Mechanical and hydraulic properties of the excavation damaged zone (EDZ) in the Opalinus Clay of the Mont Terri rock laboratory, Switzerland

Construction of cavities in the subsurface is always accompanied by excavation damage. Especially in the context of deep geological nuclear waste disposal, the evolving excavation damaged zone (EDZ) in the near field of emplacement tunnels is of utmost importance concerning safety aspects. As the EDZ differs from the intact host rock due to enhanced hydraulic transmissivity and altered geomechanical behavior, reasonable and location-dependent input data on hydraulic and mechanical properties are crucial. Thus, in this study, a hydromechanical characterization of an EDZ in the Mont Terri underground rock laboratory, Switzerland, was performed using three different handheld devices: (1) air permeameter, (2) microscopic camera and (3) needle penetrometer. The discrete fracture network (DFN), consisting of artificially induced unloading joints and reactivated natural discontinuities, was investigated by a portable air permeameter and combined microscopic imaging with automatic evaluation. Geomechanical and geophysical characterization of the claystone was conducted based on needle penetrometer testing at the exposed rock surface. Within the EDZ, permeable fractures with a mean hydraulic aperture of 84 ± 23 µm are present. Under open conditions, self-sealing of fractures is suppressed, and cyclic long-term fracture aperture oscillations in combination with closure resulting from convergence processes is observed. Based on measured needle penetration indices, a uniaxial compressive strength of 30 ± 13 MPa (normal to bedding) and 18 ± 8 MPa (parallel to bedding) was determined. Enhanced strength and stiffness are directly related to near-surface desaturation of the claystone and a sharp decrease in water content from 6.6 wt % to 3.7 wt %. The presented methodological approach is particularly suitable for time-dependent monitoring of EDZs since measurements are nondestructive and do not change the actual state of the rock mass. This allows for a spatially resolved investigation of hydraulic and mechanical fracture apertures, fracture surface roughness, and physico-mechanical rock parameters and their intra-facies variability.

1 Introduction

For all types of man-made underground structures, the formation of a so-called excavation damaged zone (EDZ) or excavation disturbed zone (EdZ) is inevitable (Pusch and Stanfors, 1992; Shen and Barton, 1997). As geologic formations are affected by regional or local stress fields, stress redistribution during excavation leads to displacement and convergence, accompanied by the formation of unloading fractures in the rock mass around the cavity (Bossart et al., 2002). The EDZ is characterized by severe hydraulic, mechanical and geochemical modifications as well as newly formed connected porosity (Dao et al., 2015; Kupferschmied et al., 2015; Labiouse and Vietor, 2014; Sato et al., 2000; Yong et al., 2017). Thus, significant changes in flow and transport properties can be observed in the EDZ due to an enhanced permeability of the connected fracture network creating preferential flow paths. In the EdZ, flow and transport properties are only scarcely affected (Bossart et al., 2002, 2004; Tsang et al., 2005).

The EDZ and its impact on hydraulic and mechanical rock properties are of particular importance for the underground storage of radioactive material (Blümling et al., 2007; Fairhurst, 2004). According to the current state of knowledge, multi-barrier systems for geological disposal are the preferred option for effectively isolating high-level nuclear waste and spent fuels (Birkholzer et al., 2012; Chapman and Hooper, 2012). A service life of up to 1 million years will essentially be guaranteed by the sealing function of a natural barrier (Apted and Ahn, 2010; Wilson and Berryman, 2010). In Switzerland, the Opalinus Clay, an overconsolidated Jurassic claystone, was selected as a host rock for deep geological storage of high-level radioactive waste (Bossart et al., 2017; Nagra, 2002). In the context of host rock characterization and site assessment, the Mont Terri generic underground rock laboratory (URL) provides a valuable site for research, testing and development of in-depth technical know-how. Since 1996, numerous studies and experiments have been conducted in order to evaluate essential properties of the undisturbed and altered rock, as well as to examine the behavior of the Opalinus Clay when exposed to short- or long-term THMC (thermal, hydrological, mechanical and chemical) impacts (Bossart et al., 2017; Pearson et al., 2003). Besides the Mont Terri URL in Switzerland, a number of underground laboratories in other countries and their potential or selected host rock formations are in operation, mainly in crystalline rocks (e.g., Äspö Hard Rock Laboratory in Sweden) and plastic or indurated clays (e.g., HADES URL in Belgium, Meuse/Haute-Marne URL and Tournemire URL in France) (Blechschmidt and Vomvoris, 2010; Delay et al., 2014). Similar to the Opalinus Clay in Switzerland, the EDZ and its impact on the hydromechanical characteristics of the rock mass in the near field of underground structures are of particular interest for the Callovo–Oxfordian claystone in France (e.g., Armand et al., 2014; Baechler et al., 2011; Menaceur et al., 2016) and for the Boom Clay in Belgium (e.g., Bastiaens et al., 2007; Dao et al., 2015).

In the Opalinus Clay of the Mont Terri URL, the EDZ is characterized by a significantly enhanced hydraulic conductivity of 1 × 10⁻¹⁴ to 1 × 10⁻⁵ m s⁻¹ (Bossart et al., 2004; Jaeggi and Bossart, 2014; Marschall et al., 2017), whereas for undisturbed conditions it ranges between 2 × 10⁻¹⁴ and 5 × 10⁻¹² m s⁻¹ (Jaeggi and Bossart, 2014; Lavanchy and Mettier, 2012). Within the EDZ, advective transport is facilitated due to fracture permeability, which is several orders of magnitude higher than the matrix permeability of the claystone (Marschall et al., 2017). Hydraulic fracture parameters such as permeability, transmissivity and flow rate are in turn directly related to the hydraulic fracture aperture a_h (Zimmerman and Bodvarsson, 1996), which therefore represents a key parameter for assessing the hydraulic characteristics of a fractured rock mass or an EDZ. The hydraulic aperture is usually derived from the cubic law (Louis, 1969; Snow, 1965) and relates to the mean opening width of a fracture accessible to advective transport. Due to the confirmed self-sealing capacity of the Opalinus Clay caused by swelling of mixed-layer illite–smectite clay minerals (e.g., Bernier et al., 2007), the hydraulic conductivity of the EDZ is expected to decline within a period of several tens to hundreds of years by progressive fracture closure (Jaeggi and Bossart, 2014). In addition, fractured rock masses are also characterized by a pronounced hydromechanical coupling; i.e., changes in the mechanical stress state result in changes in permeability and therefore hydraulic fracture aperture (Cammarata et al., 2007; Min et al., 2004; Rutqvist and Stephansson, 2003). Generally, a_h is nonlinearly linked to the mechanical fracture aperture a_m as a function of fracture surface roughness (Blum et al., 2009; Renshaw, 1995), for example via the Barton–Bandis model using the joint roughness coefficient (JRC) (Barton, 1982; Barton et al., 1985). The mechanical fracture aperture represents the average geometrical distance between the fracture surfaces (e.g., Hakami and Larsson, 1996) and is needed to examine the response of fracture networks due to normal or shear stresses (e.g., Blümling et al., 2007; Cuss et al., 2011; Zhang, 2016) and mechanical self-sealing of artificial fractures (e.g., Marschall et al., 2017; Nagra, 2002).

