Rock slope dynamics in bedrock permafrost: Insights across scales

Rock slope failure poses a potential risk to the safety of local communities and infrastructure in populated mountain regions. There is evidence that climate-related change of thermal conditions in steep bedrock permafrost leads to increased rock slope destabilization. The presence of permafrost supports the infill of fractures with ice, which can act as preferential failure planes when exposed to increasing temperatures. Permafrost also affects the hydrological and mechanical properties, which are sensitive to a changing climate. Therefore, characterizing and monitoring the dynamics of steep rock slopes is relevant for assessing rock slope stability and for detecting precursory behavior prior to slope failure. In this thesis, multi-year time series of field measurements and a comprehensive set of laboratory shear strength experiments were acquired and analyzed.

The longer-term evolution of fracture kinematics in steep bedrock permafrost was analyzed with an unprecedented level of detail thanks to a unique multi-year time series of fracture displacements, rock temperatures and environmental conditions at the Matterhorn Hörnligrat field site. In the wider context of rock slope stability assessment, a new metric was proposed to quantify irreversible displacement of fractures based on the statistical separation of reversible components, caused by thermoelastic strains, from irreversible components due to other processes.

Passive monitoring of acoustic emission and micro-seismology, recorded for the first time simultaneously in steep bedrock permafrost, covers the broad frequency range of 1-10⁵Hz. These measurements provide important subsurface information on fracturing and therefore complement surface displacement data. The analysis of artificial forcing (rebound hammer) at the Matterhorn Hörnligrat field site led to two major findings: Firstly, the lack of cross-correlation between signals indicates that waveforms change strongly with propagation distance. Secondly, a significant amplification was found in the frequency band 33-67Hz. These effects are strongly supported by evidence from additional artificial rockfall events and natural fracture events. Filtering this specific frequency band therefore enables a more reliable detection of fracture events, which is a prerequisite for rock slope stability assessment and early warning. Further, a new time-dependent index has been derived, that simulates crack growth rate caused by ice segregation. We successfully demonstrated a qualitative agreement of this index with measured acoustic emission activity.

Analysis of continuously recorded ambient seismic vibrations was for the first time applied in a mountain permafrost regime and led to two major findings: First, a sudden increase in ambient noise when rock temperatures rise above 0°C was observed. Second, seasonal variations in resonance frequencies exist and can be explained by variations in fracture infill, which in turn are related to the formation and melt of ice. Therefore, monitoring ambient seismic vibrations and tracking the deduced resonance frequencies, that are characteristic for the geometry of a rock mass, has the potential to be a powerful tool to investigate the evolution of the stability of frozen rock masses and to detect thawing-related rockfall events.

However, the mechanical processes leading to failure in ice-filled rock discontinuities are still poorly understood. To address this issue, over 140~shear tests of frozen sandwich-like rock-ice-rock samples were performed in collaboration with the Technical University of Munich. Two acoustic emission sensors were applied to monitor ice fracturing. The results demonstrate a significant decrease in shear resistance with warming and/or unloading of ice-filled discontinuities. To quantify this, we introduce a new failure criterion for ice-filled discontinuities that considers friction and cohesion changes with temperature.

In a nutshell, this thesis contributes to a better understanding and improved ability to monitor stability relevant properties in steep bedrock permafrost. Tracking the evolution of these properties and analyzing the dynamics of rock slopes allows to investigate and quantify the development of processes leading to rock slope destabilization or even to identify pre-failure precursor signs.