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This paper presents a detailed analysis of a dramatic rock slope acceleration that occurred in fall 2016 at the Moosfluh Landslide, located at the glacier tongue of the Great Aletsch Glacier (Switzerland). The acceleration that occurred in 2016 was unanticipated and exposed the valley bottom and an adjacent damned lake to high risk. This acceleration occurred in an active deep‐seated gravitational slope deformation (DSGSD) controlled primarily by deep block‐flexural toppling. In 2013, a highly accurate displacement monitoring system was developed and installed in the surroundings of the Great Aletsch Glacier, including a time‐lapse camera, GNSS, and robotic total stations. This monitoring system provided unique data during the 2016 slope acceleration which are used in this study to assess failure mechanisms, landslide volumes, and subsurface displacement geometry. Based on a novel displacement vector analysis, we find that three retrogressive secondary rockslides developed during the first six weeks of the slope acceleration, with rupture surface depths of 30 to 40 m, and estimated volumes between 1 and 4 Mm3. These rockslides display complex deformation features, including head and lateral scarps, which developed during the slope acceleration. The kinematics of these secondary rockslides changed through time, from primarily toppling to combined toppling and sliding. Our results provide a uniquely detailed understanding of the spatial and temporal evolution of deformation features and movement kinematics that occur when several sectors of a slowly moving DSGSD transitions into rapid rockslides. The slow moving (~30 cm per year) deep‐seated (up to 170‐m depth) landslide of Moosfluh experienced a substantial acceleration of slope movement in September 2016. A comprehensive monitoring system recorded that several compartments in the center of the Moosfluh Landslide suddenly started to move rapidly with velocities of up to several meters per week. This caused a strong modification of the slope surface morphology and structure which is documented in detail in this study. Based on monitored changes in the directions of slope movement over time, a new method was developed to interpret subsurface movement types and geometries. With this method we found that on top of the Moosfluh deep‐seated landslide several shallow (30–40 m) rockslides developed during the Moosfluh slope acceleration in 2016. We are able to assign depths and inclinations of the failure surfaces and failure mechanisms. In this case the deep‐seated landslide is formed by rotation (toppling) of steep rock columns, whereas the shallow secondary rockslides within the deep‐seated landslide show sliding movements along planar rupture planes and mixtures of both sliding and toppling. The rapid formation of 3 secondary rockslides within a toppling mode DSGSD (Moosfluh, CH) is monitored with DIC from webcam images, aerial photogrammetry  and total station monitoringThis transition corresponds to an acceleration of the landslide from a few dm/a to 50 m/a in the lowermost sectors of the instabilityA novel method developed to interpret total station reflector data revealed a change in landslide kinematics (toppling, sliding) through time

Water infiltration into fractures is ubiquitous in crustal rocks. However, little is known about how such a progressive wetting process affects fracture stiffness and seismic wave propagation, which are highly relevant for characterizing fracture systems in situ. We study the acousto‐mechanical behavior of a free‐standing fractured granite subjected to gradual water infiltration with a downward‐moving wetting front over 12 days. We observe significant differences (i.e., by an order of magnitude) in wave amplitudes across the fractured granite compared to an intact granite, with both cases showing a strong correlation between wave amplitudes and wetting front movement. Effects of water infiltration into the fracture and surrounding matrix on seismic attenuation are captured by a numerical model with parameters constrained by experimental data. Back‐calculated fracture stiffness decreases exponentially with the wetting front migration along the fracture. We propose that moisture‐induced matrix expansion around the fracture increases asperity mismatch, leading to reduced fracture stiffness. In the shallow layers of the Earth, hydrological cycles such as snowmelt, fog, dew, and rain have been shown to change the moisture content of crustal rocks, which can alter the elastic properties of natural fractures and affect the propagation of seismic waves. Understanding how seismic waves propagate in the near‐surface environment is crucial for the assessment of earthquake hazards and the characterization of geologic heterogeneities. In this work, we perform well‐controlled laboratory experiments to study the acousto‐mechanical behavior of a single fracture in granitic rock subjected to progressive wetting over 12 days. We report that the fracture stiffness decreases exponentially as the wetting front advances along the fracture. Our research sheds light on an important question in fracture characterization: how elastic waves propagate across a fracture undergoing moisture‐induced expansion. A laboratory study establishes a relationship among water imbibition, seismic attenuation, and stiffness evolution in a wetted fractureWave amplitudes across a fracture correlate strongly with the wetting front movement of infiltrated water within the fractureFracture stiffness exponentially decreases with the advance of the wetting front along the fracture A laboratory study establishes a relationship among water imbibition, seismic attenuation, and stiffness evolution in a wetted fracture Wave amplitudes across a fracture correlate strongly with the wetting front movement of infiltrated water within the fracture Fracture stiffness exponentially decreases with the advance of the wetting front along the fracture