Similar to the hydraulic properties, mechanical properties of the Opalinus Clay diverge significantly depending on direction, facies and stress regime (Bock, 2009; Giger et al., 2015). Furthermore, due to a clay-specific hydromechanical coupling (Amann et al., 2017; Marschall et al., 2017), geomechanical parameters such as uniaxial compressive strength, tensile strength, shear strength and the Young's modulus of the Opalinus Clay generally increase with decreasing water content (Blümling et al., 2007; Wild et al., 2015). Furthermore, geomechanical properties of the Opalinus Clay in the EDZ are also modified in comparison to the undisturbed rock mass. In the short term, a reduction in effective stress caused by pore pressure excess in the vicinity of the advancing excavation front leads to early damage of the rock around the cavity. Right after excavation, pore water drainage and increased suction of the rock mass can be observed close to the cavity (Amann et al., 2017; Giger et al., 2015). In the long term, a general decrease in water content caused by dehydration of the rock leads to locally enhanced rock strength and stiffness (Wild et al., 2015).

An accurate and comprehensive hydraulic and mechanical characterization of the EDZ is therefore essential for confirming the integrity of the host rock in terms of risk and performance assessment (e.g., Blum et al., 2005; Popp et al., 2008; Tsang et al., 2015; Xue et al., 2018). This key information serves as an appropriate starting point for numerical modeling studies investigating the development of the EDZ in the post-closure phase of the repository, and it is also useful for the selection and adaptation of engineering designs or adequate constructional measures (e.g., Hudson et al., 2005; Marschall et al., 2017; Nagra, 2019; Tsang et al., 2012). This not only applies to the issue of nuclear waste disposal, but also generally to other underground structures in different geological materials and settings (e.g., Li et al., 2012; Sheng et al., 2002; Wu et al., 2009).

Hydraulic fracture apertures are usually determined in the laboratory by permeameter tests, with either gases or liquids being used to flow through fractured rock samples (Kling et al., 2016; Li et al., 2018; Shu et al., 2019; Zhang, 2018). In the field, hydraulic properties can be derived from hydraulic or pneumatic borehole tests (Aoyagi and Ishii, 2019; Jakubick and Franz, 1993; de La Vaissière et al., 2015; Shao et al., 2008). Mechanical fracture apertures can generally be obtained by different fracture imaging methods, whereby visibility can be improved by injecting dyed or fluorescent resin into the fractured rock (Armand et al., 2014; Bossart et al., 2002).

Seismic velocity measurements can be carried out in the laboratory by using ultrasonic pulse devices (Popp et al., 2008; Wild et al., 2015) and in the field, for example, by applying mini-seismic methods (Schuster et al., 2017). Geomechanical strength and deformation parameters are usually determined by laboratory experiments. For this purpose, many different test setups are utilized such as compressive strength tests, tensile strength tests, shear tests and triaxial tests under drained or undrained conditions. For the Opalinus Clay, numerous geomechanical tests were carried out on drill cores, primarily examining bedding anisotropy in addition to the hydromechanical coupling by adapting the water content of the samples (Amann et al., 2011, 2012, 2017; Wild et al., 2015). In the field, handheld probes such as a Schmidt hammer or needle penetrometer are used to estimate the uniaxial compressive strength and other mechanical parameters of rock material (Aydin, 2009; Buyuksagis and Goktan, 2007; Erguler and Ulusay, 2009; Hucka, 1965; Okada et al., 1985; Ulusay and Erguler, 2012).

For most investigations drilling is required, either directly for performing borehole tests or for taking standard-compliant samples. However, drilling is not always feasible and boreholes also affect the EDZ by creating additional fluid pathways. Core samples do not necessarily reflect the initial state as they can suffer from disturbance or damage during extraction and transport, leading, for example, to a change in water content. Hence, the objective of this study was to investigate the hydromechanical properties of the EDZ in the Opalinus Clay of the Mont Terri URL from in situ measurements on the exposed rock surface. We carried out a nondestructive and holistic determination of hydraulic and mechanical parameters of the fractured rock mass around a small tunnel niche by combining transient-flow air permeametry, photomicroscopy and needle penetration tests. We characterized bulk rock properties of the claystone and quantified mechanical and hydraulic apertures of different fracture types of the EDZ, since these discontinuities can significantly control the overall material and flow behavior. We have also explored the alteration of the non-lined niche that was directly exposed to air for several years. By using the water content of the claystone, we compared the determined physico-mechanical parameters with data from other studies to assess the effect of desaturation directly on-site at the tunnel wall.

Figure 1 (a) Location of the Mont Terri underground rock laboratory (URL) alongside the security gallery of the Mont Terri motorway tunnel and the EZ-B niche situated in the shaly facies of the Opalinus Clay. (b) Photo and dimensions of the EZ-B niche where data acquisition was conducted. In the entrance area, shotcrete partly covers the rock surface on the left and right side wall.

