Progressive destabilization of a freestanding rock pillar in permafrost on the Matterhorn (Swiss Alps): Field observations, laboratory experiments and mechanical modeling

Abstract. Permafrost rock slopes have been extensively studied, yet the thermal and mechanical dynamics of the transition zone between the permafrost and overlying seasonally frozen layers warrant further investigation. This study investigates the progressive destabilization of a freestanding rock pillar forming the transition zone between the permafrost and the active layer on the Matterhorn Hörnligrat ridge, with an ultimate collapse on 13 June 2023. We provide a comprehensive analysis that integrates field observations, laboratory findings, and mechanical modeling from the first destabilization to the final failure of the rock pillar. Based on multi-method field observations since 2008, we analyze the kinematic, thermal, and seismic evolution leading up to failure. GNSS and inclinometer measurements reveal a strong seasonal displacement pattern with a marked acceleration beginning in 2022. Time-lapse imagery documents a visible acceleration 10 days prior to the collapse, while seismic monitoring with three nearby seismometers identifies precursors and failure dynamics. Weather data and permafrost temperature records indicate a critical role of water percolation into permafrost, driving rapid, short-term thawing at depth through non-conductive heat fluxes. Laboratory experiments show that this thawing significantly reduces the friction angle along fractures by over 50 %. The integration of the laboratory experiments into a thermo-mechanical model allows to reproduce the seasonal distinct displacement pattern observed in the field and thereby bridges the gap between experimental data and in-situ field applications. This case study provides new insights into the critical role of water percolation and highlights a widespread phenomenon in warming mountain permafrost regions, manifested in the increasing frequency of rockfall events observed in such environments.