02_clay.tex

\subsection{Opalinus Clay from Mont Terri, Switzerland}
\label{subsec:clay}
\Authors{Tilo Kneuker, Bernhard Vowinckel, Markus Furche, Gesa Ziefle, Jobst Ma{\ss}mann}

\subsubsection{The Mont Terri Rock laboratory}\label{sec:mont_terri}
\index{URL Mont Terri}
\index{Opalinus Clay}

Opalinus claystone is a very promising hostrock for the safe disposal of heat emitting nuclear waste. This type of hostrock has been investigated in the Mont Terri Rock laboratory for more than 24 years. The Mont Terri rock laboratory is a facility to conduct research in the deep geological underground at in-situ scale, such as the safe deposition of radioactive waste, where the local host rock is Opalinus Clay. The rock laboratory is located within the Jura Mountain fold belt. The development of the Jura fold belt began in the Middle Miocene around 12 million years ago, which was constrained by the first occurrence of overthrusted and folded molasse sediments \cite{bolliger1993}. The overthrust of the frontal fold and thrust belt over the allochthonous foreland (Tabular Jura) occurred ca. 10.5 million years ago \cite{becker2000}. More specifically, the Mont Terri rock laboratory is located in the southeast dipping fold limb of the NW-vergent Mont Terri anticline \cite{nussbaum2011}. The total amount of shortening of the anticlinal structure is approximately 2.1 km \cite{freivogel2003}. During the folding process, the northwestern fold limb of the Mont Terri anticlinal structure was sheared-off and now lies on top of the Tabular Jura (Figure \ref{fig:bgr_mt_sideview}). The Mont Terri anticlinal structure developed in a special structural setting at the intersection between the frontal part of the Jura fold belt (main shortening direction NW-SE) and the roughly N-S oriented structural elements of the Rhine-Bresse-graben transfer zone \cite{nussbaum2011}.

\begin{figure}[!ht]
\centering
\includegraphics[width=1\textwidth]{./figures/bgr_mont_terri_side_view.png}
\caption{Geological cross section along the motorway tunnel through the Mont Terri anticline. From: Kaufhold et al. (2016) \cite{kaufhold2016}, based on Freivogel \& Huggenberger (2003) \cite{freivogel2003}.}
\label{fig:bgr_mt_sideview}
\end{figure}

The Mont Terri rock laboratory branches off from the security gallery of the motorway tunnel near the town of St. Ursanne (NW Switzerland). The rock laboratory is located mainly within the Middle Jurassic Opalinus Clay formation. The thickness of the Opalinus Clay in the rock laboratory is around 130 m \cite{hostettler2018} and the layers are dipping with ca. 40° towards SE. The depth below ground varies between 230 m and 320 m, depending on the topography \cite{heitzmann2001}. Since 1996, a total of 1400 m of galleries and niches have been excavated in the Mont Terri rock laboratory (Figure \ref{fig:bgr_mt_topview}). The Mont Terri rock laboratory is a generic scientific research laboratory. At Mont Terri, there will be no storage of radioactive waste.

\begin{figure}[!ht]
\centering
\includegraphics[width=1\textwidth]{./figures/bgr_mont_terri_top_view.jpg}
\caption{Geological map of the rock laboratory with all in-situ experiments relevant for the GeomInt project including the locations of the two AD-boreholes (BAD-1, BAD-2), the drilling for the FS experiment (BFS-1 to 3), the EZ-B niche for the CD/LP experiment and its successor experiment CD-A in Gallery 18. The different facies types of the Opalinus Clay can be recognized by the different shades of brown and yellow, the new Gallery 18 and the new experiment niches are highlighted in green on the map.  (map modified from Mont Terri Consortium, swisstopo).}
\label{fig:bgr_mt_topview}
\end{figure}
\index{MT-CD/LP experiment}
\index{MT-FS experiment}

The Opalinus Clay in the rock laboratory is composed of a dark gray claystone that was named after the ammonite species Leioceras opalinum. This claystone formation was deposited during the period of the Toarcian/Aalenian, at an age of approximately 174 million years. The Opalinus Clay is exposed along the rim of the Swabian and Franconian Alp in Germany and stretches into northern Switzerland \cite{einsele1983}. The Opalinus Clay was deposited in a shallow-marine, epicontinental milieu in the area of the storm wave base at approximately 20 m to 50 m water depth \cite{wetzel2003}. Coarser siliciclastic components are of detritical origin. Potential sources for the detritic components are the areas of the Bohemian Massif and the Vindelician Landmass \cite{wetzel2003}. During Cretaceous burial, the Opalinus Clay experienced maximum palaeo temperatures of $75^{\circ}$~C to $90^{\circ}$~C \cite{bossart2008} at a maximum burial depth of 1.35 km. The Opalinus Clay at the Mont Terri site is underlain by  marls of Upper Toarcian age and overlain by limestones (Bajocian), some of which act as karst aquifer \cite{pearson2003}.
\index{karst aquifer}

