The role of temperature in alpine slope instabilities: numerical modelling of the historical Cimaganda Rockslide

This work explores the effect of temperature changes on the stress-strain evolution of alpine rock-slopes using thermo-mechanical (TM) numerical modelling. The role of TM stresses in the occurrence of plastic failures and fracture propagation within the rock-mass is analyzed focusing on the predisposition to large-slope instability events. The modelled slope is located on the east flank of the San Giacomo Valley (Central Italian Alps), between the village of Chiavenna and the Splügen Pass, connecting Italy to Switzerland. Along the slope, a massive rockslide event (the Cimaganda rockslide) occurred around 900 A.D. mobilizing an estimated volume of rock material of 7.5 Mm3, reaching the bottom of the valley. Interest in this historic event was raised again in recent times, as a rockslide event took place in 2012 on its active right flank, mobilizing 20.000 m3 of rock material and blocking the SS36 National Road. Geological, geomorphological and geomechanical surveys were performed on the study area, allowing to recognize features of deep-seated gravitational slope deformations and large-scale stress release. Accurate characterization involving field surveys and laboratory testing was carried out, leading to the development of a geomechanical conceptual model of the slope. Numerical modelling was performed considering the geomorphological history of the valley starting from the Last Glacial Maximum conditions. A TM semi-coupled approach was developed introducing both glacial unloading and temperature changes due to the paleo-temperature redistribution and to the gradual exposure of the slope to atmospheric conditions with seasonal fluctuations. A 2D DEM numerical approach was adopted, supported by a 2D FEM analysis to simulate heat diffusion over the valley cross-section. Results can reasonably simulate the pre-failure rock-slope conditions, giving a contribution in the comprehension of thermo-mechanical processes in rock masses, and pointing out the influence of climatic factors in the stress-strain evolution of alpine slopes.