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The fractographic analysis of plumose axes of exfoliation fracture surfaces from a preceding study in the Grimsel region of the Swiss Alps suggested complex directional trends of near-surface (i.e., within ~100 m below ground surface) maximum principal stress ( σ 1 ) trajectories within the investigated inner-trough valleys (i.e., U-shaped valleys). The stress trajectories describe a pattern governed by local topographic variations. In situ stress measurements from the region are scarce, locally scattered, based on different methods, and, thus, difficult to interpret at regional scale. In this study, we inferred that plumose structure axes form parallel to compressive σ 1 , and improve our knowledge of the near-surface three-dimensional stress field in alpine settings with complex topography. We investigated near-surface stress tensors utilising three-dimensional, elastic numerical models. Our models account for morphological details of the Grimsel area at the decametre scale. We used two models with vertical boundaries aligned N–S/W–E (0°-model) and NW–SE/SW–NE (45°-model). These models allowed investigating gravitational stresses and superimposed isotropic and anisotropic compressive stresses arising from (active) tectonic shortening and/or stresses induced by exhumation (remnant stresses). Our model results illustrate that the superposition of gravitational stresses and realistic horizontal strains reveals complex near-surface stress trajectories that widely follow the patterns of exfoliation fracture plumose axes. Our models demonstrate large variations of stress orientations within the shallow subsurface, including depth levels where exfoliation fractures formed. These variations cannot be captured by a small number of local stress measurements. Our study reveals that directional data from exfoliation fracture plumose axes of Middle to Upper Pleistocene ages can be used to constrain geologically recent and current maximum principal stress directions of the shallow subsurface of up to a few hundred metres below ground. [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: Granitic rock mass of the upper Aar valley (Grimsel area, Switzerland) contains distinct generations of exfoliation joints, which formed during different stages of the Pleistocene, subparallel to distinct glacial valley palaeotopography. The bulk of exfoliation joints shows prominent, common fractographic features: (1) radial plumose structures with distinct plume axes; (2) arrest marks superimposed by plumose striations; and (3) gradually-developing en échelon fringe cracks. Multiple arrest marks reveal that exfoliation joints formed incrementally and, together with the absence of hackle fringes, suggest stable, i.e., subcritical fracturing conditions. Smooth transitions from plumose structures on the parent plane to en échelon fringe cracks, combined with non-systematic stepping senses of fringe cracks, suggest local (vs. temporal) stress field variations. Assuming that plume axes formed parallel to the maximum principal compressive stress (σ 1) enables us to infer near-surface palaeostress orientations and compare them with classical borehole-based in-situ stress data. The majority of plume axes suggest (1) persistently subhorizontal to slightly inclined σ 1 orientations at trough valley slopes and (2) near-surface variability of σ 1 orientations originating from topographic perturbation caused by glacial valley erosion superimposed on the regional stress field. Our investigations of fracture surface morphologies yield unique insights into exfoliation fracture formation, such as directional trends of fracture propagation and associated palaeostress orientations within Alpine valley slopes. [Copyright &y& Elsevier]

Abstract: Exfoliation joints are well-known natural fractures limited to near the ground surface. Relatively few details, however, are known about their distribution and age in the Swiss Alps. Exfoliation joints follow the landscape surface at the time of their formation; the age of the associated landscape feature then provides a maximum age of exfoliation joints. While landscape forms can change through time, exfoliation joints preserve elements of former landscape morphologies by their undisturbed orientations. The Grimsel region of the Central Alps is well-suited for analyzing the impact of erosional episodes, and accompanying stress changes, on exfoliation joint formation in granitic rocks. Mapping above and below ground revealed that exfoliation joints are widespread and occur between valley bottoms and mountain crests within glacial (inner and hanging U-shaped trough valleys, glacial cirques, and steep mountain crests) and predominantly fluvial landforms (gently inclined linear slopes above the inner trough valleys, narrow inner-valley gorges, and steep V-shaped side gullies). Based primarily on their geometric properties at the ground surface, three exfoliation joint types were distinguished in our study area: (1) closely spaced (<1m) joints oriented distinctly parallel to the present-day ground surface, (2) intermediately spaced (0.6–2m) joints that are nearly parallel (<10° difference) to today's mean ground surface at a 10-m scale, and (3) widely spaced (≫2m) joints not parallel to the ground surface. Relating the mapped distribution of exfoliation joint types to identified erosional episodes and landscape features of known and inferred ages, respectively, enables us to distinguish four exfoliation joint generations in the Grimsel area, which most likely formed during the lower Pleistocene (~1.5–1Ma), middle Pleistocene (~0.7–0.4Ma), upper Pleistocene (0.1–0.02Ma), and Late Glacial/Holocene (<0.02Ma). We demonstrate that the most prominent and deepest exfoliation joint generation is associated with erosion of the inner glacial troughs of the upper Aar valley, which likely occurred during the mid-Pleistocene Revolution. Our study shows how exfoliation joint episodes can be dated, and, conversely, that better knowledge of the distribution of exfoliation joint sets can reveal unique information about the morphological evolution of an Alpine valley. [Copyright &y& Elsevier]

