Numerical modeling of hydro-mechanical coupled effects in the cyclic deformation (CD-A) experiment: First results and comparison with observations

2021 ◽  
Author(s):  
Tuanny Cajuhi ◽  
Gesa Ziefle ◽  
Jobst Maßmann ◽  
Markus Furche
Keyword(s):  
Cyclic Deformation ◽  
Unsaturated Flow ◽  
Temporal Evolution ◽  
Numerical Models ◽  
Climatic Conditions ◽  
Opalinus Clay ◽  
Northwestern Part ◽  
Tunnel Wall ◽  
Clay Formation ◽  
Mont Terri

<p>The Mont Terri rock laboratory is situated in a clay formation in the northwestern part of Switzerland and is the place of several research focused experiments. These experiments enable the study of relevant coupled effects in the Opalinus Clay formation, an important material in the context of radioactive waste management due to its possible use as geological barrier. Our study focuses on the cyclic deformation (CD-A) experiment, which aims at investigating the coupled hydro-mechanical (HM) behavior of the material, e.g. shrinkage, swelling, changes in permeability. These processes can affect the stability and integrity of the rock. The experiment encompasses seasonal variations such as natural cyclic humidity changes due to winter and summer and consists of two niches. While one niche is open to the influence of the surroundings and hence, subjected to the effects of the seasonal changes of air humidity, the other niche is kept under controlled, high humidity conditions. Long-term quasi-continuous as well as repeating measurements for parameters such as relative humidity, water content, temperature, electrical resistivity and deformation, e.g. tunnel wall convergence via laser scans, are carried out amongst others (start in October 2019). These monitoring data are used as input for calibration and validation of numerical models.</p><p>In this contribution we numerically model the HM coupled effects in the context of the CD-A experiment using a macroscopic poromechanical approach. The mathematical model consists of the mass balance of the solid and the liquid phases with displacements and pore pressure as independent variables. Furthermore, it considers unsaturated flow by the Richards approximation. The model is solved numerically with the finite element method using the open-source software OpenGeoSys (OGS 6). Based on a literature review on the material properties of the clay sandy facies and experimental data, a two-dimensional model has been setup stepwise considering (i) in-situ initial pore water pressures and stresses, (ii) the effect of excavation, (iii) the experimentally based seasonal climatic conditions in the niches and (iv) the effect of bedding-induced anisotropy on the HM coupled behavior. With this numerical investigation, we evaluate the temporal evolution of the unsaturated zone. The Nuclear Magnetic Resonance (NMR), Electric Resistivity Tomography (ERT) and Taupe measurements indicate the spatial and temporal evolution of the seasonal hydraulic effects near the niches within the first experimental year. A first comparative study indicates qualitative agreement between monitored ERT data and simulation results and offers paths for model improvement and extension such as in the context of shrinkage-induced cracking.</p>

scholarly journals Deformation mechanisms and evolution of the microstructure of gouge in the Main Fault in Opalinus Clay in the Mont Terri rock laboratory (CH)

Solid Earth ◽  
2018 ◽  
Vol 9 (1) ◽  
pp. 1-24 ◽  
Author(s):  
Ben Laurich ◽  
Janos L. Urai ◽  
Christian Vollmer ◽  
Christophe Nussbaum
Keyword(s):  
Electron Microscopy ◽  
Fault Zone ◽  
Shear Zones ◽  
Deformation Mechanisms ◽  
Opalinus Clay ◽  
Rock Laboratory ◽  
Main Fault ◽  
Clay Formation ◽  
Mont Terri ◽  
Mont Terri Rock Laboratory

Abstract. We studied gouge from an upper-crustal, low-offset reverse fault in slightly overconsolidated claystone in the Mont Terri rock laboratory (Switzerland). The laboratory is designed to evaluate the suitability of the Opalinus Clay formation (OPA) to host a repository for radioactive waste. The gouge occurs in thin bands and lenses in the fault zone; it is darker in color and less fissile than the surrounding rock. It shows a matrix-based, P-foliated microfabric bordered and truncated by micrometer-thin shear zones consisting of aligned clay grains, as shown with broad-ion-beam scanning electron microscopy (BIB-SEM) and optical microscopy. Selected area electron diffraction based on transmission electron microscopy (TEM) shows evidence for randomly oriented nanometer-sized clay particles in the gouge matrix, surrounding larger elongated phyllosilicates with a strict P foliation. For the first time for the OPA, we report the occurrence of amorphous SiO2 grains within the gouge. Gouge has lower SEM-visible porosity and almost no calcite grains compared to the undeformed OPA. We present two hypotheses to explain the origin of gouge in the Main Fault: (i) authigenic generation consisting of fluid-mediated removal of calcite from the deforming OPA during shearing and (ii) clay smear consisting of mechanical smearing of calcite-poor (yet to be identified) source layers into the fault zone. Based on our data we prefer the first or a combination of both, but more work is needed to resolve this. Microstructures indicate a range of deformation mechanisms including solution–precipitation processes and a gouge that is weaker than the OPA because of the lower fraction of hard grains. For gouge, we infer a more rate-dependent frictional rheology than suggested from laboratory experiments on the undeformed OPA.