2 Material and methods

2.1 Study site

Fieldwork was performed in the EZ-B niche of the Mont Terri underground rock laboratory (URL) in St. Ursanne, Switzerland (Fig. 1a). The axis of the niche is oriented almost normal to Gallery 04 and to the minimum principal stress direction of the in situ stress field (Yong et al., 2010). The niche is located in the upper shaly facies of the Opalinus Clay, which consists of dark gray, mostly mica and pyrite containing calcareous silty–sandy claystones (Hostettler et al., 2017). Bedding dips 45^∘ towards 150^∘, and thus the niche axis is oriented perpendicular to the strike of the bedding. As the URL is located in the southern limb of the Mont Terri overthrust anticline, the Opalinus Clay has experienced tectonic deformation (Nussbaum et al., 2011). As a consequence, pre-existing natural discontinuities, i.e., bedding-parallel tectonic faults, steeper splays and bedding planes, are present (Nussbaum et al., 2005). The EZ-B niche provides direct access to the overconsolidated claystone of the shaly facies of the Opalinus Clay and to the excavation-induced fracture network of Gallery 04. It was excavated from December 2004 to March 2005 mainly by a road header and pneumatic hammering (Nussbaum et al., 2005). Numerous experiments were carried out in the niche, focusing, for example, on determining the extent and degree of damage of the EDZ (Schuster et al., 2017), fracture network analysis and small-scale mapping (Nussbaum et al., 2011; Yong, 2007), or long-term hydromechanical coupling processes (Möri et al., 2010; Ziefle et al., 2017).

Excavation-induced unloading joints (EDZ fractures) that are related to the construction of Gallery 04 are present within the first 1.3 m of depth into the EZ-B niche (Nussbaum et al., 2005). Strike direction is mostly parallel to Gallery 04 and therefore perpendicular to the axis of the niche. At greater distances, artificial EDZ fractures that originate from the excavation process of the EZ-B niche itself are mainly oriented parallel to the side walls. In addition to the artificially induced unloading fractures, the EDZ also includes tectonic faults and splays, referred to as tectonic fractures. These tectonic fractures were reactivated by stress redistribution and convergence processes after the niche excavation and therefore show measurable fracture apertures (Nussbaum et al., 2005, 2011). In contrast, tectonic discontinuities outside the EDZ are completely closed. In the entrance area of the EZ-B niche, the rock is partly covered by shotcrete, making a section of the EDZ inaccessible (Fig. 1b). The on-site measurements in the Mont Terri URL were carried out on 16–17 April 2019. At that time, the average air temperature in the EZ-B niche was 16.5 ^∘C, while relative humidity was in the range of 67 %–72 %.

2.2 Air permeameter

A handheld transient-flow air permeameter (model TinyPerm 3, New England Research, Inc.) was used to measure the hydraulic aperture (a_h) of accessible fractures in the EZ-B niche. The working principle of the device was outlined by Brown and Smith (2013) and illustrated in Fig. 1 of Hale et al. (2020b). Further specifications are provided by New England Research, Inc. (2015). For each fracture, measurement was repeated at least three times. In the case that the mean absolute deviation of measured values was above 10 µm, the measurement was continued. Clear outliers were rejected in order to eliminate erroneous data, e.g., caused by fracture fillings (dust or loose material) or by leaks at the rubber nozzle tip of the air permeameter. Hydraulic fracture apertures are determined directly based on the time-dependent pressure equilibration and the internal calibration of the device (Brown and Smith, 2013; New England Research, Inc., 2015). Thus, no post-processing of data is required for the air permeameter.

For most rocks, the hydraulic aperture derived from air permeameter measurements agrees with the hydraulic aperture available for advective flow. For sandstone, this was demonstrated by Cheng et al. (2020), wherein the air permeameter was validated by steady-state flow tests and different types of artificial fractures with apertures ranging between 7 and 62 µm. For all tested samples, hydraulic apertures were in excellent agreement, with deviations below 5 µm (Cheng et al., 2020). Since clay minerals represent the main constituents of the Opalinus Clay (Bossart and Thury, 2008), diffusive double layers (DDLs) are formed on exposed clay mineral surfaces in water-saturated fractures (Soler, 2001), which could potentially reduce the hydraulic aperture of fractures in argillaceous rocks. For the Opalinus Clay, the maximum thickness of the DDL is only 22 nm, which can be approximated by the Debye length (Wigger and Van Loon, 2018) using representative pore water ionic strength values (e.g., Pearson et al., 2003; Van Loon et al., 2003). Thus, in this case the DDL effect on a_h is negligible.

2.3 Microscope camera

For the same set of fractures (Sect. 2.2), high-resolution images of fracture traces were taken with a microscope camera (DigiMicro Mobile, dnt GmbH) in order to estimate mechanical fracture apertures (a_m) in the EZ-B niche. The digital camera, with an image resolution of up to 12 million pixels, is comprised of a microscope with an adjustable magnification factor of up to 240. By adjusting the focus dial, the rock surface can be brought into sharp focus. Subsequently, the set magnification factor has to be recorded to evaluate the images. While taking the photo, the field of view should be aligned parallel to the fracture axis and the camera should look vertically into the fracture.

Microscope camera images can be evaluated both manually and automatically. The arithmetic mean of distances measured evenly along the fracture trace corresponds to the mechanical fracture aperture a_m, whereas the associated standard deviation (${\mathit{\sigma }}_{a_{\text{m}}}$) provides a reasonable measure for fracture surface roughness (e.g., Brown, 1987; Kling et al., 2017). The manual evaluation method uses image analysis software to determine the distance between the two fracture edges regularly along the imaged segment. For a detailed description of the manual image analysis approach, we refer to Hale et al. (2020b). A minimum of 20 distance measurements was needed to gain representative mechanical apertures. Additionally, an automatic approach for determining a_m and ${\mathit{\sigma }}_{a_{\text{m}}}$ was applied in this study. The code for running the workflow in Fig. 2 is written in MATLAB (see “Data and code availability”). As input data, microscopic grayscale images with specified magnification factors are used. For an applied image resolution of 9 million pixels, the resulting image size is 3456 pixels in the x direction. As the images should be cropped adequately before analysis according to the extent of the fracture void area, the image size in the y direction is variable (Fig. 2).

Figure 2 Workflow of the automatic approach for determining the mechanical fracture aperture based on microscope camera images of fracture trace segments.

The automated workflow is delineated in Fig. 2 and involves four steps. Based on the grayscale values of the image (0–255), a suitable threshold value T is first defined in order to segment the void area (region of interest) as precisely as possible. As a second step, the image is binarized; i.e., a value of 1 is assigned to the pixels of the void area and a value of 0 is assigned to all remaining pixels. Based on the resulting binary pixel matrix, the total number of void pixels (v) is determined column-wise. In order to convert the number of void pixels into aperture values a (in µm), the real length of one image pixel is required as a conversion factor. The pixel length directly depends on the magnification of the microscope camera that was set when taking the photo. It can be determined by using the software PortableCapture (Hale et al., 2020b). Finally, the mechanical aperture of the analyzed fracture segment corresponds to the arithmetic mean of the computed aperture values in a. If the fracture trace deviates from the x direction of the image (denoted by angle α), a_m is corrected accordingly. Using a_m and ${\mathit{\sigma }}_{a_{\text{m}}}$, hydraulic fracture apertures a_h can be subsequently estimated by applying different empirical equations (Table 1).

