The water adsorption into pore spaces in brittle rocks affects wave velocity and transmitted amplitude of elastic waves. Experimental and theoretical studies have been performed to characterize moisture-induced elastodynamic variations due to macroporous effects; however, little attention has been paid to the manner in which wetting of nanopores affect elastic wave transmission. In this work, we extend our understanding of moisture-induced elastic changes in a microcracked nanopore-dominated medium (80 \% of the surface area exhibits pore diameters below 10 nm). We studied acousto-mechanical response resulting from a gradual wetting on a freestanding intact Herrnholz granite specimen over 98 hours using time-lapse ultrasonic and digital imaging techniques. Linkages between ultrasonic attributes and adsorption-induced stress/strain are established during the approach of wetting front. We found that Gassmann theory, previously validated in channel-like nanoporous media, breaks down in predicting P-wave velocity increase of microcracked nanopore-dominated media. However, squirt flow – a theory recognized to characterize wave velocity increase and attenuation in microcracked macropore-dominated media at pore scale – also accounts for the observed increase of P-wave velocity in microcracked nanopore-dominated media. The transmitted amplitude change in direct P waves are explained and predicted by the elastic wave propagation within P-wave first Fresnel zone and reflection/refraction on the wetting front.

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.

We study the physical mechanisms that drive alpine slope deformation during water infiltration and depletion into fractured bedrocks. We develop a fully coupled hydromechanical model at the valley scale with multiscale fracture systems ranging from meter to kilometer scales represented. The model parameterized with realistic rock mass properties captures the effects of fractures via an upscaling framework with equivalent hydraulic and mechanical properties assigned to local rock mass blocks. The important heterogeneous and anisotropic characteristics of bedrocks due to depth-dependent variations of fracture density and stress state are taken into account and found to play a critical role in groundwater recharge and valley-scale deformation. Our simulation results show that pore pressure actively diffuses downward from the groundwater table during a recharge event, rendering a critical hydraulic response zone controlling surface deformation patterns. During the recession, the hydraulic front migrates downwards and the deformation recorded at the surface (up to ~4 cm) rotates accordingly. The most essential parameters in our model are the fracture network geometry, initial fracture aperture (controlling the rock mass permeability), and regional stress conditions. The magnitude and orientation of our model’s transient annual slope surface deformation are consistent with field observations at our study site in the Aletsch valley. Our research findings have important implications for understanding groundwater flow and slope deformations in alpine mountain environments.

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.