Characterisation of potentially unstable mountain permafrost - A multidisciplinary approach

A multidisciplinary study including geophysical, geotechnical and glaciological techniques has been conducted on an Alpine rock glacier. Lateral extensions were obtained from air photographs, whereas verti- cal extensions of seismic refraction tomography provided critical information on major internal features, as well as bedrock topography. Four boreholes were drilled down to bedrock in a zone of degrading permafrost. Detailed information on the rock glacier stratigraphy was obtained, undisturbed cores were extracted to study mechanical properties and temperature monitoring, deformation measurements and georadar cross-hole tomography were carried out. The combined analysis of these data leads to a unique interpretation. Significant shear deformation occurs at the interface between an ice rich layer and a zone of sandy gravel at 20 m depth. Below the sandy gravel, a conspicuous, 5 m layer of boulders with large voids was found. This prevents accumulation of high pore pressures, which are a prerequisite for rock glacier instability.