The Opalinus Clay at the Mont Terri rock laboratory can be subdivided into three main facies types \cite{bossart2008}. First, the lower shaly facies occupies the largest area of the rock laboratory (Figure \ref{fig:bgr_mt_topview}). It dominates the lower part of the Opalinus Clay formation. It consists of mica-bearing clay and marly shales as well as flasery, marly layers characterized by bioturbation. The upper shaly facies contains a higher volumetric content of quartz grains. Second, the sandy facies occurs in the middle and upper part of the profile (lower and upper sandy facies). It includes medium gray marly claystones with intercalated, bioturbated marly layers or lenticular, gray sandy limestones and pale sand layers of approximately 1-10 mm thickness that include pyrite as well. Third, a carbonate-rich sandy facies of approx. 5 m thickness occurs in the middle part of the rock formation. It consists of calcareous sandstones with intercalated bioturbated limestone layers, which show a relatively high proportion of detritic quartz and white mica. The different facies types of the Opalinus Clay can be attributed to varying sedimentation conditions in a shallow marine environment (like variations in depth and current directions). The carbonate-rich facies is typical for the Jura region in western Switzerland and it does not occur in the proposed siting regions for a deep geological repository in Northern Switzerland.
\index{bioturbation}

The mineralogical composition of the Opalinus Clay was examined by Traber \& Blaser (2013) \cite{traber2013} for several locations in Northern Switzerland. For the shaly facies, the clay mineral content varies between 40 wt\% and 75 wt\%. The clay minerals determined include illite, kaolinite and smectite-illite mixed layer minerals, the proportion of swellable clay minerals is around 10 wt\%. Detritic components such as quartz and feldspars typically make up to 20 wt\% of the investigated samples. The carbonate content (calcite and dolomite) is around 20 wt\%. The sandy facies of the Opalinus Clay is composed of up to 40 wt\% clay minerals and ca. 30 wt\% quartz; it shows a lower amount of clay minerals in favor of a higher quartz content, compared to the shaly facies \cite{heitzmann2001}. 
\index{Opalinus Clay}

The BGR has been involved in a number of campaigns to study in-situ conditions of clay rock, of which the following four are particularly noteworthy within the context of the GeomInt-Project. First, in the CD experiment the long-term cyclic deformation (CD) due to seasonally induced cyclic swelling and shrinkage is investigated in a niche of the rock laboratory. These measurements are continued in the LP (long-term monitoring of pore parameters) experiment. In addition, a follow-up project, the CD-A experiment, has been prepared in recent years, to distinguish between deformation processes due to stress redistribution and seasonal variations in air humidity that cause saturation (swelling) and desaturation (shrinkage) of the rock and stress redistribution alone. To this end, two identical niches were excavated, one sealed towards the gallery and with a high humidity inside to minimize desaturation and one open to the general air circulation of the rock laboratory. The measurement campaign was started in October 2019 \cite{ziefle2019}. Third, the AD experiment (experimental-numerical Analysis of Discontinuities) intends to provide an improved process understanding for the experimental-numerical analysis of discontinuities. Finally, the Fault Slip (FS) – experiment addresses the fault reactivation due to pressure-induced percolation in a low-permeability, large-scale discontinuity in the Mont Terri rock laboratory. The AD is directly relevant to the numerical and experimental investigations presented in Sections \ref{sec:mex05} - \ref{sec:mex12}, because the rock material used in these experiments were drilling samples from the AD experiment. Hence, we provide a brief overview for the campaign in the following. 



\subsubsection{The CD/LP Experiment in the Mont Terri Rock laboratory}
\index{MT-CD/LP experiment}

The Mont Terri Rock laboratory in Switzerland hosts a multitude of in-situ experiments that investigate the response of Opalinus Clay to various geotechnical applications. An overview of the rock laboratory is given in Section \ref{sec:mont_terri}. In particular, the CD (Cyclic Deformation) experiment has been a valuable site to gather experimental data at the in-situ scale to investigate the hydraulic-mechanial coupling induced by swelling and shrinking of Opalinus Clay due to cyclic variations of air humidity. Section \ref{sec:mex10} focuses on the numerical investigation of these processes. Here, we provide a brief overview of the experimental CD campaign at the Mont Terri Rock Laboratory. 

