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Six hydraulic shearing experiments have been conducted in the framework of the In-situ Stimulation and Circulation experiment within a decameter-scale crystalline rock volume at the Grimsel Test Site, Switzerland. During each experiment one out of two different shear zone types were hydraulically reactivated. An extensive monitoring system of sensors recording seismicity, pressure and strain was spatially distributed in eleven boreholes around the injection locations. As a result of the stimulation, the near-wellbore transmissivity increased up to three orders in magnitude, while jacking pressures of the stimulated structures reduced during most of the experiments. Transmissivity change, jacking pressure and seismic activity were different for the two shear zone types, suggesting that the shear zone characteristics govern the seismo-hydro-mechanical response. The elevated fracture-fluid-pressures associated with the stimulations propagated mostly along the stimulated shear zones. The absence of high-pressure signals away from the injection point for most experiments (except two out of six experiments) is interpreted as channelized flow within the shear zones. The observed deformation field within 15 m-20 m from the injection point is characterized by variable extensional and compressive strain, produced by fracture normal opening and/or slip dislocation, as well as stress redistribution related to these processes. At greater distance from the injection location, strain measurements indicate a volumetric compressive zone, in which the strain magnitude decreases with increasing distance. This compressive strain signals are interpreted as a poro-elastic far-field response to the emplacement of fluid volume around the injection interval. The exceptional hydro-mechanical data reveal that the overall stimulation effected volume is significantly larger than implied by the seismicity cloud, and can be subdivided into a primary stimulated and secondary effected zone. [ABSTRACT FROM AUTHOR]

Abstract Large catastrophic rock slope failures are difficult to predict because the underlying mechanisms causing slope accelerations are difficult to study under in-situ conditions. For slope failures with compound basal rupture planes, not only fracture propagation and slip in the basal rupture surface, but also within the landslide body control the displacement evolution and time-to-failure. The Preonzo instability complex in southern Switzerland failed several times since 2002 and offers a unique opportunity to study these mechanisms under in-situ conditions. The largest failure at Preonzo occurred in May 2012 and was conditioned by large slope parallel (NN E -SSW) fracture zones, which are intersected by NE and SW and E-W striking faults creating diverging lateral boundaries. Whereas in the head scarp region oblique and flexural toppling along pre-existing fracture sets is the prevailing mechanism, sliding of either planar or wedge type is the preferred kinematic mode facilitated by the new discontinuities developed within the basal rupture surfaces. These dominating mechanisms, leading to a classical rock slope collapse, could only be revealed in retrospect. We compare fracture patterns in stable ground (tectonic and unloading fractures) with new fractures related to rock slope failure observed on basal rupture surfaces and in the head scarp. Simplified mechanical and kinematic analyses show that old pre-existing tectonic fractures cannot accommodate substantial deformations under the in-situ stress conditions and propagate/connect through curved wing cracks forming at high angles. This situation favors the development of new nearly slope parallel fractures, which are interpreted as synthetic P-shears along localized shear zones. Fractographic markings on these fractures indicate a continuous and relatively fast formation, which presumably has taken place during a period of about 60 years prior to catastrophic failure. This detailed description of the multi-stage evolution of slope failure at Preonzo represents a unique data set for future numerical studies of progressive failure in crystalline rocks. Highlights • Structural evolution of retrogressive failures with fracture orientation change • Interplay of pre-existing fractures and progressive failure in basal rupture planes • Multi-stage kinematic analyses reveal change in kinematics of a retrogressive slope • Possible analogies of tectonic shear zones with fracture reactivation + propagation [ABSTRACT FROM AUTHOR]

The Gotthard Base Tunnel (GBT) is a 57 km long and up to 2500 m deep railway tunnel constructed between 2000 and 2011 in the Central Alps of Switzerland. As drainage of fractured rocks by deep tunnels accompanied by significant decrease in groundwater pressure causes large-scale deformations even in hard crystalline rocks, a comprehensive surface deformation and tunnel inflow monitoring system has been established and operated for more than ten years. This paper presents the results from this monitoring system and explains the observed hydro-mechanically coupled and transient rock mass behavior based on detailed assessments of geological, geomechanical and hydrogeological conditions and conceptual continuum models. The collected data show that significant tunnel-drainage induced surface deformations also develop in rock masses with moderate hydraulic conductivity (2E−9 m/s) and small cumulative tunnel inflows (a few liters per second per kilometer). In this case deformations are caused by pore pressure reductions and rock mass deformations around the draining tunnel at depth, and not by groundwater table elevation changes. The pattern of surface settlements observed along the tunnel axis is very irregular (up to 11 cm in 2013) and strongly influenced by hectometer scale hydro-mechanical heterogeneities of steeply dipping geological units striking at large angle to the tunnel axes. At the depth of the studied tunnel section (1500–2500 m) about 50% of the surface settlements can be recorded. The surface settlements are connected to horizontal displacements and strains directed towards the tunnel axes or advancing tunnel face. The resulting horizontal displacement at the Nalps dam has reached about 65 mm in 2013. Compressive strains in the order of 20–50 microstrain are typically observed within a corridor of about 1 to 1.5 km width. Outside the reversal point of the settlement trough, extensile strains of similar magnitude develop. [ABSTRACT FROM AUTHOR]

