Characterization and simulation of permafrost rock slope failures in mica-rich rocks - An integrated laboratory study of temperature-resistivity measurements and direct shear tests

Degrading permafrost is an increasingly important problem in high altitude alpine mountains and Arctic regions around the world because it exerts a primary control on rock falls and landslides in these areas. The present study focuses on the effect of ice in permafrost conditions on i) the electrical resistivity of rocks; and ii) the shear strength of ice-filled joints. Freezing gradients in metamorphic rocks have previously been described by linear temperature-resistivity (T-p) paths. Electric resistivity measurements of low porosity mica-rich rocks from the Mannen active landslide and the Ramnanosi active landslide in southwestern Norway have been used to verify the linear relationship. The two samples were drilled and fitted with electrodes in linear Wenner arrays. The experiment were conducted with a resistivity meter automatically measuring every 15 minutes. The samples were cooled at temperature steps of 2 °C, -2 °C, -8 °C, and subsequently thawed by turning the cooling machine off. The measured T-p paths can be described by a bilinear relationship separated at 0 °C. The results showed one order of magnitude higher frozen gradients compared to unfrozen gradients, and one order of magnitude difference in resistivity values of the two samples. The results emphasize the importance of calibrated T-p measurements when characterizing permafrost as frozen conditions resistivity values in one sample represents unfrozen conditions in the other. Rock type dependence on the failure criterion of permafrost rock joints has not yet been studied. The present study focuses on mica-rich rocks with well developed foliation sampled from the Ramnanosi active landslide in southwestern Norway, the Nordnesfjellet active landslide in northern Norway and the Matterhorn mountain rock falls in the Swiss Alps. In these three areas, the cohesion of the rock volume is partly controlled by the presence of ice in mica-rich rock joints. The samples have been sheared to test the potential rock type dependent shear strength. The samples sliding surface were ground to ensure reproducibility of the initial roughness. A direct shear machine, developed at the Technical University of Munich, was used to conduct 36 tests on mica-rich rock-ice-mica-rich rock sandwich samples. A constant mean strain rate of 9.16 ± 5.9 × 10^-4 s^-1 was applied, provoking fracturing while a constant normal stress equivalent to 4 or 15 meters overburden was maintained. The temperature was constant and controlled at -10 °C, -6 °C and -2 °C. Results showed a 45\% and 39\% reduction in the shear strength for a temperature rise from -10 °C to -2 °C for 100 and 400 kPa normal stress, respectively. During the same temperature range, cohesion degrades by 47\%. The static friction coefficient were found independent of temperature. The results were compared to Wetterstein limestones, and a possible porosity dependent static friction is proposed. The mechanism is explained by an increase in rock-ice contact surface with increased porosity. A reduced dynamic friction coefficient with increased normal load were also found. The dynamic friction is suggested to decrease with increasing normal load because of frictional heating. The integration of rocks electric properties and shear properties may improve estimations of rock slope failures in the degrading permafrost rock slopes in Arctic and alpine mountain areas, where the effects of global warming are known to be large