The experiment itself is located in the EZ-B niche (Figure \ref{fig:bgr_mt_topview}). The experiment has been conducted for more than 13 years to provide information on the swelling and shrinkage behavior of Opalinus Clay in the Mont Terri rock laboratory. The idea was to analyze a niche that is not covered by shotcrete. Instead, the clay rock remains in direct contact with the atmospheric conditions of the main gallery for the entire time. Consequently, the swelling and shrinkage is induced by changes in temperature and relative humidity, which can decrease to values as low as 65\% in the winter and reaches values of up to 100\% in the summer. 

\begin{figure}[!ht]
\centering
\includegraphics[width=1\textwidth]{./figures/bgr_CD_experiment.jpg}
\caption{The EZ-B niche in the Mont Terri rock laboratory, where the CD/LP experiment has been conducted since 2006 (photo: Mont Terri consortium, swisstopo).}
\label{fig:bgr_CD_experiment}
\end{figure}

A special focus of this experiment was to investigate the long-term impact of these seasonal variations on the temporal evolution of the cracks that occur during the excavation process and make up the Excavation Damaged Zone (EDZ). To this end, the EZ-B niche was excavated in the years 2004/2005 (Figure \ref{fig:bgr_CD_experiment}). Subsequently, the niche was equipped with a comprehensive set of measurement devices to record the evolution of temperature, water content, convergence of the niche and crack development at the tunnel walls over time. This measurement campaign was started in 2006 and has been continued until today to investigate long-term effects. Note that the experiment was transferred into the LP-A experiment to explicitly focus on the long-term monitoring of pore pressure. The CD/LP experiment under in-situ conditions was supplemented by laboratory experiments with drill cores to determine hydraulic-mechanical properties of the clay rock, such as porosity, grain density, etc. \cite{matray2013}. 

Characteristic macroscopic cracks on the tunnel walls have been monitored and the field data of the crack opening show a good correlation with the seasonal variation of temperature and humidity. The cyclic deformation of the crack opening yields a re-occurring compression perpendicular to the crack during summer, which typically is a time of high relative humidity and, hence, the swelling causes an increase of rock volume \cite{jaeggi2012}. This characteristic behavior of swelling and shrinking was successfully reproduced by means of hydraulic-mechanically coupled numerical simulations \cite{ziefle2018}, which provides valuable benchmark data for future investigations of the cyclic deformation of clay rock.

\subsubsection{The AD Experiment}
\index{MT-AD experiment}

The aim of the experiment is to provide core samples from the sandy facies of the Opalinus Clay as a typical example of an argillaceous host rock for the safe disposal of nuclear waste. These samples were used for experimental-numerical analysis in the framework of the GeomInt project. Additionally, a geological characterization of the cores and seismic (Interval Velocity Measurements - IVM) and geolelectrical measurements (Electrical Resistivity Tomography - ERT) in the boreholes were performed. The results of the experimental campaign yield a valuable description of the sandy facies in addition to the well characterized shaly facies of the Opalinus Clay \cite{bossart2008,jahn2016}. 
\index{ERT Electrical Resistivity Tomography}

From a geological perspective, the AD experiment gave opportunity to study the lower sandy facies of the Opalinus Clay at the Mont Terri rock laboratory in detail. The two fully cored boreholes BAD-1 and BAD-2 with a diameter of 131 mm (yielding samples of 101 mm diameter) were drilled parallel and perpendicular to the sedimentary bedding, respectively. The 15.35 m long horizontal borehole BAD-1 was drilled from 7th-10th of July 2018 by the BGR. It is located entirely in the lower sandy facies. The geological mapping was performed by swisstopo \cite{galletti2019}. The core material of BAD-1 was entirely sampled for laboratory experiments performed by the Christian-Albrechts University of Kiel (Germany) and the Institute of Geomechanics (IfG) Leipzig (Germany). The core samples were conditioned in aluminum foil and pressurized in special nitrogen-filled BGR-liners (autoclaves) to avoid further alteration. 