Table 1 Equations for estimating the hydraulic fracture aperture based on the mean mechanical fracture aperture and the standard deviation of measured distance values along a fracture trace.

2.4 Needle penetration test

A needle penetrometer device (model SH-70, Maruto Corporation Limited, Japan) was used to determine the needle penetration index (NPI) of the Opalinus Clay normal and parallel to bedding, which is directly dependent on the strength of the rock (e.g., Ulusay and Erguler, 2012). For testing, the needle is pushed into the rock by manually applying a maximum load of 100 N. The quotient of the applied load and the attained needle penetration depth (in N mm⁻¹) corresponds to the NPI (Aydan et al., 2014; Ulusay et al., 2014). Needle penetrometer tests were carried out at different measurement points on the rock surfaces in the EZ-B niche. If microcrack formation around the needle hole or tensile splitting along bedding planes was observed, the measured value was excluded from the dataset. For a detailed description of the working principle and testing procedure we refer to the ISRM-suggested method for needle penetration testing by Ulusay et al. (2014).

Several physico-mechanical parameters are directly related to the NPI. For example, a strong correlation between the NPI and the uniaxial compressive strength of intact rock was proven (e.g., Aydan, 2012; Uchida et al., 2004). Established empirical equations were used in this study to estimate uniaxial compressive strength (UCS), Brazilian tensile strength (BTS), Young's modulus (E), elastic P-wave (v_P) and S-wave velocity (v_S), cohesion (c), and friction angle (φ) (Table 2). In order to enable a direct comparison of the estimated parameters with existing literature data, the water content of the Opalinus Clay was additionally determined by oven drying according to DIN EN ISO 17892-1 (2015-03) using two representative rock specimens from the walls of the EZ-B niche, which were sampled at the time of the on-site measurements.

Figure 3 (a) Structural maps of the EZ-B niche with measurement points for hydraulic and mechanical aperture determination on the left (closed symbols) and right side wall (open symbols), modified after Nussbaum et al. (2005). (b) Hydraulic fracture apertures measured by the air permeameter plotted against the distance to Gallery 04. On the right side, the distribution of a_h is visualized by a probability density function obtained by kernel density estimation (KDE).

3 Results and discussion

3.1 Hydraulic and mechanical fracture properties

3.1.1 Measured hydraulic fracture aperture

The hydraulic aperture a_h of artificially induced unloading joints, reactivated fault planes, and bedding-parallel desiccation or unloading cracks of the EDZ in the EZ-B niche of the Mont Terri URL was determined at 43 measuring points on both side walls using the handheld transient-flow air permeameter (Fig. 3a). The mean hydraulic fracture aperture in the EZ-B niche was 84 ± 23 µm, with values in the range of around 100 µm occurring most frequently (Fig. 3b). On average, artificially induced unloading fractures (hereinafter referred to as EDZ fractures), mainly oriented sub-parallel to the axis of Gallery 04, showed the smallest hydraulic apertures of 61 ± 30 µm (n=9) compared to reactivated fault and bedding planes. They were also characterized by the largest range of measured aperture values from 20 to 100 µm, which is also evident from the high standard deviation.

Table 2 Equations for the estimation of physico-mechanical rock parameters using the needle penetration index (NPI), taken from Ulusay and Erguler (2012), Ulusay et al. (2014), and Aydan et al. (2014). The relations are based on compiled experimental data obtained from various lithologies.

^a Clay. ^b Mudstone, sandstone, siltstone, marl, lignite, tuff, soapstone, pumice, soft limestone, sheared shale. ^c Marl, siltstone, mudstone, tuff. In Ulusay and Erguler (2012), the term needle penetration resistance (NPR) is used instead of NPI.

Reactivated tectonic discontinuities, namely fault planes and splays of the SSE-dipping thrust system (hereinafter referred to as tectonic fractures), showed an average hydraulic aperture of 89 ± 18 µm (n=31). The hydraulic fracture aperture of bedding-parallel cracks was highest (94 ± 8 µm), although the obtained average value cannot be considered representative due to a small number of measurements (n=3). On the right side wall of the EZ-B niche, most of the sampling points were arranged near borehole BEZ-B1 (Fig. 3b) due to good accessibility and beneficial surface conditions. Based on the measured aperture values, no indication of a “borehole damaged zone” (Amann et al., 2017) is observable, which is related to the fact that borehole BEZ-B1 is oriented perpendicular to the strike of the bedding.

The on-site aperture measurements in the EDZ clearly show that about 15 years after excavation, hydraulically open fractures and therefore accessible fluid pathways are still present in the EZ-B niche. In the Opalinus Clay, fractures are successively closed by self-sealing processes, which lead to a significant permeability reduction in the EDZ, finally approaching the hydraulic conditions of undeformed rock again (Bernier et al., 2007; Jaeggi and Bossart, 2014; Nagra, 2002). However, this only applies for saturated conditions after backfilling and sealing, when progressive re-saturation of the host rock around the underground facility is initiated (Bossart et al., 2017; Marschall et al., 2017). Under open conditions the EDZ is an unsaturated zone, as shown, for example, by Ziefle et al. (2017). Since 2006, the temporal evolution of the EDZ in the EZ-B niche was assessed by jointmeter time series obtained from a single tectonic fracture. Based on this dataset, a cyclic long-term closure of the monitored fracture was demonstrated (Ziefle et al., 2017), but this is probably primarily due to niche convergence.

For shales or argillaceous rocks, changes in the saturation state are directly linked to structural modifications (Valès et al., 2004; Yurikov et al., 2019). In the highly saturated state, a large portion of the pore water is adsorbed onto the clay mineral surfaces, while for high external suction, i.e., desaturation, it is extracted from the rock through the pore network (Zhang et al., 2007). Dehydration of claystone leads to a decrease in total porosity (Yurikov et al., 2019) and induces a reduction of pore and swelling pressure, which in turn impedes self-sealing processes (Tsang et al., 2005; Zhang et al., 2007). In addition, secondary shrinking-induced tension fractures can develop parallel to bedding with progressive dehydration (Delage, 2014). In contrast, hydration of claystones leads to a significantly enhanced creep and swelling capacity (Yurikov et al., 2019; Zhang et al., 2007). In the case of the non-lined EZ-B niche in Mont Terri, self-sealing is inhibited due to a sharp decrease in water content of the rock mass close to the niche caused by sustained ventilation since tunnel excavation. This observation is of particular importance for the second phase in repository development, the open drift stage, where ventilation-induced damage and dehydration in the tunnel systems are also expected (Tsang et al., 2005). The presented results therefore serve as a valuable analog and provide information on the state of the EDZ in a non-lined niche in indurated clay after prolonged exposure.

