How percolating snowmelt water progressively destabilizes a free-standing rock pillar on permafrost: Field observations from Matterhorn (CH), laboratory experiments and mechanical modeling

Permafrost rock slopes have been extensively studied, but seasonally frozen zones are often neglected. However, these rocks are subject to progressive destabilization driven by complex thermal and mechanical interactions. Their thickening in response to atmospheric warming is critical as pressurized water within them can induce short-term warming and thawing at depth through non-conductive, more efficient heat transport, potentially enhancing the destabilization of the rock slope.This study focuses on the collapse of a 20 cubic meter, free-standing rock pillar on the Matterhorn Hörnligrat ridge on 13 June 2023, leveraging a unique long-term, multi-method monitoring dataset initiated in 2008. The pillar’s behavior was assessed through differential GNSS measurements, inclinometers, seismic monitoring, time-lapse imagery, weather data, and permafrost ground temperature records. These data reveal a strong seasonality in displacement patterns, with significant acceleration starting in 2022 and visually detectable changes two weeks before the collapse. Seasonal snowmelt infiltration into frozen fractures emerged as the primary driver of observed displacement patterns, a hypothesis corroborated by controlled laboratory experiments and thermo-mechanical modeling.A 2D mechanical modeling framework (UDEC) was employed to evaluate the effects of seasonal freezing and thawing on fracture behavior, integrating results from laboratory shear tests conducted on Matterhorn rock samples under dry/wet and frozen/unfrozen conditions. The results highlight the critical role of a thawing-induced drop in the coefficient of friction along fractures, which drives shear stress changes and kinematic responses.By integrating long-term field monitoring, laboratory experiments, and numerical modeling, this research provides insights into the destabilization of permafrost-affected rock slopes. It underscores the importance of incorporating seasonally frozen layers and their thermo-mechanical behavior into stability assessments, particularly under accelerating climate change.