Abstract: A systematic pre-drilling campaign was undertaken during the excavation of the Sedrun South lot of the Gotthard Base Tunnel. This tunnel section is located 897 to 2026m below a high alpine topography and located in fractured crystalline basement rocks of the Gotthard massif. The section covered by the systematic drilling campaign extends over 5km in the Nalps hydropower lake region and aimed to support decisions and counter measures related to tunnel drainage induced surface deformations. The pre-drillings conducted in both tunnel tubes included 30 cored and 94 destructive drillings with a length ranging from 17.5 to 358.5m. This paper presents in detail the results from pre-drilling and tunnel inflow monitoring. A new methodology is presented and discussed that allows for quantitative analysis of fracture transmissivity and equivalent continuum hydraulic conductivity from basic pre-drilling inflow and pressure observations, using a simplified form of the Jacob and Lohman solution for transient inflow rate to a well of constant drawdown in a confined aquifer. Uncertainties with respect to the derived hydraulic conductivity values are estimated through Monte Carlo simulation and compared with results from digitally controlled borehole outflow and pressure build-up tests. The spatial distribution of fracture transmissivity and equivalent continuum hydraulic conductivity is presented and compared with tectonic and lithological properties. It is suggested that both steeply dipping, regional brittle faults and lithologic contacts with significant stiffness contrasts impact the distribution of preferential groundwater pathways at depth. Along fault strike strong small scale variations in brittle fault architecture lead to low hydraulic conductivity correlations between the two tunnel tubes separated by 40m. Normal to fault strike larger groundwater inflows cluster within domains of several hundred meters in length. The hydraulic conductivities from pre-drilling analysis are used for inflow prediction for the excavated tunnel. Compared to the small-scale (50m) variations of hydraulic conductivity as derived from pre-drilling analysis the sampled volumes of long term tunnel inflows are at least one order of magnitude larger (>500m). This leads to very uniform steady state inflow rates of 500–1000m long monitoring sections for cumulative tunnel inflows. These inflows range between 3 and 5l/s·km and compare very well with the predicted tunnel inflows from geometric mean hydraulic conductivity as derived from pre-drilling analysis. [Copyright &y& Elsevier]

Abstract: In 1991, the steep slope above the village of Randa in southern Switzerland failed during two successive rockslide events three weeks apart. The failures, which resulted in the release of approximately 30×106 m3 of material, occurred in fractured gneissic rock whose principal foliation, faults and fracture zones dip into the slope. Above the scarp of the second rockslide, the mountain continues to move at rates of up to 2 cm/year. In an attempt to understand failure mechanisms in fractured crystalline rock masses, we have carried out a comprehensive multidisciplinary study that included detailed geological mapping, borehole logging and imaging, and surface and borehole geophysical surveys. An integrated interpretation of the resultant data has revealed the presence of three prominent fault- and fracture zone sets that define the internal structure of the rock mass. Some faults and fracture zones could be traced up to 85 m into the subsurface using borehole radar reflection methods. The dips of the faults and fracture zones estimated from surface mapping matched those from radar reflection and borehole televiewer surveys. No evidence was found for significant faults or persistent weakness planes that paralleled the 1991 rockslide failure surfaces. Instead, the new observations were consistent with dominant failure mechanisms that involve the progressive development of major shear and brittle fracture zones and the eventual destruction of intact rock bridges immediately prior to catastrophic collapse. This integrated study provides the first reliable model of the 3-D fault structure associated with and contributing to a large rock slope instability in a brittle crystalline rock mass. [Copyright &y& Elsevier]