With increasing distance to Gallery 04, a general decrease in hydraulic fracture aperture a_h was expected. When leaving the EDZ, the degree of damage or disturbance generally decreases as deconfinement, displacement and deviatoric stresses within the rock mass are highest directly next to the cavity (Lisjak et al., 2016; Yong et al., 2010). Based on the results of the air permeameter measurements, however, a weak positive correlation between a_h and the horizontal distance could be observed (correlation coefficient r=0.43). A decrease in hydraulic aperture with greater distance to Gallery 04 could not be observed, since the fractures in the EZ-B niche originate from two different excavations. Due to the applied excavation technique and favorable orientation of the niche (Sect. 2.1), the EDZ of the EZ-B niche is less pronounced compared to the EDZ around Gallery 04. However, two EDZs, i.e., two fracture systems, are superposed. Hydraulic apertures in the immediate vicinity of Gallery 04 are comparatively small (Fig. 3b). Presumably, this is caused by shotcrete application to the exposed claystone surface associated with increased water availability, leading to partial re-saturation of the rock. This water supply most likely promoted swelling of clay minerals and fracture closure to a certain degree, resulting in a general reduction of hydraulic apertures in the entrance area of the EZ-B niche.

Due to the limited measuring range of the air permeameter, hydraulic apertures of widely opened fractures (a_h>2 mm) could not be quantified. Thus, the mean a_h of open fractures in the EZ-B niche was rather underestimated with this method. It is also noticeable that for 49 % of the measurement points, the first measured value was smallest. This can be explained by dust or loose material inside the fracture, which was removed by the first stroke with the air permeameter. As outlined above and supported by continuously recorded jointmeter data from the EZ-B niche, the saturation state of the Opalinus Clay has a major influence on measured fracture apertures. From 2015 to 2019, the investigated single fracture was subject to annual aperture fluctuations of up to 500 µm due to seasonal fluctuations of relative humidity (RH) (Ziefle et al., 2017). However, over the long period since the niche was completed in 2005, the observed trend of cyclic aperture closure has decreased substantially, and seasonal fluctuations of fracture aperture are by now far less pronounced. In the EZ-B niche, relative humidity is highest between July and October (RH = 100 %), while the lowest values are usually recorded in February (RH ≈ 60 %). Since the measurement campaign for the present study was carried out in April, it can be assumed that the obtained hydraulic and mechanical aperture values roughly represent an annual average state of the continuously changing fracture system.

For the Opalinus Clay in the Mont Terri URL, no direct information on hydraulic fracture apertures is available, which further illustrates the difficulty of conducting practicable and accurate a_h measurements in the field. In order to compare the measured values with literature values in terms of plausibility, fracture transmissivities (T_f) obtained from extensive hydraulic testing in the Mont Terri URL were utilized (Table 3). Based on the cubic law, an equivalent hydraulic fracture aperture (a_h,eq) can be calculated from T_f using the relation (Brown, 1987)

$$\begin{array}{}\text{(9)}& a_{\text{h,eq}}=\sqrt[\mathrm{3}]{T_{\text{f}}\cdot {\displaystyle \frac{\mathrm{12}\cdot {\mathit{\mu }}_{\text{W}}}{{\mathit{\rho }}_{\text{W}}\cdot g}}},\end{array}$$

where μ_W is the kinematic viscosity of water, ρ_W is the density of water and g is the gravitational acceleration. It should be noted that hydraulic tests are generally used to characterize certain borehole intervals. Transmissivity values that are derived from these tests therefore relate to a certain rock volume, while the number of hydraulically active fractures intersecting the test interval is usually unknown (Gustafson and Fransson, 2006). In this case, fracture densities must be considered for calculating equivalent hydraulic apertures. However, some studies also provide single-fracture transmissivities, which could therefore directly be converted to a_h,eq using Eq. (9) for direct comparison with a_h measured by the air permeameter (Table 3).

Table 3 Comparison of measured hydraulic fracture aperture (this study) and equivalent hydraulic aperture values derived from reported single-fracture transmissivities from the Opalinus Clay in the Mont Terri URL.

^a Assumption of a single hydraulically dominant fracture (short test intervals, typically 5 cm).

Although slightly larger, the hydraulic fracture apertures measured by the air permeameter are of the same order of magnitude compared to equivalent hydraulic apertures that were derived from single-fracture transmissivity testing (Table 3). Due to the previously outlined processes related to the successive dehydration of the claystone, hydraulic apertures in the EZ-B niche were expected to differ slightly from the literature values due to the elongated exposure time. Nevertheless, the measured values are plausible and clearly show that the air permeameter is suitable for the measurement of hydraulic fracture apertures within the EDZ of the Opalinus Clay.

Figure 4 Comparison of measured mechanical and hydraulic fracture apertures in the EZ-B niche with probability density function for a_m. Two outliers with mechanical apertures of 789 and 833 µm are not shown for better visibility.

3.1.2 Measured mechanical fracture aperture

For the same set of fractures, the mechanical fracture aperture a_m was determined based on microscope camera images. For seven measurement points the mechanical aperture could not be evaluated due to poor quality of the microscopic images. The measured mechanical fracture apertures in the EZ-B niche showed a widespread range between 19 and 833 µm, while most values were clustered around 115 µm (Fig. 4). Artificial EDZ fractures showed the lowest values among the studied fracture types (127 ± 92 µm), whereas the mean mechanical aperture in the EZ-B niche was 233 ± 205 µm. Again, no distinct trend with increasing distance to Gallery 04 could be observed. For the vast majority of sampled fractures, a_m was greater than or equal to a_h as expected (Fig. 4). As is the case for hydraulic apertures, almost no information on mechanical fracture apertures within the EDZ of the Opalinus Clay was available for comparison. Bossart et al. (2004) mention unloading fractures with mechanical apertures of up to 1 cm. However, as the specified measurement range of the air permeameter is limited to hydraulic apertures of 2 mm, such widely opened fractures were not investigated in this study.