The BAD-2 borehole has a length of 27.0 m. It was drilled by the BGR team from 9th-17th of May 2018. The borehole is oriented perpendicular to the bedding (with a dip of 43°), thus crossing several facies types of the Opalinus Clay. The geological mapping by Swisstopo reported by Galletti \& Jaeggi (2019) \cite{galletti2019} revealed the following sequence with varying quantities of quartz, carbonates (cements and fossils) and clay minerals: 

\begin{list}{-}{\leftmargin=1em \itemindent=0em \itemsep=0em}
\item 0.0 m to 4.75 m depth: upper shaly facies,
\item 4.75 m to 19.4 m depth: lower sandy facies,
\item 19.4 m to 24.57 m depth: carbonate-rich sandy-facies, 
\item 24.57 m to 27.0 m depth: lower shaly facies.
\end{list}

This subdivision is confirmed by petrographic-structural studies and geoelectrical resistivity measurements (ERT) performed by the BGR. The BAD-2 drillcores were sampled from 4.0 m to 14.8 m (lower sandy facies) for laboratory experiments by the Universities of Kiel and IFG Leipzig. Following the procedure employed for the BAD-1 drilling, the core material was conditioned in aluminum foil and pressurized in special nitrogen-filled BGR-liners (autoclaves). The drillcore material from the intervals between 0.0 m to 4.0 m and 14.8 m to 27 m, including the transition towards the underlying carbonate-rich facies, are stored at the BGR facility in Hannover (Germany) for further geological characterization. The first results revealed a good core quality and confirm a rather uniform appearance of the sampled profile inside the lower sandy facies, the drillcore material is thus suitable for the planned experiments (cf. Figure \ref{fig:bgr_AD_experiment}).

\begin{figure}[!ht]
\centering
\includegraphics[width=1\textwidth]{./figures/bgr_AD_experiment.jpg}
\caption{Schematic profile of borehole BAD-2 as marked in Figure \ref{fig:bgr_mt_topview} (left), macrostructural (on drillcore scale) and microstructural features (on thin section scale) of the different facies types of Opalinus Clay encountered in the BAD-2 borehole.}
\label{fig:bgr_AD_experiment}
\end{figure}

\textbf{High resolution ERT measurements in borehole BAD-2[Markus Furche]}
\index{ERT Electrical Resistivity Tomography}
The task of geophysics was to characterise the rock unit that had been drilled through, to precisely determine the locations of the facies boundaries and to describe the variability, especially of the sandy facies. The electrical resistivity tomography (ERT) method was used for this purpose.

\textbf{Principle}
To determine the spatial electrical resistivity distribution (or its reciprocal $-$ the electrical conductivity) in the ground, a direct current (DC) is injected into the ground through two point electrodes (A, B), see Figure \ref{fig:four-electrode-array}.
\begin{figure}[!ht]
	\centering
		\includegraphics[width=1\textwidth]{./figures/fig-ERT-1.jpg}
	\caption{Principle of resistivity measurement with a four-electrode array (modified after Kn{\"o}del et al., 2007 \cite{knoedel2007}).}
	\label{fig:four-electrode-array}
\end{figure}
The resulting electrical field is measured using two other electrodes (M, N). A point electrode introducing an electrical current $I$ will generate a potential $V_{r}$ at a distance $r$ from the source. In the case of the four-electrode array shown in Figure \ref{fig:four-electrode-array}, the two current electrodes (A, B) introduce a current $I$. When assuming a homogeneous half-space, the potential difference $\Delta V$ between the electrodes M and N can be calculated as:
\begin{equation}
	\Delta V=\rho I \left[\frac{1}{2\pi}\left(\frac{1}{\overline{\rm{AM}}}-\frac{1}{\overline{\rm{AN}}}-\frac{1}{\overline{\rm{BM}}}+\frac{1}{\overline{\rm{BN}}}\right)\right]
\end{equation}
Here, $\overline{\rm{P}_{1}\rm{P}_{2}}$ denotes the distance between two points $\rm{P}_{1}$  and $\rm{P}_{2}$. Replacing the factor in square brackets with $1/K$ , we obtain the resistivity of the homogeneous half space as follows:
\begin{equation}
	\rho=K\frac{\Delta V}{I}.
\end{equation}
The parameter $K$ is called configuration factor or geometric factor. For inhomogeneous conditions, it yields the resistivity of an equivalent homogeneous half-space. For this value the term apparent resistivity $\rho_{a}$ is introduced, which is normally assigned to the center of the electrode array. Multi-electrode resistivity meters enable the measurement of 2D resistivity surveys (2D imaging). The advantages of this kind of measurements are their high vertical and horizontal resolution along the profile. 
An inversion process of the measured data is necessary for the final interpretation. This process transforms the apparent resistivities into a reliable model discretised into a distinct number of elements of homogeneous resistivity. All inversions are performed using the non-commercial software BERT (Boundless Electrical Resistivity Tomography \footnote{https://gitlab.com/resistivity-net/bert}) developed by Th. G{\"u}nther (Leibniz Institute of Applied Geophysics, Hannover) and C. R{\"u}cker (Technical University of Berlin).