Information on structural features of a fracture network at early stages of Enhanced Geothermal System development is mostly restricted to borehole images and, if available, outcrop data. However, using this information to image discontinuities in deep reservoirs is difficult. Wellbore failure data provides only some information on components of the in situ stress state and its heterogeneity. Our working hypothesis is that slip on natural fractures primarily controls these stress heterogeneities. Based on this, we introduce stress‐based tomography in a Bayesian framework to characterize the fracture network and its heterogeneity in potential Enhanced Geothermal System reservoirs. In this procedure, first a random initial discrete fracture network (DFN) realization is generated based on prior information about the network. The observations needed to calibrate the DFN are based on local variations of the orientation and magnitude of at least one principal stress component along boreholes. A Markov Chain Monte Carlo sequence is employed to update the DFN iteratively by a fracture translation within the domain. The Markov sequence compares the simulated stress profile with the observed stress profiles in the borehole, evaluates each iteration with Metropolis‐Hastings acceptance criteria, and stores acceptable DFN realizations in an ensemble. Finally, this obtained ensemble is used to visualize the potential occurrence of fractures in a probability map, indicating possible fracture locations and lengths. We test this methodology to reconstruct simple synthetic and more complex outcrop‐based fracture networks and successfully image the significant fractures in the domain. We introduce stress‐based tomography as a method to characterize the fracture network and its heterogeneity in deep geothermal reservoirsStress heterogeneities at borehole scale are applied to calibrate DFN realizationsThe methodology is successfully tested to image simple synthetic and outcrop‐based fracture networks

Cycles of glaciation alter the temperature field in proximal alpine valley flanks, driving rock slope damage through thermomechanical stresses. Here we extend simplified assumptions of glacial debuttressing to quantitatively examine how paraglacial bedrock temperature changes, acting in concert with changing ice loads during Late Pleistocene and Holocene glacial cycles, create damage in adjacent rock slopes and prepare future slope instabilities. When in contact with temperate glacier ice, valley walls maintain near isothermal ~0 °C surface temperatures and are shielded from daily and annual cycles. With retreat, rock walls are rapidly exposed to strongly varying temperature boundary conditions, a transition we term “paraglacial thermal shock.” Using detailed, conceptual numerical models based on the Aletsch Glacier in Switzerland, we show that including thermomechanical stresses during simulated glacial cycles creates significantly more rock slope damage than predicted for purely mechanical ice loading and unloading. Glacier advances are especially effective in generating damage as rapid cooling drives contraction of the rock mass reducing joint normal stresses. First time exposure to annual temperature cycles during deglaciation induces a shallow damage front that follows the retreating ice margin, generating damage in a complementary process at shorter time scales. Acting on a reduced strength rock mass, modeled thermomechanical cycles enhance the development of a slope instability with similar attributes as observed in our study area. Our results demonstrate that thermomechanical stresses acting in conjunction with changing ice loads are capable of generating considerable rock slope damage with spatial and temporal patterns controlled by glacier extents. Subsurface temperature changes on glacial time scales can generate rock mass damage through thermomechanical coupled stressesGlacier advances are most effective in producing rock slope damage as cooling drives contraction of subglacial bedrockThermal shock during deglaciation creates a shallow damage front as bedrock is first exposed to annual temperature cycles

