Investigation of the Frost-Induced Shallow Alpine Rockfall Process Using 3D FDEM

Increasing rockfall activities in alpine areas have been observed under the global climate change circumstance. One widely accepted explanation of the alpine rockfall is that the warming and thawing of ice-rich bedrock permafrost decreases effective shear strength and destabilizes rock substrates in unusual hot summers. However, small but frequent shallow collapses are also widely observed in mountainous areas when the temperature decreases in late autumn or cold winter. The research work presented herein proposes a numerical framework for simulating the frost heaving/cracking process and revealing the related shallow alpine rockfall mechanism, based on the three-dimensional (3D) combined finite discrete element method (FDEM), where complex coupled thermal-mechanical processes at low-temperature are considered, e.g., the heat transfer, water-ice transition, ice-rock interaction, and cracking. Results suggest that shallow collapses are observed when the temperature decreases in late autumn or cold winter. Cracks initiate and propagate in the rock mass, driven by the ice-rock interaction in cold seasons, deteriorate the structure stability and induce shallow rockfall. Knowledge of long-term thermal/water evolution of a rock slope therefore provides valuable insight into progressive rock deformation and related alpine rockfall hazards. 1. BACKGROUND Increasing rockfall activities in alpine areas have been observed under the global climate change circumstance (Harris et al. 2009; Ravanel and Deline 2011), and are linked to the frost heaving/cracking behavior (Hallet 2006; Draebing and Krautblatter 2019). One wildly accepted explanation of the alpine rockfall is that the warming and thawing of ice-rich bedrock permafrost decreases effective shear strength and destabilize rock substrates in unusual hot summers (Gruber et al. 2004; Hasler et al. 2012). However, small but frequent shallow collapses are also widely observed in mountainous areas when the temperature decreases in late autumn or cold winter (Gruber and Haeberli 2007; Wu et al. 2021). Frost cracking is hypothesized to play an important role in the destabilization of steep permafrost (Matsuoka and Murton 2008), which can be inferred from the evidence that ice is common in fractures of bedrock and the fresh detachment scars of rockfall. Therefore, understanding the frost heaving/cracking mechanism is critical to shallow rockfall disasters in cold regions. To address this knowledge gap, a 3D numerical framework is developed to study the frost heaving and cracking process in fractured rock masses and further investigate the shallow rockfall mechanism based on the 3D FDEM (Munjiza 2004), which takes the advantage of simulating both continuum and discontinuum behavior.