Crystalline rock has been tested/selected by many countries for developing underground nuclear waste repositories at ~500 m depth to achieve geological isolation of high-level, long-lived radioactive waste. During the assessment period of up to one million years, large earthquakes may occur around the repository and trigger coseismic displacements along secondary fractures, jeopardising the integrity of the buffer-waste canister system. It is, therefore, of great importance to understand the coseismic behaviour of the repository site during large earthquakes. In this paper, we develop a finite element method-based seismo-mechanical model to simulate the response of fractured rocks subject to both in-situ stresses and seismic activities. The model can well capture the fracture displacement behaviour under dynamic loadings involving alternating regimes of contact loss, partial slip and total sliding. We model the earthquake-induced displacement field according to the seismic source theory in combination with a generic source time function. We apply the model to the nuclear waste repository site at Forsmark, Sweden, and analyse the coseismic responses of both single fractures and fracture networks during a potential post-glacial earthquake with a moment magnitude of M w = 5.6. The shear dislocation of a single fracture is strongly dependent on the fracture length and dip angle, while the displacement pattern of a fracture network is dominated by its "backbone" structures. We observe that significant coseismic shear displacement occurs if the fractured rock is close to the hypocentre and located in the dilational quadrant of the primary fault due to reduced shear resistance. However, the earthquake-induced shear displacement decreases drastically with the increased distance to the hypocentre and a distance of ~700 m may be needed for a fracture up to 100 m long to not displace beyond 5 cm. If two repeated earthquakes would occur, a distance of ~1200 m may be necessary. [ABSTRACT FROM AUTHOR]

Most natural rock slope failures result from long-term strength degradation. It is often hypothesized that glacial retreat leads to enhanced progressive damage accumulation in adjacent rock slopes, due to mechanical unloading and changes to the thermal and hydraulic boundary conditions. However, direct observations of subsurface processes in a rock slope subject to glacial retreat are rare. In this paper, we present the design, implementation, and performance of a new borehole monitoring system installed on a rock slope in fractured crystalline rock located beside the glacier tongue of the retreating Great Aletsch Glacier (Valais, Switzerland). The three vertical, 50 m deep boreholes were instrumented to continuously monitor groundwater pressure, temperature and deformation at high resolution, in order to investigate thermo-hydro-mechanical coupled slope processes that drive progressive rock mass damage. We show that the system is capable of measuring both reversible and irreversible displacements along single fractures at magnitudes ranging between 0.001 mm to >2 mm in the studied rock slope, and that it is often possible to identify drivers of these deformation signals. The transient subsurface temperature field shows clear indications of former ice occupation in form of cold temperatures preserved at depth and superimposed annual temperature cycles penetrating down to a depth of about 17 m. The variability of the pressure head in the slope is driven by annual snowmelt infiltration cycles, rainfall events, and the assumed connection to englacial water of the temperate glacier. The new and continuously growing dataset presented here will enable us to relate the changing boundary conditions caused by glacial retreat and fatigue from daily to annual thermal and hydraulic loading cycles to progressive rock mass weakening, which may ultimately result in rock slope failure. • We installed a borehole monitoring system in a glacier adjacent rock slope. • We measure subsurface temperature, pore pressure and strain during deglaciation. • This data is used to analyze thermo-hydro-mechanical coupled rock slope processes. • We document rock mass damage at the micrometer-scale linked to deglaciation. [ABSTRACT FROM AUTHOR]

Fractured rock-masses deform in response to changes in pore pressure. In alpine catchments, groundwater flow dynamics is generally dominated by strong recharge signal from snowmelt infiltration in spring / early summer and in early winter, and punctuated by large rainstorms in snow-free periods. In crystalline rock slopes, infiltration occurs along discontinuities, such as faults and joints. Under constant normal stress joint apertures increase non-linearly during pore pressure increase, increasing the volume of saturated rock masses and leading to surface deformation. The latter can be measured with high-resolution geodetic monitoring systems and remote sensing methods such as DInSAR.In this work, we discuss temporal and spatial variations of reversible ground surface deformation measured in the Aletsch region, Switzerland. The study area is situated in the vicinity of the tongue of the Great Aletsch Glacier, central Swiss Alps, in the gneissic and granitic bedrock of the Aar massif. The valley (1550 to 2300 m.a.s.l.), is roughly oriented parallel to the main Alpine foliation (NE-SW) and to major fault zones. Total annual precipitation is less than 1 m/y, but snow depth frequently reaches 2 m during winter, with strong spatial variability. We use a network of 4 continuous Global Positioning System (cGPS) stations and 90 reflectors linked to 2 total stations (TPS), operating since 2014, to track deformation at millimetric to centimetric scale. We use weather and hydrological data from surrounding stations to estimate the timing of infiltration.At some locations in our study area we observe peak-to-peak magnitude of seasonal deformation over 2 cm, whereas other areas show no significant seasonal deformation. Measured displacement vectors are generally oriented towards the valley center in spring, when deformation rates are the highest (>0.5 mm/d). The start of the high-rate of deformation period also varies spatially. The South-facing slope starts deforming around 20 days earlier than the North-facing slope. The vertical component of motion is generally smaller than the horizontal one, and often below the noise level. Finally, we discuss possible factors (e.g. topography and slope orientation, discontinuities' orientation and density, lithology, groundwater recharge magnitudes and durations) that might influence the spatial and temporal variations of reversible deformation on the slopes adjacent to the tongue of the Great Aletsch Glacier. [ABSTRACT FROM AUTHOR]