The range of options for investigation of hydraulic behavior of aquifers from boreholes has been limited to rigid, cumbersome packers, and inflatable sleeves. Here we show how a new temporary borehole sealing technique using soft grains of polyacrylamide (PAM) gel as a sealing material can be used to investigate natural groundwater flow dynamics and discuss other possible applications of the technology. If no compressive stress is applied, the gel packing, with a permeability similar to open gravel, suppresses free convection, allowing for local temperature measurements and chemical sampling through free-flowing gel packing. Active heating laboratory and field experiments combined with temperature measurements along fiber optic cables were conducted in water-filled boreholes and boreholes filled with soft grains of polyacrylamide gel. The gel packing is shown to minimize the effect of free convection within the well column and enable detection of thin zones of relatively high or low velocity in a highly transmissive alluvial aquifer, thus providing a significant improvement compared to temperature measurements in open boreholes. Laboratory experiments demonstrate that under modest compressive stress to the gel media the permeability transitions from highly permeable to nearly impermeable grouting. Under this configuration the gel packing could potentially allow for monitoring local response pressure from the formation with all other locations in the borehole hydraulically isolated. We propose a new temporary borehole sealing technique using soft grains of PAM gel as a sealing materialPAM gel compresses free convection in boreholes thus allowing to characterize natural groundwater dynamicsWe demonstrate that low-permeable grouting material could be created by compressing PAM

Cycles of glaciation impose mechanical stresses on underlying bedrock as glaciers advance, erode, and retreat. Fracture initiation and propagation constitute rock mass damage and act as preparatory factors for slope failures; however, the mechanics of paraglacial rock slope damage remain poorly characterized. Using conceptual numerical models closely based on the Aletsch Glacier region of Switzerland, we explore how in situ stress changes associated with fluctuating ice thickness can drive progressive rock mass failure preparing future slope instabilities. Our simulations reveal that glacial cycles as purely mechanical loading and unloading phenomena produce relatively limited new damage. However, ice fluctuations can increase the criticality of fractures in adjacent slopes, which may in turn increase the efficacy of fatigue processes. Bedrock erosion during glaciation promotes significant new damage during first deglaciation. An already weakened rock slope is more susceptible to damage from glacier loading and unloading and may fail completely. We find that damage kinematics are controlled by discontinuity geometry and the relative position of the glacier; ice advance and retreat both generate damage. We correlate model results with mapped landslides around the Great Aletsch Glacier. Our result that most damage occurs during first deglaciation agrees with the relative age of the majority of identified landslides. The kinematics and dimensions of a slope failure produced in our models are also in good agreement with characteristics of instabilities observed in the field. Our results extend simplified assumptions of glacial debuttressing, demonstrating in detail how cycles of ice loading, erosion, and unloading drive paraglacial rock slope damage. Simply adding then removing glacier ice from an alpine valley has little net effect on rock wall damageGlacial erosion, i.e., rock debuttressing, creates significant new rock slope damage during first deglaciationDamage kinematics vary during a glacial cycle: ice advance favors toppling, while retreat promotes sliding

Catastrophic landslides characterized by runaway slope failures remain difficult to predict. Here, we develop a physics‐based framework to prospectively assess slope failure potential. Our method builds upon the physics of extreme events in natural systems: the extremes so‐called “dragon‐kings” (e.g., slope tertiary creeps prior to failure) exhibit statistically different properties than other smaller‐sized events (e.g., slope secondary creeps). We develop statistical tools to detect the emergence of dragon‐kings during landslide evolution, with the secondary‐to‐tertiary creep transition quantitatively captured. We construct a phase diagram characterizing the detectability of dragon‐kings against “black‐swans” and informing on whether the slope evolves toward a catastrophic or slow landslide. We test our method on synthetic and real data sets, demonstrating how it might have been used to forecast three representative historical landslides. Our method can in principle considerably reduce the number of false alarms and identify with high confidence the presence of true hazards of catastrophic landslides. Catastrophic slope failures that pose great threats to life and property remain difficult to predict due to the strong variability of slope behavior. As a result, only a limited number of large rock slope failures have been so far successfully forecasted with associated risks mitigated. Here, we propose a novel predictive framework to prospectively and quantitatively detect slope failure precursors with high confidence. Our research sheds light on one of the most challenging questions in landslide prediction: Would an active landslide slowly move or catastrophically fail in the future? Our method adds a new conceptual framework and operational methodology with a significant potential to support existing early warning systems and hence reduce landslide risks. Tertiary creeps of catastrophic landslides accommodate dragon‐kings showing statistically different properties than secondary slope creepsA predictive framework is developed to forecast catastrophic landslides by detecting signatures typical of the emergence of dragon‐kingsA phase diagram characterizes the detectability of dragon‐kings against black‐swans and discriminates catastrophic and slow landslides Tertiary creeps of catastrophic landslides accommodate dragon‐kings showing statistically different properties than secondary slope creeps A predictive framework is developed to forecast catastrophic landslides by detecting signatures typical of the emergence of dragon‐kings A phase diagram characterizes the detectability of dragon‐kings against black‐swans and discriminates catastrophic and slow landslides

