Geotechnical and Geophysical Characterization of Frozen Granular Material

Degrading or thawing permafrost has been identified as being an issue of national importance with respect to its potential for causing severe damage or even loss of life in densely populated Alpine regions due to climatic change (Gruber, 2004, 2007). On this basis, a joint study has been initiated by the Safe Mountain Foundation (Fondazione Montagna Sicura, Cormayeur, Italy) and some Universities to investigate variations in the geotechnical behaviour of Alpine permafrost caused by changes in temperature. Permafrost is found in Alpine regions at elevations higher than about 2500 m above sea level, depending on the location and exposure of the slope. Its mechanical features are determined by the coupled contribution of the soil particles and of the ice. The water content can have an important influence on the resistance of the frozen soil: if the water content is low, the ice will not be able to include the soil particles, however, if it is too high the resistance will be determined above all by the ice itself (Andresland, 1987). Temperature is another driving factor that can influence the mechanical behaviour of ice (Fish, 1997). On this basis this work has been dedicated to analysing the geotechnical and geophysical behaviour of the widespread morainic deposits found in Alpine areas, subjected to different climatic conditions when the temperature increases. The work is dedicated, in particular, to the geotechnical and geophysical characterization of two different glacial deposits present in the Aosta Valley region: the first one generated by Tsanteleina glacier (Rhêmes Valley, 2690 m a.s.l.) and the second one surrounding the Blu lake (Ayas Valley, 2214 m a.s.l.). The characterization has been carried out at different scales: at a laboratory scale, on the thinner components of the granular material and at an in situ scale, through geotechnical measurement and geophysical testing. The geotechnical measurement were performed through photographical dimension analysis of the deposit and geophysical measurement by means of seismic and electrical surveys. Specimens have been reconstructed, saturated, and frozen in the laboratory and subjected to laboratory tests to evaluate whether both the mechanical (mono-axial compressive strength and Young modulus) and geophysical (P and S wave velocities) behaviour of this kind of soils are affected by temperature variations under 0°C (-35/-5 °C). This work has been realized and funded in the framework of the project GlaRiskAlp (2007–2013 Operational programme for cross-border cooperation Italy – France, Alps – ALCOTRA)