Abstract: This paper summarizes the results from a subsurface exploration campaign carried out in fractured and karstified limestones and marls of the Lötschberg Base Tunnel in the Swiss Alps. The exploration program in the 3.2km long Doldenhorn Section consisted of 50 cored and 13 destructive horizontal predrillings of 250–300m length each (in total 14,853m of predrillings). These boreholes were used to locate possible tunnel inflows and characterize their hydraulic properties. A new efficient hydrogeological test protocol has been developed and was systematically and successfully applied. The test protocol consists of short constant rate outflow and recovery tests in open boreholes. The on-site test analysis allowed reliable predictions of the hydrogeological conditions ahead of the advancing tunnel face in brief period of time, leading to decisions regarding further reconnaissance drillings, tunnel driving or sealing measures. Detailed analysis of the transient pressure responses of the hydrogeological tests carried out after tunnel completion led to important new findings regarding flow model identification (type and orientation of conductive fractures), aquifer parameters and boundary effects. Even though the hydrogeological tests had to be carried out within a short time frame (a few hours) they allowed characterization of large scale aquifer properties relevant for long term tunnel inflow behavior. [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]

[Display omitted] • We explore THM coupled processes in a new TBM tunnel in the Opalinus Clay shale. • We present the evolution of in-situ state variables acquired over four years. • Radial total pressures (RTP) on the tunnel support are heterogeneous. • RTP reflect construction activities and long-term tunnel climate variations. • RTP reveal swelling, gap grout yielding, and are consistent with lab swelling tests. The old Belchen tunnel tubes in the Swiss Jura Mountains were excavated with drill-and-blast in swellable sedimentary rocks, i.e., anhydrite-rich marls (Gipskeuper) and Opalinus Clay shale (OPA). Already during construction in the 1960s both rock formations caused substantial damage to the tunnel support through high swelling pressures and heave, and in later years the tubes had to be refurbished again. Important maintenance and repair prompted the construction of a new, third Belchen tunnel tube (2016–2021) with a tunnel-boring machine (TBM). In this study we present in-situ datasets acquired to investigate the stress evolution and controlling mechanisms over more than four years at a monitoring section located in a strongly faulted OPA section of the new Belchen tunnel tube. The main datasets include time series of total radial pressure, radial strain, rock water content, rock and concrete temperatures, as well as details of the geological structures obtained from analyses of borehole logs and three-dimensional photogrammetric excavation face models. Finally, a series of idealised numerical simulations explore the impact of measured temperature variations on the measured total pressures, which confirm a strong temperature effect on radial pressures related to the setting of concrete and seasonal climatic variations. We find that in our monitoring section radial pressures on the tunnel support are very heterogeneous, i.e., they range between 0.5 MPa and 1.5 MPa, and still gently increasing 4 years after excavation. The measured pressures are 2–5 times greater than measured in the old Belchen tunnel tubes and similar in magnitude to swelling pressures obtained in laboratory tests. EDZ permeability measurements, water content evolution, and radial strain data from the tunnel invert suggest that swelling processes contribute to the long term radial pressure build-up. Thermo-elastic deformation and swelling might be superimposed by local reactivation of tectonic faults and gap grout cracking at crack-initiation stress levels. [ABSTRACT FROM AUTHOR]

We present a combined experimental and numerical study of failure and damage in faulted Opalinus Clay shale around an opening at the Mont Terri Rock Laboratory, Switzerland. An experiment borehole with a diameter of 0.6 m and a length of 12.9 m intersecting a major fault zone at an acute angle of ∼ 40 ° was drilled, with subsequent overbreaks closely monitored. To investigate the underlying mechanisms that lead to the observed overbreaks, we developed a 3D geomechanics model to simulate the deformation and failure behaviour of faulted Opalinus Clay shale. To represent geological structures including the major fault zone and secondary fracture sets, a site-specific fracture network was constructed. Our model captured many important geomechanical properties and responses of the faulted Opalinus Clay shale, such as anisotropy of the shale matrix, deformation of the fault zone, dislocation of secondary fractures, and growth of new cracks as well as generation of overbreaks around the opening. We compared our simulation results with in-situ experimental observation of short-term overbreak patterns along the borehole and found that the overbreak occurrence is strongly controlled by both stress conditions and geological structures. Our results indicate that the damaged zone is characterised by an inner shell and an outer shell, where the former (dominated by extensive tensile/shear cracks) has a thickness of about one quarter of the borehole diameter and the latter (dominated by sparse shear cracks) extends by half to one borehole diameter into the surrounding rock. We further elucidated the impacts of the major fault zone and secondary fracture sets as well as the borehole–fault intersection angle on the deformation, damage, and failure characteristics. The findings and insights obtained in this study have important implications for the stability evaluation of waste emplacement drifts and long-term safety assessments of nuclear waste galleries in faulted argillaceous rocks. • 3D simulation captures overbreak and damage of Opalinus Clay around an opening. • Interplay of stress and heterogeneity drives overbreak formation and pattern. • Damaged zone of a two-shell structure rotates with the distance to a fault zone. • Acute angled faults exert strong controls on overbreak and damage generation. [ABSTRACT FROM AUTHOR]