Data from continuously recording permanent seismic networks can contain information about rockslide occurrence and timing complementary to eyewitness observations and thus aid in construction of robust event catalogs. However, detecting infrequent rockslide signals within large volumes of continuous seismic waveform data remains challenging and often requires demanding manual intervention. We adapted an automatic classification method using hidden Markov models to detect rockslide signals in seismic data from two stations in central Switzerland. We first processed 21 known rockslides, with event volumes spanning 3 orders of magnitude and station event distances varying by 1 order of magnitude, which resulted in 13 and 19 successfully classified events at the two stations. Retraining the models to incorporate seismic noise from the day of the event improved the respective results to 16 and 19 successful classifications. The missed events generally had low signal-to-noise ratio and small to medium volumes. We then processed nearly 14?years of continuous seismic data from the same two stations to detect previously unknown events. After postprocessing, we classified 30 new events as rockslides, of which we could verify three through independent observation. In particular, the largest new event, with estimated volume of 500,000?m3, was not generally known within the Swiss landslide community, highlighting the importance of regional seismic data analysis even in densely populated mountainous regions. Our method can be easily implemented as part of existing earthquake monitoring systems, and with an average event detection rate of about two per month, manual verification would not significantly increase operational workload. Rockslide seismic signals are detected and classified using hidden Markov modelsSixteen and 19 successful classifications of 21 known events at two seismic stationsThirty previously unknown events are rockslides detected in 14?years of continuous seismic data

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. The formation of rock slope instabilities is a long‐term process related to continuous rock mass weakening. For rock slopes in glaciated valleys, increased rates of rock mass damage are assumed to occur during ice retreat due to a reduction of the glacier weight and concurrent changes in rock temperatures and slope groundwater pressures. However, because direct field measurements are missing, governing processes and their relative importance are still poorly understood. Here, we analyze borehole monitoring data and compare it to englacial water levels, climate data, and decreasing ice levels. We find that the glacial hydrology influences the dynamics of the groundwater pressures in the adjacent rock slope at various timescales. We record micrometer‐scale reversible (elastic) deformation and irreversible rock mass damage and identify the involved drivers. Several processes contribute to progressive rock mass weakening at different timescales and magnitudes, but most damage is related to mechanical unloading from ice downwasting and is strongest directly at the ice margin. Locations with many retreat/advance cycles are sensitive to increased damage accumulation and landslide formation. The current climate warming impacts very sensitive valley sectors, which is confirmed by landslide distributions and activity in the study area. We monitored subsurface pore pressures and micrometer scale strain in a rock slope during strong glacial retreat caused by climate warmingWe identified thermo‐hydro‐mechanical drivers for reversible deformation and irreversible rock mass damage acting at various timescalesSlope damage intensity can be related to landslide spatial and temporal distribution We monitored subsurface pore pressures and micrometer scale strain in a rock slope during strong glacial retreat caused by climate warming We identified thermo‐hydro‐mechanical drivers for reversible deformation and irreversible rock mass damage acting at various timescales Slope damage intensity can be related to landslide spatial and temporal distribution

Very intensive rainfall in August 2005 (>300 mm/3 days) triggered moderately deep (2–10 m) landslides of about 50'000 m3volume each in two mountain torrent catchments above the village of Brienz (Berner Oberland, Switzerland). These landslides – originating in Trachtbach and Glyssibach catchments – transformed into extremely rapid (>5 m/s) debris flows, which caused significant damage in inhabited areas; two persons lost their lives and about twenty-five families became homeless. The Brienz case was the most damaging one among many landslide disasters occurring during those rainy days in the Swiss Alps. In this paper we study in detail the predisposition and causes of the 2005 landslides in the Brienz area, based on field mapping, analysis of high resolution images and digital terrain models, derived from LIDAR and infrared measurements taken before and after the event. The features of these landslides are compared with past and dormant landslides in the mid-slope portion of the mountain chain north of Brienz, which has been the source of many catastrophic mass wasting events during the last centuries.

