In deglaciating environments, rock mass weakening and potential formation of rock slope instabilities is driven by long-term and seasonal changes in thermal- and hydraulic boundary conditions, combined with unloading due to ice melting. However, in-situ observations are rare. In this study, we present new monitoring data from three highly instrumented boreholes, and numerical simulations to investigate rock slope temperature evolution and micrometer-scale deformation during deglaciation. Our results show that the subsurface temperatures are adjusting to a new, warmer surface temperature following ice retreat. Heat conduction is identified as the dominant heat transfer process at sites with intact rock. Observed non-conductive processes are related to groundwater exchange with cold subglacial water, snowmelt infiltration, or creek water infiltration. Our strain data shows that annual surface temperature cycles cause thermoelastic deformation that dominate the strain signals in the shallow thermally active layer at our stable rock slope locations. At deeper sensors, reversible strain signals correlating with pore pressure fluctuations dominate. Irreversible deformation, which we relate with progressive rock mass damage, occurs as short-term (hours to weeks) strain events and as slower, continuous strain trends. The majority of the short-term irreversible strain events coincides with precipitation events or pore pressure changes. Longer-term trends in the strain time series and a minority of short-term strain events cannot directly be related to any of the investigated drivers. We propose that the observed increased damage accumulation close to the glacier margin can significantly contribute to the long-term formation of paraglacial rock slope instabilities during multiple glacial cycles.

Rock slope failures often result from progressive rock mass damage which accumulates over long timescales, and is driven by changing environmental boundary conditions. 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 is rare, the different processes and their relative contribution to slope damage remain poorly understood. Here we present detailed analyses of subsurface pore pressures and micrometer scale strain time series recorded in three boreholes drilled in a rock slope aside the retreating Great Aletsch Glacier (Switzerland). Additionally, we use monitored englacial water levels, climatic data, and annually acquired ice surface measurements for our process analysis. Pore pressures in our glacial adjacent rock slope show a seasonal signal controlled by infiltration from snowmelt and rainfall as well as effects from the connectivity to the englacial hydrological system. We find that reversible and irreversible strains are driven by hydromechanical effects from diffusing englacial pressure fluctuations and pore pressure reactions on infiltration events, stress transfer related to changing mechanical glacial loads from short-term englacial water level fluctuations and longer-term ice downwasting, and thermomechanical effects from annual temperature cycles penetrating the shallow subsurface. We relate most observed irreversible strain (damage) to mechanical unloading from ice downwasting. Additionally, short-term stress changes related to mechanical loading from englacial water level fluctuations and hydromechanical effects from pore pressure variations due to infiltration events were identified to contribute to the observed irreversible strain.

A comprehensive surface displacement monitoring system installed in the recently deglaciated bedrock slopes of the Aletsch Valley shows systematic reversible motions at the annual scale. We explore potential drivers for this deformation signal and demonstrate that the main driver is pore pressure changes of phreatic groundwater in fractured granitic mountain slopes. The spatial pattern of these reversible annual deformations shows similar magnitudes and orientations for adjacent monitoring points, leading to the hypothesis that the annually reversible deformation is caused by slope-scale groundwater elevation changes and rock mass properties. Conversely, we show that the ground reaction to infiltration from snowmelt and summer rainstorms can be highly heterogeneous at local scale, and that brittle-ductile fault zones are key features for the groundwater pressure-related rock mass deformations. We also observe irreversible long-term trends (over the 6.5 yr dataset) of deformation in the Aletsch valley composed of a larger uplift than observed at our reference GNSS station in the Rhone valley, and horizontal displacements of the slopes towards the valley. These observations can be attributed respectively to the elastic bedrock rebound in response to current glacier mass downwasting of the Great Aletsch Glacier and gravitational slope deformations enabled by cyclic groundwater pressure-related rock mass fatigue in the fractured rock slopes.