The newly implemented analysis algorithm for microscopic images (Fig. 2) provided similar results compared to the manual approach used in Hale et al. (2020b) (correlation coefficient r=0.93). Thus, automatic image analysis is highly recommended. In addition to significant time savings, more representative results for a_m and ${\mathit{\sigma }}_{a_{\text{m}}}$ of the imaged fracture trace segment can be obtained due to the large number of distance measurements. For images that could not be evaluated automatically, manual analysis was employed. This particularly applied to images that were not sufficiently focused and fissures with very small mechanical apertures. In these cases, the void area could not be properly distinguished from the rock, thus hampering the selection of an appropriate threshold value for automatic analysis.

Figure 5 Comparison of hydraulic fracture apertures measured by the air permeameter (TinyPerm 3) and estimated based on mean mechanical fracture apertures and standard deviations from microscopic fracture trace analysis. For each dataset, the arithmetic mean (bold black line), the median (thin purple line) and the probability density function (obtained by kernel density estimation) are shown. Values above 550 µm are excluded from the graph.

3.1.3 Estimated hydraulic fracture aperture

Based on the mean mechanical aperture (a_m) and the corresponding standard deviation (${\mathit{\sigma }}_{a_{\text{m}}}$), which provides a statistical measure for fracture surface roughness, hydraulic apertures were estimated using established empirical relations (Eqs. 1 to 8, Table 1). For eight different equations, the results are shown in Fig. 5. With regard to the mean hydraulic fracture aperture, the full dataset of the discrete fracture network (DFN) in the EZ-B niche was best reproduced by Eq. (1) (Kling et al., 2017), which is therefore recommended for future studies. The median of this dataset was also in excellent agreement with the measured data (Fig. 5). However, there were generally large deviations between the measured and estimated mean a_h. With regard to the frequency distribution of data points, Eqs. (6) to (8) performed better because a_m is reduced to a lesser extent. Similar to the dataset of the TinyPerm 3 (air permeameter), most values were clustered around 100 µm. However, few very high values led to a significant overestimation of the arithmetic mean (216–221 µm). In order to provide a representative measure of the central tendency of the estimated hydraulic aperture for a given fracture set or DFN, the use of the median is highly recommended. For EDZ fractures in particular, a_h was best estimated by Eq. (2) (Rasouli and Hosseinian, 2011).

Higher deviations between estimated and measured hydraulic apertures were observed for fractures with either very low or very high mechanical apertures in relation to the measured value range. This was confirmed by a strong correlation between a_m and the resulting deviation between estimated and measured a_h values; for example, for Eq. (1) the correlation coefficient was 0.96. It was noticeable that for EDZ fractures far higher agreement of estimated and measured hydraulic apertures could be observed in comparison to tectonic fractures, independently of the relation that was used for conversion. This is most likely related to smaller mechanical apertures. While EDZ fractures seem to better correspond to the general model concept of an “ideal plane-parallel fracture”, tectonic fractures are most probably characterized by a different a_m–a_h relation. While implied by the presented measurement data, this issue should still be examined based on larger datasets.

Despite the good performance of the microscope camera method for EDZ fracture analysis, direct measurement of a_h with the air permeameter is always preferable. Nevertheless, the data derived from microscopic imaging can provide explicit information on fracture geometry and formation mechanism. Presumably due to their formation mechanism, EDZ fractures (artificial unloading joints) showed comparatively little variation in aperture along the fracture traces, resulting in a rather low mean standard deviation ${\mathit{\sigma }}_{a_{\text{m}}}$ of 58 µm. For EDZ fractures, a_m was on average 1.7 times larger than a_h. In contrast, for tectonic fractures (reactivated fault planes), an average ${\mathit{\sigma }}_{a_{\text{m}}}$ of 90 µm and an $a_{\text{m}}/a_{\text{h}}$ ratio of 2.4 were determined in the EZ-B niche. However, the higher mean standard deviation for the measured aperture values along the fracture segment obtained for tectonic fractures is not necessarily caused by a higher roughness of the fault surfaces themselves. Namely, observations in the Mont Terri URL showed that fault surfaces are generally polished with slickensides, while EDZ fracture surfaces often show plumose structures (Nussbaum et al., 2005, 2011; Yong, 2007). On the other hand, since EDZ fractures originate from tensile stresses, these fracture surfaces also show a high degree of matching. This directly leads to a comparatively small standard deviation of measured aperture values along an imaged EDZ fracture segment. Fault planes in the EZ-B niche were reactivated by stress redistribution during excavation and subsequent convergence, which is why the investigated tectonic fractures even show measurable apertures. Due to shear loading, relative displacement led to a higher mismatch of the two fracture surfaces. For the tectonic fractures, the higher mean standard deviation ${\mathit{\sigma }}_{a_{\text{m}}}$ that was observed by microscopic image analysis therefore primarily reflects the increased variance of measured distances due to mismatched fracture walls rather than the actual fracture surface roughness.

3.2 Geomechanical and geophysical properties

Needle penetration testing (NPT) was performed at 47 sampling points on both side walls of the EZ-B niche, including 18 tests normal to bedding and 29 tests parallel to bedding. The measured needle penetration index (NPI) clearly confirmed the significant strength anisotropy of the Opalinus Clay (Jaeggi and Bossart, 2014). Normal to bedding, strength was significantly higher than parallel to bedding, indicated by an NPI of 98 ± 29 and 59 ± 19 N mm⁻¹, respectively. For all measurements a constant load of 100 N was selected. Thus, the NPI only depended on the observed needle penetration depth. Measurements normal to bedding resulted in several invalid tests due to formation of microcracks around the needle hole. In this case, the measured value was discarded (Sect. 2.4). The results of the parameter estimation based on the measured NPI are listed in Table 4.

Figure 6 Comparison of estimated geomechanical and geophysical parameters of the Opalinus Clay in the EZ-B niche of the Mont Terri URL (black symbols, this study) with literature data for the shaly facies normal (a–g) and parallel (b–i) to bedding. In addition, a second NPT dataset of Blum et al. (2013) from various locations in the URL (shaly facies) is shown (gray symbols). Literature data (green and blue symbols) originate from other niches in the Mont Terri URL and are mainly adapted after Jaeggi and Bossart (2014) and references herein (the full list of source documents is provided in Appendix A).