Abstract: This paper presents a detailed field investigation and hazard assessment for an unstable rock slope above an industrial site in the Riviera valley in the Canton of Ticino in southern Switzerland. An integrated framework was used to counter issues of geological complexity and uncertainty, linking geological mapping and numerical modeling to develop an understanding of the acting instability mechanism to 2-D and 3-D dynamic runout simulations to predict the travel path and reach in the event of a large volume rockslide. The results from the numerical stability analysis provided a means to constrain failure volume estimates, whereas a series of calibration simulations were used to constrain the input parameters required by the rheological model used for the runout analysis. Results from this assessment suggest that current protection measures in place may not be sufficient, helping local authorities to define hazard zones and aid further development plans for the region.

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. Glaciers impose mechanical loads on the bedrock below and influence temperature and groundwater conditions in adjacent valley flanks. When glaciers retreat, the boundary conditions change, and related stress changes can weaken the adjacent rock mass. These processes potentially contribute to the long‐term formation of rock slope instabilities. In this study, we investigate such processes based on rare borehole monitoring data with a focus on the subsurface temperature evolution and related deformations. Collected subsurface rock temperatures reveal a warming trend following ice retreat that is strongly controlled by heat conduction. Observed non‐conductive processes are related to groundwater exchange with cold subglacial water, snowmelt infiltration, or creek water infiltration. Our deformation data shows that thermomechanical deformations related to annual surface temperature cycles are significant in the shallow 10–15 m. At greater depth, deformations related to changes in groundwater pressure dominate. Progressive rock mass damage, which is identified as the irreversible portion of the monitored deformation, occurs as fast and slow/continuous deformation. Precipitation events and groundwater pressure changes have been identified as important drivers for fast damage events. Drivers for continuous irreversible deformation remain uncertain but fatigue processes and presumably changing glacial loads are considered as potential causes. We monitor and model the subsurface temperature evolution and deformation in a rock slope during deglaciationWe observe a conduction dominated transient subsurface temperature regime adjusting to a new surface temperature following ice retreatWe document reversible deformation and irreversible rock mass damage and compare thermal and hydrological drivers causing these deformations We monitor and model the subsurface temperature evolution and deformation in a rock slope during deglaciation We observe a conduction dominated transient subsurface temperature regime adjusting to a new surface temperature following ice retreat We document reversible deformation and irreversible rock mass damage and compare thermal and hydrological drivers causing these deformations

In low to moderate tectonically active areas, such as intracontinental regions with deep reaching zones of crustal weakness, direct observations of surface faulting offsets are very scarce and the seismic record is often incomplete and small. This complicates the study of a possible surface faulting hazard and requires new analysis concepts. In the case study presented in this paper, the seismic hazard potential of an already defined nuclear power plant site located in such a moderate active region had to be reassessed, and the possible effects of surface faulting had to be quantified. Dans les secteurs à activité tectonique faible ou moyenne, telles que les régions intracontinentales avec des zones de faiblesse de la croûte très profonde, des observations directes des fracturations de surface sont très dispersées et les données sur la seismicité incomplètes. Ceci complique l'étude d'un éventuel risque dû à la fracturation de surface et nécessite d'utiliser de nouveaux concepts d'analyse. Dans l'exemple présenté dans cet article, le risque sismique lié à un site de centrale nucléaire déjà défini a dû être réexaminé et les effets éventuels d'une fracturation de surface ont été quantifiés.