Tritium and Iodide Diffusion through Opalinus Clay

2000 ◽  
Vol 663 ◽  
Author(s):  
A. Yllera de Llano ◽  
M. Mingarro Sainz-Ezquerra ◽  
M. García Gutiérrez
Keyword(s):  
Methodological Approach ◽  
Time Lag ◽  
Tritiated Water ◽  
Effective Diffusion ◽  
Opalinus Clay ◽  
Small Scale ◽  
Northwestern Part ◽  
Large Variability ◽  
Rock Laboratory ◽  
Mont Terri

ABSTRACTThe International Mont Terri Project [1, 2] started in 1995 under the patronage of the Swiss National Hydrological and Geological Survey (SNHGS), and has the authorization of the République et Canton du Jura. The underground rock laboratory is located at the northwestern part of Switzerland (Canton Jura), in and beside the reconnaissance gallery of the Mont Terri motorway tunnel, one of the several tunnels of the A16 “Transjurane” motorway. The depth of overburden above the rock laboratory is approximately 300 meters.The project is aimed to investigate the geological, hydrogeological, geochemical and rock mechanical properties of the Opalinus Clay for assessing the feasibility and safety of a repository for radioactive waste placed in this type of host rock. One of the issues under study is radionuclide migration by diffusion through clays. As a part of this investigation, an interlaboratory comparison on small-scale diffusion experiments was carried out by three research laboratories: AEA Technology (UK), SCK·CEN (Belgium) and CIEMAT (Spain). The radionuclides investigated were tritium and iodine. This paper concerns to the methodological approach and results of the experiments undertaken by CIEMAT.The effective diffusion coefficients were measured for tritiated water and iodine (as I-), resulting larger for tritium [(1.7±0.4)×10−11 m2/s] than for iodide [(2.7±0.3)×10−12 m2/s]. The porosity available for diffusion was calculated by using the time-lag method, but some results seemed unrealistic and showed a large variability. In general, tritium exhibited higher values of porosity than iodide (17 to 26% and 12 to 17%, respectively), which were consistent with the anion exclusion affecting the distribution of iodide into the clay pores.

scholarly journals A MiniSandwich Experiment with Blended Ca-Bentonite and Pearson Water—Hydration, Swelling, Solute Transport and Cation Exchange

Minerals ◽  
2021 ◽  
Vol 11 (10) ◽  
pp. 1061
Author(s):  
Katja Emmerich ◽  
Eleanor Bakker ◽  
Franz Königer ◽  
Christopher Rölke ◽  
Till Popp ◽  
...  
Keyword(s):  
Solute Transport ◽  
Cation Exchange ◽  
Numerical Models ◽  
Chloride Concentration ◽  
Injection Pressure ◽  
Sodium Concentration ◽  
Opalinus Clay ◽  
Hydraulic Permeability ◽  
Data Set ◽  
Clay Formation

Shaft seals are geotechnical barriers in nuclear waste deposits and underground mines. The Sandwich sealing system consists of alternating sealing segments (DS) of bentonite and equipotential segments (ES). MiniSandwich experiments were performed with blended Ca-bentonite (90 mm diameter and 125 mm height) to study hydration, swelling, solute transport and cation exchange during hydration with A3 Pearson water, which resembles pore water of Opalinus Clay Formation at sandy facies. Two experiments were run in parallel with DS installed either in one-layer hydrate state (1W) or in air-dry two-layer hydrate (2W) state. Breakthrough at 0.3 MPa injection pressure occurred after 20 days and the fluid inlet was closed after 543 days, where 4289 mL and 2984 mL, respectively, passed both cells. Final hydraulic permeability was 2.0–2.7·10−17 m2. Cells were kept for another 142 days before dismantling. Swelling of DS resulted in slight compaction of ES. No changes in the mineralogy of the DS and ES material despite precipitated halite and sulfates occurred. Overall cation exchange capacity of the DS does not change, maintaining an overall value of 72 ± 2 cmol(+)/kg. Exchangeable Na+ strongly increased while exchangeable Ca2+ decreased. Exchangeable Mg2+ and K+ remained nearly constant. Sodium concentration in the outflow indicated two different exchange processes while the concentration of calcium and magnesium decreased potentially. Concentration of sulfate increased in the outflow, until it reached a constant value and chloride concentration decreased to a minimum before it slightly increased to a constant value. The available data set will be used to adapt numerical models for a mechanism-based description of the observed physical and geochemical processes.

