large.