We present a new inventory of large rock slope instabilities and an extensive structural data set for northwestern Bhutan. Our data set is largely based on satellite observations, such as optical images and high‐resolution digital surface models, complemented with field observations. Kinematic analyses over seven different structural domains recognized in the study region were carried out to investigate structural control over the landslide distribution at a regional scale. In order to account for the sampling heterogeneity in the different data sets, a weighting system to the structural data is proposed. The results show that structural control exists in five out of seven domains, with foliation strongly influencing the sliding mechanisms. A lithological control appears also evident, with sedimentary rocks showing the highest landslide densities in the region. This study demonstrates how methodologies built on remote sensing data are suitable to investigate landslide predisposition at regional scales especially in largely inaccessible areas. Rock slope instabilities and structural inventories for NW Bhutan based on optical images and high‐resolution DSMWeighting system applied to the data set improves its reliability and the assessment of limitationsLarge‐scale kinematic analyses for rock slope instabilities reveal structural control over a large part of the study region

Subglacial water pressures influence groundwater conditions in proximal alpine valley rock slopes, varying with glacier advance and retreat in parallel with changing ice thickness. Fluctuating groundwater pressures in turn increase or reduce effective joint normal stresses, affecting the yield strength of discontinuities. Here we extend simplified assumptions of glacial debuttressing to investigate how glacier loading cycles together with changing groundwater pressures generate rock slope damage and prepare future slope instabilities. Using hydromechanical coupled numerical models closely based on the Aletsch Glacier valley in Switzerland, we simulate Late Pleistocene and Holocene glacier loading cycles including long‐term and annual groundwater fluctuations. Measurements of transient subglacial water pressures from ice boreholes in the Aletsch Glacier ablation area, as well as continuous monitoring of bedrock deformation from permanent Global Navigation Satellite Systems stations, help verify our model assumptions. While purely mechanical glacier loading cycles create only limited rock slope damage in our models, introducing a fluctuating groundwater table generates substantial new fracturing. Superposed annual groundwater cycles increase predicted damage. The cumulative effects are capable of destabilizing the eastern valley flank of our model in toppling‐mode failure, similar to field observations of active landslide geometry and kinematics. We find that hydromechanical fatigue is most effective acting in combination with long‐term loading and unloading of the slope during glacial cycles. Our results demonstrate that hydromechanical stresses associated with glacial cycles are capable of generating substantial rock slope damage and represent a key preparatory factor for paraglacial slope instabilities. Glacial melt produces large amounts of water keeping water pressures at the base of alpine glaciers high. This meltwater in turn influences the groundwater table in adjacent valley flanks. Water seeps into the subsurface through joints and faults reducing the strength of these discontinuities. In a glaciated alpine valley, the position of the glacier changes over time and the groundwater table changes in accordance. Meanwhile, seasonal infiltration from snowmelt raises and lowers the water table in valley flanks, creating measureable movements. These changes in groundwater can alter the strength of a rock mass and together with the changing ice load generate damage in the floors and walls of alpine valleys. This damage helps prepare future slope instabilities, creating fractures that may ultimately define a landslide failure surface. We present new data and models from the Great Aletsch Glacier in Switzerland exploring our hypothesis that groundwater‐glacier coupled interactions are an important mechanism generating damage in alpine valley rock slopes. Our results show that higher water pressures reduce the strength of rock discontinuities and can result in failure, especially as glaciers retreat. This mechanism is an important part of the strength and stress changes in valley slopes that accompany glacial cycles. Subglacial water pressures in ablation areas are high and vary with ice thickness, affecting the groundwater table in adjacent valley flanksIncreasing groundwater pressure during glacier advance reduces effective stresses in joints, partly offsetting the increased ice loadHydromechanical coupled stresses during glacial cycles create and propagate fractures in rock slopes, preparing future instabilities

The Schöllenen Gorge in the Reuss Valley of the Central Swiss Alps (Figs. 1 and 2) is a famous tourist attraction and ideal location for the study of the properties and formation mechanisms of upliftand post-upliftunloading joints. The gorge is situated in the southern part of the Central Aar Granite, a granitic batholith which intruded about 300 million years ago. The magmatic fabric of this batholith (Fig. 4) has only been locally modified during Alpine tectonic and metamorphic overprinting, mainly in the vicinity of ductile-brittle shear zones. The up to 600 m deep gorge provides an ideal opportunity to study the complex fracture systems of the batholith, and tunnels of the Göschenen hydropower system allow the study of the fracture patterns below ground surface.