Figure 6 provides a graphical overview and enables comparison with available literature data to assess the quality of the applied in situ parameter estimation. As discussed in Sect. 1, geomechanical and geophysical parameters in the EZ-B niche are significantly affected by the desaturation of the rock. Compared to the natural water content of the shaly facies of about 6.6 wt % (Bossart and Thury, 2008), the water content of the Opalinus Clay in the EZ-B niche has decreased drastically to 3.7 wt % due to direct air contact and long-time ventilation of the URL (Jaeggi and Bossart, 2014; Ziefle et al., 2017). Since samples for water content determination were taken directly from the rock surface at the walls of the EZ-B niche, this value represents the state of the Opalinus Clay after 15 years of direct atmospheric exposure. The NPI, and therefore estimated geomechanical and geophysical parameters, are not influenced by the distance to Gallery 04.

Based on the needle penetrometer data obtained in the EZ-B niche, the estimated uniaxial compressive strength (UCS) was 29.7 MPa for a water content of 3.7 wt % normal to bedding (Fig. 6a). For water contents below 4.5 wt % , no literature data on UCS are available for the shaly facies of the Opalinus Clay. Due to the fact that UCS is increasing linearly with decreasing water content (Wild et al., 2015), the estimated mean value can be considered reasonable and complements the literature dataset in Fig. 6a. The additional NPT dataset of Blum et al. (2013) also confirms the negative linear correlation of UCS and water content. Parallel to bedding, the estimated UCS of 18.2 MPa is consistent with literature data on the shaly facies for a similar water content (Fig. 6b). The two NPT datasets, acquired from the EZ-B niche and different locations in the Mont Terri URL (Blum et al., 2013), exactly reproduce the trend that is evident from the available literature data. The estimated UCS values in Fig. 6a and b were obtained by using different empirical equations, leading to a comparatively high standard deviation. The individual results for each empirical equation are listed in Table 4. As needle penetration testing is widely used for estimating the UCS of rocks (Ulusay et al., 2014), several functions are available which have been deduced from various rock types (Table 2). For the Opalinus Clay, the applied combination of different established equations has proven to be suitable.

The Brazilian tensile strength (BTS) of the Opalinus Clay normal to bedding could also be estimated by the needle penetration tests. The mean value of 2 MPa that was obtained from the on-site measurements in the EZ-B niche is located approximately in the center of the literature value range for a comparable water content (Fig. 6c). Apparently, the literature data for the shaly facies are divided into two subgroups. Lower BTS values normal to bedding are most likely related to pre-damaged sample material, i.e., desiccation cracks, which preferentially form parallel to the bedding features in the drill core and therefore reduce the tensile strength of the tested sample perpendicular to bedding. Parallel to bedding, the estimated BTS deviates by about 2 MPa from the available literature data and is therefore underestimated (Fig. 6d).

Similar to UCS, the Young's modulus E is increasing monotonically with declining water content as evident from both literature data and the NPT dataset of Blum et al. (2013). For water contents below 5.7 wt %, no literature data are available for the shaly facies normal to bedding. As the derived Young's modulus of 4.9 GPa normal to bedding for a water content of 3.7 wt % was significantly higher than the available literature values for natural water contents (maximum of 2.4 GPa), the estimation was assumed to be applicable for the Opalinus Clay (Fig. 6e). With a mean value of 3 GPa, E was largely underestimated parallel to bedding compared to the available literature data ranging between 13 and 31 GPa for similar water contents (Fig. 6f). This is most probably due to the fact that rather well-preserved drill core specimens are compared to the long-exposed and highly damaged rock surface of a niche.

The elastic P-wave velocity (v_P) is slightly overestimated normal to bedding (Fig. 6g). Similar to Fig. 6c, two sub-datasets can be identified within the literature data. Here, pre-damage of the core sample material or an insufficient coupling of the ultrasonic source was most likely responsible for these reduced P-wave velocities, which are therefore not representative for the intact rock body (Jaeggi and Bossart, 2014). According to Jaeggi and Bossart (2014), the core samples belonging to the lower sub-dataset of literature values in Fig. 6g were partially penetrated by microcracks. The estimated v_P data seem to represent the upper subset of the literature data. Since the needle penetrometer only samples a small area on the rock surface, the NPT is able to reflect the actual in situ conditions. Parallel to bedding, v_P is slightly underestimated but fairly represents the data range of the shaly facies (Fig. 6h). The observed trend of a slight P-wave velocity increase with decreasing water content of the Opalinus Clay normal and parallel to bedding is also well represented by the datasets obtained from needle penetration testing.

For the elastic S-wave velocity v_S of the Opalinus Clay, a mean value of 1.9 km s⁻¹ was estimated normal to bedding. Due to strong absorption and attenuation of wave energy and insufficient signal retrieval during ultrasonic velocity measurement (Gräsle and Plischke, 2010; Schnier and Stührenberg, 2007), few literature data are available. For the shaly facies of the Opalinus Clay, Bock (2009) reports v_S values of 1.51 km s⁻¹ for a natural water content of 6.4 wt %. Wileveau (2005) provides S-wave velocities of 1.45–1.58 km s⁻¹ for water contents varying between 2.4 and 2.9 wt %. Hence, normal to bedding v_S is most likely overestimated by the NPT. Parallel to bedding, a value of 1.5 km s⁻¹ was determined. In comparison to existing literature data, v_S is underestimated in this direction (Fig. 6i). As ultrasonic velocity is explicitly dependent on the internal structure of the rock, such as cementation, anisotropy and porosity structure (Jaeggi and Bossart, 2014; Schuster et al., 2017), it can be assumed that the relationship of ultrasonic velocity and NPI is probably rather weak.

Table 4 Estimated geomechanical and geophysical parameters for the Opalinus Clay in the EZ-B niche normal and parallel to bedding based on the needle penetration index.

For the cohesion c and the friction angle φ, reliability of the estimated values (Table 4) could not be assessed due to limited literature data, especially for samples with low water contents. In Bock (2009), mean values for the cohesion (3.7 and 5.4 MPa) and friction angle (22 and 23^∘) of the Opalinus Clay for an average water content of 6.7 wt % are given normal and parallel to bedding, respectively. In contrast, Lisjak et al. (2015) used a cohesion of 24.8 MPa (normal) and 2.4 MPa (parallel) as an input parameter for their calibrated finite–discrete-element model. The implemented friction angle of 22^∘ complies with Bock (2009), but no information on the assumed water content is included. Cohesion values of 3.9 MPa (normal) and 2.4 MPa (parallel) derived from needle penetrometer measurements in the EZ-B niche were of the same order of magnitude as the data given by Bock (2009). However, the estimated friction angles of 42 and 37^∘ normal and parallel to bedding, respectively, were significantly higher in comparison to the literature data.