Abstract: The monitoring of slope displacements over time provides the basis for most rockslide early warning systems, yet the prediction of catastrophic failure from these records is highly problematic and tenuous, especially if the underlying kinematics and instability mechanism are poorly understood. An example is the moving slope above the town of Randa in the Swiss Alps. This slope is considered typical of those in crystalline rock that lacks a natural, highly persistent, weakness plane dipping towards the valley that can serve as a through-going detachment surface for kinematic release. In Part I (the companion paper to this), the findings from a comprehensive geological and geophysical investigation to image the internal structures of the unstable rock mass were presented. In this paper we develop a kinematic model that describes the pattern of displacement vectors for the rock mass along these structures, both on surface and at depth. The displacements were estimated from 5 years of data from an extensive monitoring system which included surface geodetic and crackmeter measurements, borehole inclinometer and extensometer measurements (up to 120 m depth), and microseismicity. The results showed that the displacement field is highly heterogeneous. Internal deformation is accommodated by both shear and opening-mode dislocation of faults and fracture zones which dip moderately to steeply into the slope. Microseismicity is most intense near the front of the scarp where surface translation in large part reflects the toppling of blocks accommodated by slip along the steeper dipping fractures. The small displacement rates at the study site of up to 2 cm/year coupled with a modest deviation of up to 15° from vertical of the boreholes posed severe problems for the estimation of horizontal displacements from the inclinometer data. We describe the error analysis of these data in some detail since it is relevant for similar installations elsewhere. Conclusions drawn from this work highlight the importance of integrating various types of data in order to better understand the complex block-kinematic processes whose evolution governs the long-term progressive failure of unstable rock slopes. [Copyright &y& Elsevier]

In February 2017, a total of six in-situ hydraulic shearing (HS) experiments were conducted in the framework of the decameter scale In-situ Stimulation and Circulation (ISC) project in crystalline rocks at the Grimsel Test Site, Switzerland. During these experiments, three ductile (containing very few distinct fractures) and three brittle-ductile (containing multiple fractures) shear zones were the target for high-pressure fluid injections. The injected fluid volume was ~1 m³ per experiment. The experiment volume has an overburden of ~480 m and the maximum principal stress is dipping 40° towards East with a magnitude of 13 MPa. In this contribution, we present the observations and preliminary interpretations of the hydromechanical rock mass response during the different experiments and compare them to the target shear zone type. To monitor the hydromechanical response, six boreholes were drilled into the rock mass. Three of them are dedicated to pressure monitoring in seven intervals covering the target shear zones. In the other three boreholes, a total of sixty longitudinal Fiber-Bragg Grating strain sensors were installed. These sensors have a base length of one meter and are installed across intact rock, fractures or shear zones. During most of the HS experiments, the near-wellbore injectivity was successfully increased to a magnitude that is similar for all experiments. The same was observed for the near-wellbore transmissivity. For both parameters, a stronger increase was observed at the ductile shear zones, where the initial parameter magnitudes were smaller. The jacking pressure was reduced during most experiments and the induced slip dislocation at the injection point was in the range of 0.5 - 1.5 mm. We observed a pressure increase up to a radial distance of ~16 m from the injection well during all experiments. Further away from injection location the pressure monitoring data suggest poro-elastic effects. Pressure fronts that were expected to propagate along the target shear zone were only observed during two experiments. The strain monitoring revealed a zone of transient variable deformation (i.e., mix of compression and extension) close to the injection location and a compressive zone further away from the injection location. The extent of the variable zone seems to depend on the target shear zone type and is larger for the ductile shear zone. During the experiments, a meter scale Poisson effect was directly measured in the rock mass. Additionally, the formation of new fractures was detected.Our data highlight the complexity of the rock mass and fracture response during a decameter scale hydraulic stimulation and provide a basis for the development of future large scale hydromechanical-coupled numerical models of stimulation experiments. [ABSTRACT FROM AUTHOR]