Abstract

In this work, a thermo-mechanical (TM) numerical approach is presented and applied to investigate the stress-strain evolution of an alpine rock-slope located in the Central Italian Alps (Sondrio Province). Along the "Cimaganda" slope a massive rockslide event occurred around 900 A.D. mobilizing an estimated volume of 7.5 Mm3 of material, and reaching the bottom of the valley. Interest in this historic event was raised again in recent times, as a new rockslide took place in 2012, mobilizing 20.000 m3 of rock material and blocking the SS36 National Road.

To understand the general evolution of the Cimaganda rock slope, the recent geomorphological history of the Valley (post Last Glacial Maximum) was considered. In particular, to explore how glacial loading and unloading, associated with thermo-mechanical processes can promote rock mass damage, a 2D DEM numerical approach was adopted, calibrated upon the collected experimental and field data, and supported by a 2D FEM analysis to simulate transient heat diffusion over the Valley cross-section due to ice retreat and paleo-temperature evolution. Results show a clear relation between TM stresses and the occurrence of rock-mass damage and slip propagation along discontinuities. Simulated displacement and the development of a deep region of shear strain localization, allow to highlight the significance of temperature influence in preparing the rock slope to instability.

Introduction

Large rockslide events and deep-seated gravitational slope deformations (DSGSD), result from a complex time-dependent interaction among different geological, geomorphological and climatic factors [1]. Even if the role of temperature in alpine slope instabilities has been widely recognized [2,3], the role of Thermo-Mechanical (TM) and Thermo-Hydro-Mechanical (THM) couplings in driving rock-slope failure is still little explored. In alpine slopes, glacial unloading is often suggested to be the predominant mechanism leading to the development of rock mass tensile damage and large-scale stress release [1,4,5,6]. The deglaciation process, however, is also associated with variations in climatic conditions that promote rock-mass progressive degradation. In this work, a TM semi-coupled approach is applied to investigate the stress-strain evolution of an alpine rock slope, considering both glacial unloading and paleo-temperature evolution resulting from the Last Glacial Maximum (LGM) conditions.

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 (figure 1a). 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 [7] (figure 1b-c). The Cimaganda rockslide involved gneissic rocks of the Tambo nappe Unit belonging to the Upper-Pennidic alpine domain. The slope is characterized by high sub-vertical rock cliffs leading to a complex stress-strain evolution. The presence of highly persistent and opened fracture systems parallel to the Valley axis (figure 1c-e), result in periodic slope failure phenomena which involved limited volumes of material. In September 2012, on the right flank of the historical Cimaganda rockslide, a failure event mobilized 20.000 m3 of rock material blocking the SS36 national road (figure 1b) [8].

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