Generally, the applied empirical functions performed better normal to bedding. The poor estimation of parameters parallel to bedding (apart from UCS) may be linked to unperceived (possibly micro-scale) shrinking or desiccation cracks, which preferentially form parallel to the bedding features (e.g., Amann et al., 2017; Schnier and Stührenberg, 2007). Microcracks would facilitate needle penetration, thus reducing the measured NPI and estimated parameter values. It should also be noted that the existing empirical relations for estimating physico-mechanical parameters do not specifically apply to claystones or shales, but were derived based on compiled experimental data obtained for various types of soft rocks (Aydan et al., 2014). In addition to the factors mentioned above, sedimentary heterogeneity might also be responsible for geomechanical variability as well as the observed deviations between the NPI-based values and the considered literature data. However, heterogeneity of the shaly facies of the Opalinus Clay in Mont Terri is comparatively low (Jaeggi and Bossart, 2014). More likely, the observed deviations are due to variant surface constitution caused by the hugely varying exposure time of the rock walls in the non-lined EZ-B niche in contrast to sampled specimens from drill cores. Although the measurements were conducted at the upper end of the specified application range of the needle penetrometer, physico-mechanical parameter estimation based on needle penetration testing can be recommended for indurated clays, especially for determining the anisotropic uniaxial compressive strength.

4 Conclusions

An excavation damaged zone (EDZ) in the Opalinus Clay of the Mont Terri rock laboratory, Switzerland, was characterized with regard to hydraulic and mechanical properties using three different methodological on-site approaches: (1) air permeameter, (2) microscope camera and (3) needle penetration test. About 15 years after excavation, artificially induced unloading joints (EDZ fractures), reactivated fault planes (tectonic fractures) and bedding-parallel desiccation cracks with a mean mechanical aperture of 233 µm and a mean hydraulic aperture of 84 µm were observed in the EZ-B niche, serving as potential flow paths for advective transport in the indurated clay formation. This is not only limited to the area of the strongly pronounced EDZ around Gallery 04, where a dense network of interconnected fractures is encountered, but also applies to potentially reactivated tectonic discontinuities at greater distances, e.g., due to large-scale stress redistribution or injection of fluids. After an initial continuous aperture closure observed by long-term jointmeter data records in the non-lined niche, which can be attributed to seasonally controlled shrinkage and swelling cycles in combination with niche convergence, this process seems to be decelerating significantly after 15 years of monitoring. Locally, fractures are influenced by shotcrete application, leading to reduced hydraulic and mechanical apertures due to enhanced water availability and swelling of clay minerals in the immediate vicinity. Among the studied discontinuity types, the EDZ fractures showed the smallest hydraulic apertures. However, as 60 % of all measured values were within the range of 80–120 µm, a clear distinction was not possible.

From direct measurement with the portable transient-flow air permeameter, plausible hydraulic aperture data could be acquired on-site, even if the entire range of fractures in the EZ-B niche could not be reproduced due to a limited measuring range. This means that the permeameter measurements tend to overrepresent smaller fractures. Here, indirect determination of hydraulic fracture apertures based on the automatic evaluation of high-resolution microscope camera images of fracture traces offers a practical alternative. Due to the smaller mean mechanical aperture of the artificially induced unloading fractures compared to the investigated tectonic fractures, conversion was most appropriate for the EDZ fractures. Tectonic fractures on average exhibit a higher variance of measured distances along imaged fracture traces, which can be explained by a higher degree of mismatch between the fracture surfaces due to the reactivation of fault planes during excavation. However, the statistical significance of the observed differences between the different fracture types would have to be tested based on a larger dataset. For specifying the mechanical aperture of a fracture network based on fracture trace micro-imaging, the median of measured values is most representative.

The needle penetration test proved to be a valuable tool, especially for accurate estimation of the anisotropic uniaxial compressive strength, as the needle penetration index satisfactorily reflects the in situ conditions of the intact rock mass. For the shaly facies of the Opalinus Clay in the EZ-B niche, a mean uniaxial compressive strength of 30 MPa (normal to bedding) and 18 MPa (parallel to bedding) was determined. While parameter estimation based on needle penetration indices normal to bedding showed high agreement with available literature data, physico-mechanical parameter values were mostly underestimated in the bedding-parallel direction. Due to damage of the exposed rock surface associated with the formation of microcracks parallel to stratification, needle penetration was facilitated in this case. Due to direct air contact and ventilation of the rock laboratory, the desaturation of the claystone in the near field of the niche led to a sharp decrease in water content to 3.7 wt %, which is directly linked to an increased uniaxial compressive strength, Young's modulus and elastic P-wave velocity normal to bedding.

The applied on-site measurement methodology and evaluation approach are suitable for the hydraulic and mechanical characterization of excavation damaged or excavation disturbed zones in different geological environments, especially since drilling is not always feasible and the validity of estimated parameters is limited to the investigated location. With comparatively little effort, nondestructive analysis of time- and location-dependent variability of important parameters is permitted. Besides confirming the suitability of the methodological approach for flexibly determining hydraulic and mechanical properties, the study assesses the state of an EDZ in a non-lined niche after long-term exposure and therefore serves as an important guideline for diverse tunneling projects and future performance assessments of nuclear waste disposal sites in argillaceous rocks.

Appendix A

The literature data included in Fig. 6 (Sect. 3.2) were mainly taken from Jaeggi and Bossart (2014). This expert report offers a compilation of safety-related rock parameters determined for the Opalinus Clay in the Mont Terri URL. All source documents for experimentally derived data on geomechanical and geophysical properties that were utilized in this study are listed in Table A1. Technical notes (TNs) and technical reports (TRs) for the Mont Terri Project are accessible via https://www.mont-terri.ch/en/documentation/technical-reports.html (last access: 5 July 2021).

Table A1 Source documents for experimental data on uniaxial compressive strength (UCS), Brazilian tensile strength (BTS), Young's modulus (E), and elastic P-wave (v_P) and S-wave velocity (v_S) of the Opalinus Clay (shaly facies) in Mont Terri.

^a Unpublished final theses, ETH Zurich, Switzerland.