The Bellecombes Rock Glacier Case Study, 2 Alpes, France

Abstract

Permafrost is soil or rock that is frozen for at least two successive years. Permafrost is composed by an active layer that thaws during the summer and freezes again during the winter. Temperature changes (2 °C increase of minimum temperatures during the 20th century in the Alps) imply changes in mechanical properties of permafrost that reach a disequilibrium point, enhancing gravitational hazards. As these property changes cannot be detected from outside, the risk assessment is not straightforward. In order to detect and anticipate this increasing hazard, geophysical approach was performed to investigate the permafrost body of the Bellecombes rock glacier. Rock glacier is a kind of permafrost composed of rock debris and ice. The Bellecombes site is located in the French Alps, at the “2 Alpes” winter ski resort. The rock glacier affects a ski lift. This work aims at finding methodology to determine (i) the thickness of the permafrost and the bedrock depth which is important to resolve geotechnical problems; and (ii) the thickness of the active layer which is highly instable. In this paper, investigations were composed of borehole, electrical and seismic tomography, ambient noise measurements and georadar measurements. Results show that geophysical investigations improve knowledge of internal structure of rock glacier when they are combined with preliminary geomorphological study and boreholes.