scholarly journals Claystone formations in Germany: what we (don't) know about them and how we can change this

2021 ◽  
Vol 1 ◽  
pp. 47-48
Author(s):  
Bernhard Schuck ◽  
Tilo Kneuker
Keyword(s):  
Phase I ◽  
Nuclear Waste ◽  
Site Selection ◽  
Numerical Models ◽  
Selection Procedure ◽  
Opalinus Clay ◽  
Nuclear Waste Repository ◽  
Fracture Density ◽  
Clay Formation

Abstract. Deep geological formations are considered for safe long-term disposal of high-level radioactive waste. Such a repository would be requested to prevent radionuclides from entering the biosphere for a period of 1 million years (StandAG, 2017). Consequently, a holistic characterization including lithological, mineralogical, geochemical, hydrological, structural and geomechanical properties of any potential repository-hosting rock formation is required. Nine claystone formations have been identified as “sub-areas” within the German site-selection procedure (BGE, 2020). The area covered by these formations comprises about half of the total area considered as being qualified for further exploration. However, despite its relevance to act as a geological barrier for, e.g. hydrocarbons or radionuclides, the characterization of clay-rich formations at depths exceeding 300 m in Germany has attained substantially less attention than economically more relevant units hosted by, e.g. sandstones or rock salt, which have been intensively explored. The BGR project BASTION aims at contributing to characterizing these claystone formations and emphasizes properties relevant to host a repository for nuclear waste. Investigations comprise (micro)structural/petrographic, mineralogical, geochemical, geophysical, hydraulic and thermomechanical analyses. In project phase I (2013–2019), claystones deposited in Northern Germany during the Lower Cretaceous were studied. These rocks belong to the fourth largest sub-area hosting claystones. Two of the main foci were to explore variations in lithology, mineralogy and geochemistry, and to identify deformation mechanisms (natural and artificial) by microstructural analyses. Although rocks appeared to be quite homogeneous on the 10–100 m scale, the results revealed distinct structural and sedimentary heterogeneities on the meter scale affecting fracture density. Another sub-area located in Southern Germany hosts the Opalinus Clay Formation (OPA). This up to 150 m thick claystone formation was deposited during the Middle Jurassic (Franz and Nitsch, 2009). Owing to its self-sealing capacity and ability to retain fluids, it is supposed to host the nuclear waste repository of Switzerland (Bossart et al., 2017). The OPA is quite well understood in terms of its lithology and (bio)stratigraphy, and there have been mineralogical, hydrological and petrophysical analyses, mostly documented in university theses a few decades old and sometimes difficult to access. However, it is questionable to what extent these investigations reflect the situation at depths relevant for the site-selection procedure. Well-documented data on the OPA and its properties relevant for nuclear waste disposal are available via the Swiss site-selection procedure (Bossart et al., 2017). However, as there remain substantial questions regarding the nature of the German portions of the OPA (e.g. spatial distribution of lithology, mineralogy, microstructures) at depths greater than a few decameters, it is unclear to what degree insights obtained in the Swiss site-selection procedure also account for Germany. Therefore, phase II of BASTION, which began in 2020, aims to use the multidisciplinary approach developed during phase I to characterize properties of the OPA relevant for the save long-term disposal of nuclear waste by identifying and quantifying structural and rheological heterogeneities. This will constitute important input for numerical models in any long-term safety assessment.

scholarly journals Deformation mechanisms and evolution of the microstructure of gouge in the Main Fault in Opalinus Clay in the Mont Terri rock laboratory (CH)

