Denis Fabre

The distribution of permafrost in rock glaciers of the southern Alps (France)

Abstract Forty electrical resistivity surveys were performed on ten rock glaciers in the southern Alps of France in the summer of 1986. Taking a resistivity value exceeding 25,000 ωm as indicative of a frozen layer suggests that seven of these rock glaciers still contain permafrost. Values up to 1,000,000 ωm were encountered. In some cases, the usual resistivity interpretation is inadequate because of the oversteepening effect due to the shape of the frozen bodies. In general, resistivity survey interpretation enables permafrost to be separated from massive glacier ice and gives information on the ice content of the permafrost and the thickness of the layer. The resulting conclusion that these rock glaciers are essentially inherited forms, testifying the passage of recent cold periods, is confirmed by the discovery on three sites of rock glaciers with a heterogeneous structure, a thawed central part and a accumulation of ice at the snout. It suggests that the rock glaciers in the southern Alps were formed, for the most part, during the Subboreal period.

Prospection électrique sur les glaciers rocheux du cirque de Sainte‐Anne (Queyras, Alpes du Sud, France)

The Sainte-Anne cirque in the Queyras area contains several active rock glaciers. The internal structure of the main two (Ste-Anne and Petite-Part) have been studied by electrical means (about 15 resistivity soundings of the Schlumberger type). The apparatus and the methodology have been tested in the Alps since 1986. The shape of the resistivity curves clearly shows the occurrence of ice in the ground. The interpretation of these curves leads us to estimate the ice content and geometry of the investigated layers. In the Sainte-Anne area, the internal structure of the rock glaciers is somewhat complex, with coexistence of rather pure ice from the upper glacierets and ice-cemented sediments of periglacial origin. The tongue of the rock glacier can be iceless (Sainte-Anne). The active terminal lobe always contains a core of poorly ice-cemented sediments. This is responsible for the measured movements (35 cm/a) and the observed ridges. This core, actually melting, is about 20 m thick at Sainte-Anne and 10 m at Petite-Part, under a few metres of ice-free sediments.

Detection of Permafrost and Foundation Related Problems in High Mountain Ski Resorts

The ground ice contained in screes with various rock debris (“rock glacier”) is the most common form of permafrost currently encountered in the Alps between 2500 and 3200 m asl. This altitudinal range is exactly the one which is concerned with the development of major ski resorts. In the manner of glaciers, rock glaciers will move on the slopes as a result of creep of the ice. However, because of the presence of rocky debris, the internal friction slows the movement. Confirming the presence of ice in the ground can be based at first on ground temperature measurements. But regarding the distribution with depth, in the absence of soundings technically and economically difficult to achieve, the geophysical methods provide good results. In the current context of climatic change, the problem of foundations of structures is increasingly taken into account. The observed warming in the Alps causes a slow melting of permafrost; settlements and collapses are observed. Geotechnical studies to illustrate this behavior are of the two following types: (i) feasibility study upstream projects, in order to define areas of permafrost and to avoid them for optimizing the implementation of the works; (ii) diagnostic studies giving solutions for reinforcement works after disorders due to the evolution of permafrost. Two specific examples are given: (i) the choice to draw a line for a chairlift, and (ii) the reinforcement of a ski-lift station with differential settlements.