Hydro-Mechanical Interactions of a Rock Slope With a Retreating Temperate Valley Glacier

Rock slope failures often result from progressive rock mass damage which accumulates over long timescales. In deglaciating environments, rock slopes are affected by stress perturbations driven by mechanical unloading due to ice downwasting and concurrent changes in thermal and hydraulic boundary conditions. Since in-situ data are rare, the different processes and their relative contribution to slope damage remain poorly understood. Here, we analyze borehole monitoring data from a rock slope adjacent to the retreating Great Aletsch Glacier (Switzerland) and compare it to englacial water levels, climate data, and decreasing ice levels. Rock slope pore pressures show a seasonal signal controlled by infiltration events as well as effects from the connectivity to the englacial hydrological system. We find that reversible and irreversible strains are driven by: (a) hydromechanical effects caused by englacial pressure fluctuations and infiltration events, (b) stress transfer related to changing mechanical glacial loads from short-term englacial water level fluctuations and longer term ice downwasting, and (c) thermomechanical effects from annual temperature cycles penetrating the shallow subsurface, which primarily result in reversible deformation. We relate most observed irreversible strain (damage) to mechanical unloading from ice downwasting. Damage is strongest directly at the ice margin and moves through the slope at the pace of glacial retreat and advance. Locations with many retreat/advance cycles are very sensitive to landslide formation. The current climate warming impacts very sensitive valley sectors, which is confirmed by landslide distributions and activity in the study area.

Automated summary

Rock slope failures are often caused by long-term progressive rock mass damage. In deglaciating environments, stress changes resulting from mechanical unloading due to ice downwasting and concurrent changes in thermal and hydraulic boundary conditions affect rock slopes. However, due to the lack of in-situ data, the different processes and their contributions to slope damage are not well understood. In this study, borehole monitoring data from a rock slope adjacent to the retreating Great Aletsch Glacier in Switzerland are analyzed, and the results are compared with englacial water levels, climate data, and decreasing ice levels. The findings indicate that rock slope pore pressures show a seasonal signal controlled by infiltration events and the connectivity to the englacial hydrological system. Most of the observed irreversible strain (damage) is related to mechanical unloading from ice downwasting. The study also confirms that the current climate warming impacts sensitive valley sectors, as evidenced by landslide distributions and activity in the study area.