Abstract: In this study, the rock mass response ahead of an advancing test tunnel in the Opalinus Clay at the Mont Terri Rock Laboratory (Switzerland) was investigated. Characterisation of the excavation-induced damage zone at Mont Terri is a challenging task due to the anisotropic and heterogeneous nature of the shale: pronounced bedding leads to intact rock anisotropy and prevalent small-scale tectonic shears lead to rock mass heterogeneity. Rock mass damage ahead of an experimental tunnel or niche was characterised through single-hole seismic wave velocity logging, borehole digital optical televiewer imaging, and geological drillcore mapping. Three-dimensional elastic stress analyses were completed and showed that rock mass degradation can be correlated to changes in the maximum to minimum principal stress ratio (i.e., spalling limit). Numerical results showed that close to the niche boundary, unloading lowers stress ratios, which correspond with decreasing seismic wave amplitudes and velocities; thus, indicating that strength degradation resulted from increasing crack-induced damage. Considerations of tectonic shears and distance from a previously stressed volume of rock were necessary in understanding both the damage state and extent ahead of the face. By integrating field and numerical data, the investigation showed that geological structures (i.e., bedding and bedding-parallel tectonic shears) were most influential near the entrance but played a lesser role as the niche deepened. Additionally, a portion of the niche is located in the perturbed zone of the intersecting Gallery04. [Copyright &y& Elsevier]

Abstract: The Opalinus Clay is currently under investigation as a potential host rock for deep geological disposal of nuclear waste at the Mont Terri Rock Laboratory in Switzerland. Bedding in the Opalinus Clay at Mont Terri is ubiquitous and highly persistent leading to mechanical transverse isotropy. Adding to the complexity at the Rock Laboratory is the frequent occurrence of small-scale tectonic shears. This paper explores the influences of millimetre-thick tectonic shears and bedding on the development of induced fractures mapped in the EZ-B field experiment at the research facility. A series of numerical analyses were carried out by increasing the geological complexity of the host rock and comparing the redistributed stress field with geological maps of the induced fractures. The analyses show that if tectonic shears are not kinematically constrained, mobilisation of the shears can play a key role in the development of the induced fracture network and therefore, be a primary factor in the development of the excavation damaged zone. This illustrates that under certain conditions rock mass heterogeneity (in this case, resulting from the tectonic shears) may dominate over rock matrix anisotropy (in this case, resulting from bedding) and must be considered when predicting the induced fracture network of the excavation damaged zone. [ABSTRACT FROM AUTHOR]

The Gotthard Base Tunnel (GBT), constructed between 2000 and 2011, is a 57 km long and up to 2.5 km deep high-speed railway tunnel located in the Swiss Alps. Significant ground surface displacements reaching about 10 cm were observed during and after the tunnel construction. To gain a better understanding of the causal mechanism of such conspicuous ground deformations, we develop a three-dimensional (3D) fully-coupled hydro-mechanical model to simulate GBT-induced groundwater drainage, stress redistribution, rock mass consolidation, and fault zone deformation at the regional scale. First, we construct a geological model with topographical features, lithological units, and natural faults realistically represented. We constrain the material properties of fault zones and rock masses based on available extensive laboratory testing results and site characterisation datasets. We then simulate the tunnelling process over time, with the resulting coupled hydro-mechanical responses of faulted rock masses well captured and ground surface/subsurface displacements quantitatively analysed. The simulation results in general show a good agreement with the field monitoring data of ground surface displacement, subsurface tunnel settlement, and groundwater inflow into the tunnel. Our model indicates that ground surface displacements originate from GBT-induced water drainage and rock mass consolidation in the deep subsurface. Our results also show that the GBT construction could trigger faults to shear via drainage-induced pressure diffusion and poroelastic stressing. The research findings from our work have important implications for many groundwater drainage-related geoengineering activities such as underground excavation in alpine mountains and fluid withdrawal in subsurface reservoirs. [Display omitted] • 3D simulation captures tunnelling-induced groundwater flow and rock mass deformation. • The simulation explains ground subsidence in crystalline rock during deep tunnelling. • Drainage and consolidation in deep subsurface cause ground surface displacements. • Tunnelling-induced pressure diffusion and poroelastic stressing drive fault movement. [ABSTRACT FROM AUTHOR]

