Martin Franz: La falaise des Bornes du Diable : de l’Instabilité au Tsunami.

Martin Franz
Co-supervisors: Prof. Michel Jaboyedoff, Prof. Yury Podladchikov, MSc. Clément Michoud
Expert: Dr. Marc-Henri Derron

The case study, “les Bornes du Diables” cliff, 160 meters high, is located directly above the Mauvoisin dam’s reservoir in south western Switzerland. This study is conducted due to recent rock falls that reached the lake and damaged an alpine road and due to the clear presence of back cracks. The important rock fracture, the thin schistosity and the highly weathered surface contribute to a Geological Strength Index betweeb 35 and 45. The entire case study is located in the Penninic domain. The “marbres phyllteux roux” (foliated marbles) and the base complex that compose the cliff belong to the Tsaté nappe. The albitic gneiss is part of the Mt.Fort nappe. The boundary between both these nappes is the base of the cliff itself.

In order to assess the consequences of a potential collapse of the instability for the road, the dam and the reservoir of Mauvoisin, the aim of this study is to characterize the structural settings, to quantify the movements occurring in the area as well as to compute the propagation of a possible landslide generated impulse wave.

The structural analysis is performed using both field observations and TLS point clouds processed with Coltop3D. Seven discontinuity sets are identified. A kinematic analysis results in the identification of three failure mechanisms they are wedge and planar sliding as well as toppling.

Three discontinuity sets match the observed back cracks. They divide the cliff into five compartments basally delimited by the contact between the Mt Fort nappe and the Tsaté nappe. These blocks volumes range from 2’000 m3 to 500’000 m3. The mane mechanisms controlling the blocks are planar sliding, toppling and a composite mechanism regarded as a step-like sliding (pseudo-circular failure).

In 2009 and 2010, DGNSS measurements were performed on seven determined points in order to characterize recent movements. Five of these measurements are “RTK fixed” it allows a 2 mm horizontal and a 4 mm vertical precision. The suitable calculated displacements measure 3 to 7 cm.

To determine the longer trend displacements, simultaneously orthorectified aerials measurements and stereophotogrammetry are respectively applied to 1979 and 2000 aerials and 1961 and 2009 aerials pairs. ERDAS IMAGINE® software and more specifically the LPS and StereoAnalyst® components were used. The resulting displacements measured are of around 1 cm per year.

Using both movement quantification and structural analysis allows explaining the complete potential collapse mechanism through a 3D static model. It is supported by a model performed with the UDEC® software. The model supports the failure mechanism and the displacements measured.

The potential collapse could generate an impulse wave in the dam’s reservoir that could lead to catastrophic consequences. The wave’s characteristics i.e. its height, run-up and over topping are calculated using the VAW model (ETH Zurich). In addition the wave propagation is calculated using a 2.5D model developed on MATLAB®, which is based on the shallow water equation. The results show the propagation and associated consequences of such a wave. Those latter are characterized by a 3 m high overtopping, in the condition of the lake at its maximum level.