Geophysical investigations of a landslide to interpret the distortion of a railway tunnel

Summary The Aspe Valley (Pyrenees Atlantiques, France) is an old glaciated valley of the Wurm period. Steep rock escarpments alternating with slipped less marked slopes are presented along the valley. The Peilhou rock slide is an example of gravitating instability related to glacial erosion. A consequence is the deformation of a railway tunnel, unexploited and instrumented since 1979. Geological and geophysical approaches allow observations to be correlated to disorders affecting masonry, then to interpret sliding mode of the slope. Schist and sandstone basement is fractured, folded and partly covered by moraine. The slide affects primarily a rock material, and mobilized superficial debris. Various zones are distinguished according to their geomorphological, geological and geomechanical behaviours. Use of electric profiles allows the in-depth extension of the surface observations, and complete investigations. These zones testify to the structural heterogeneity of the rock mass, in conformity with deformation observed of the railway tunnel.