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 affects elastic wave transmission. In this work, we extend our understanding of moisture-induced elastic changes in a microcracked nanopore-dominated medium where 80 % of the surface area exhibits pore diameters (also include microcrack widths) below 10 nm. We study acousto-mechanical response resulting from a gradual wetting on a free-standing intact Herrnholz granite specimen over 98 h using time-lapse ultrasonic and digital imaging techniques. Linkages between ultrasonic attributes and adsorption-induced stress/strain are established during the approach of the wetting front. We find that Gassmann theory, previously validated in channel-like nanoporous media, does not work properly in predicting the P-wave velocity increase of microcracked nanopore-dominated media at ultrasonic frequency. However, squirt flow – a theory recognized to characterize wave velocity increase and attenuation in microcracked macropore-dominated media at the pore scale – also accounts for the observed increase of P-wave velocity in microcracked nanopore-dominated media. The transmitted amplitude changes 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. • Acousto-mechanical response of a free-standing intact specimen to gradual wetting. • Squirt flow dominates P-wave velocity increase in microcracked nanoporous media. • Amplitude changes are explained by elastic wave propagation around Fresnel zone. [ABSTRACT FROM AUTHOR]

Abstract: Surface subsidence associated with the construction of a headrace tunnel in the Swiss Alps at more than 2000m above sea level (a.s.l.) has been detected at two locations with satellite differential Synthetic Aperture Radar (SAR) interferometry. At the first location, a subsidence trough of about 4cm in the satellite line-of-sight direction following the headrace tunnel axes has been measured between August 1995 and August 1996. Similar values from SAR data of ascending and descending orbits indicate displacements in the vertical direction of the movement. In the second case, a symmetric cone of depression with a maximum displacement of about 4cm between 1995 and 1997 has been observed above the tunnel. Differences in the results from satellite SAR data of ascending and descending orbits indicate that the direction of displacement in this second case was not entirely vertical. Large-scale consolidation associated with pore-pressure reduction in the rock mass arising from tunnel drainage at about 200–400m depth beneath the topographical surface is believed to be the contributing mechanism (Zangerl et al., 2008a, 2008b). Evidence for this process is based on pore pressure recordings in nearby deep wells. In both areas, the subsidence was followed by a small uplift of about one centimeter between 1997 and 1999, after the tunnel was cased with permeable concrete segments. This partial recovery is also visible in pore pressure records and can be related to the elastic components of rock mass deformation. [Copyright &y& Elsevier]

Slope deformation in mountainous terrain can be driven by different processes, the nature of which is either gravitational and irreversible or seasonal and reversible, the latter induced by permafrost variations or by hydromechanical or thermomechanical effects. The importance of identifying such deformations is not only related to the hazard they can pose, but also to the understanding of changes that permafrost or local hydrological conditions undergo. Here, we carry out a multi-temporal InSAR analysis over a mountainous area 8000 km2 large, straddling north-western Bhutan and southern Tibet. We propose a methodology to separate locally deforming areas from the effects of long-wavelength signals through the analysis of the spatio-temporal behaviour of 4-years long time series of surface displacements. We present the mapping of hundreds of small-scale features that appear to be actively deforming, as well as several examples of reversible deformation rarely detected at this scale in such a challenging and vast region. The analysis of the multi-annual trend of ground deformation shows a relatively small number of irreversible gravitational movements clearly related to rock slides which attests a low level of recent activity of large rock slope instabilities in the region. In the southernmost, lower elevation parts of the study area, we quantify reversible surface displacements with amplitudes ranging between 5 and 17 mm, and showing maximum displacements towards the satellite in summer months, thus compatible with hydro-mechanical effects related to groundwater table variations. In addition, the ground movement induced by the active layer's response to thawing and freezing over the gentler slopes and high-elevation permafrost regions of Bhutan and southern Tibet is on average around 10 mm, with maxima up to 28 mm. The localised displacements appear to be largely associated to braided stream plains, glacier outwash plains or low angle, fine sediment covered slopes. • Inventory of gravitational and reversible surface displacements for NW Bhutan • Classification of slope processes made on the basis of time series trends • Reversible, mostly vertical deformation, observed in the permafrost region • Reversible deformation with horizontal component observed at lower elevations • Rock glaciers appear to have lower creep rates than in the Alps. [ABSTRACT FROM AUTHOR]