\textbf{Measurements and Results}

\begin{figure}[!ht]
	\centering
		\includegraphics[width=1.35\textwidth, angle=90]{./figures/fig-ERT-2.jpg}
	\caption{Borehole BAD-2: Two-dimensional distribution of the resistivity as a result of the inversion calculation.}
	\label{fig:ERT-2d-model}
\end{figure}
The measurements were performed on May 21$^{\rm{st}}$ and 22$^{\rm{nd}}$ 2019. Since the borehole is perpendicular with respect to the bedding, measurements were only taken in one orientation (0$^{\circ}$, i.e. the electrodes are oriented upwards). Along the borehole, 35 individual profiles of 1.5 m length with half-sided overlapping were measured.

The data processing consists of the following steps: First thing is the scaling of the data in order to eliminate the geometry effects of the borehole. Then data points with high statistic error ($>3 \%$) or high phase angle (absolute value $>$ 100 mrad) were eliminated. 12 or 13 consecutive single data sets were combined to three cumulative ones (00.10 m - 09.85 m, 08.36 m - 18.10 m, 16.61 m - 26.98 m; the last record of the previous section is the first of the following). With these three data sets the inversion was performed using standard parameters. The resulting resistivity models are shown in Figure \ref{fig:ERT-2d-model}.

\begin{figure}[!ht]
	\centering
		\includegraphics[width=1\textwidth]{./figures/fig-ERT-3.jpg}
	\caption{Borehole BAD-2: Curve of the resistivity at a distance of 5 cm from the borehole wall.}
	\label{fig:ERT-model-curve}
\end{figure}
Up to 50 cm borehole depth, the shotcrete can be recognised as a high-resistance structure. Beyond that, the resistivity is well below 10 $\Omega$m and rises slowly with increasing borehole depth. Embedded in this basic structure are thin layers with high resistivity ($>$ 30 $\Omega$m). Between 18.5 m and 19.5 m, there is a very extensive high-resistance range, after which the resistance level drops significantly, again with embedded high-resistance layers. Between 24.0 m and 24.7 m a second distinct high-resistance range is reached, beyond which a sharp drop to values below 10 $\Omega$m can be observed. This level is maintained down to the bottom of the borehole without further intermediate structures.

From the calculated 2D models of the resistivity, curves can be extracted for specific distances from the borehole wall. Figure \ref{fig:ERT-model-curve} shows the corresponding resistivity curve for a distance of 5 cm. The different facies types are indicated as colour background. It can be seen that both the upper and lower shaly facies are characterised by resistivities below 10 $\Omega$m with little variation. The mean level in the sandy facies (4.9 m to 19.7 m) is significantly higher (about 15 $\Omega$m), and the variability is also considerably greater. Both, the upper and the lower transition towards the carbonate-rich sandy facies are clearly indicated as a sharp drop in the resistance curve. Here, the average level of resistivity lies between the values for shaly and sandy facies, the amplitudes of the variations are highest.

\textbf{Implications of the geological-geophysical investigations}

Petrographic-structural studies form the basis for rock characterization and provide first indications for the compositional-structural variability, which independently is confirmed by geophysical borehole measurements.

ERT is able to characterise structures close to the borehole and to resolve rock variability with high accuracy. The individual facies types of Opalinus Clay can be distinguished based on mean resistivities and the type of heterogeneity (amplitudes and sequence). Especially the important transitions towards the carbonate-rich sandy facies can be precisely located.

The results confirm that the upper shaly facies is closely related to the most homogenous facies type of Opalinus Clay (the lower shaly facies). In contrast, the lower sandy facies and especially the carbonate-rich sandy facies represent the more heterogeneous endmembers of the investigated claystone formation.
The new results are consistent with published data and support the classification of the Opalinus Clay at the Mont Terri site into several major facies types. Further investigations will focus on the characterization of intra-facies variability using the sub-facies concept.