2017 ◽  
Author(s):  
Ben Laurich ◽  
Janos L. Urai ◽  
Christian Vollmer ◽  
Christophe Nussbaum
Keyword(s):  
Shear Zones ◽  
Deformation Mechanisms ◽  
Opalinus Clay ◽  
Reverse Fault ◽  
Amorphous Sio2 ◽  
Rock Laboratory ◽  
Main Fault ◽  
Clay Formation ◽  
Mont Terri ◽  
Mont Terri Rock Laboratory

Abstract. We studied gouge from an upper-crustal, low offset reverse fault in slightly overconsolidated claystone in the Mont Terri rock laboratory (CH). The laboratory is designed to evaluate the suitability of the Opalinus Clay formation (OPA) to host a repository for radioactive waste. The macroscopically dark gouge displays a matrix-based, P-foliated microfabric bordered and truncated by μm-thin shear zones consisting of aligned clay grains, as shown by BIB-SEM and optical microscopy. TEM-SAED shows evidence for randomly oriented nm-sized clay particles in the gouge matrix, surrounding larger elongated phyllosilicates with a strict P-foliation. For the first time in OPA, we report the occurrence of amorphous SiO2 grains within the gouge. Gouge has lower SEM-visible porosity and almost no calcite grains, compared to undeformed OPA. We present two hypotheses to explain the origin of gouge in the Main Fault: (i) "authigenic generation": fluid-mediated removal of calcite from deforming OPA during shearing, (ii) and "clay smear": mechanical smearing of calcite-poor (yet to be identified) source layers into the fault zone. Based on our data we prefer the first or a combination of both, but more work is needed to resolve this. Microstructures indicate a range of deformation mechanisms including solution-precipitation processes and a gouge which is weaker than OPA because of the lower fraction of hard grains. We infer that the long-term rheology of gouge is more strongly rate-dependent than suggested from laboratory experiments.

scholarly journals Building the bridge between safety requirements and numerical modeling: an example considering crack development of Opalinus clay in laboratory and field scales

2021 ◽  
Vol 1 ◽  
pp. 165-167
Author(s):  
Tuanny Cajuhi ◽  
Jobst Maßmann ◽  
Gesa Ziefle
Keyword(s):  
Numerical Modeling ◽  
Numerical Model ◽  
Large Scale ◽  
Numerical Models ◽  
Laboratory Scale ◽  
Opalinus Clay ◽  
Air Humidity ◽  
Mont Terri

Abstract. Salt, crystalline and clay formations are under discussion as potential host rocks for storage of heat-generating radioactive waste. Each of these rocks has a different structure and composition, and consequently a different material behavior. The latter needs to be studied and evaluated with respect to the main aim: to find a place to store the waste in a safe and sustainable manner. Several requirements in the context of the safety of a repository need to be fulfilled, concerning the long-term as well as the operational phase. One key point in this matter is the integrity, which refers to retention of the isolating rock zone's containment capabilities. With the focus on some experimental and numerical investigations on the excavation influenced near-field behavior of Opalinus clay (OPA), this contribution aims to illustrate an example for the role of numerical modeling in safety assessment. Once, e.g. anthropogenic action such as excavation starts, the natural state of equilibrium in the formation is disturbed. Trying to restore it, the rock deforms (convergence) and/or releases energy in other ways such as cracking. This could lead to loss of integrity since crack nucleation and propagation can affect the mechanical stability and create paths to transport contaminants. During operation in the excavated rock, environmental changes, e.g. temperature and humidity, further affect its behavior. The understanding of these dynamic phenomena ideally needs to occur at the in situ scale; however, performing an experiment in the spatial and time scales of interest is not always possible. For this reason, the in situ problem needs to be formulated, abstracted and mathematically modeled. The interpretation of the results must take place with simplifying assumptions and complementary laboratory scale experiments can be used to improve understanding of the system. The real problem is approached stepwise, each step associated to the size of the model and its complexity. The gradually obtained knowledge is necessary to achieve a better understanding of the process and to evaluate the capacities and limitations of the models. This contribution aims at showing the basic practical steps for numerical modeling with particular focus on the preparation and interpretation of the models and results, e.g. model calibration, verification and validation. As an example, the OPA at the Mont Terri site is chosen. The material parameters are obtained either experimentally or from the literature. We choose and perform laboratory scale simulations that are related to nearly the same mechanism as in the in situ scale. To have a first impression on the latter, a simplified, large-scale numerical model is prepared. The mechanism in study is drying and wetting, which is associated with shrinkage and swelling. We analyze the pore pressure and stress development in both scales. Thus, hydraulic mechanically coupled approaches are essential. The concept of effective stress is used, which combines the contributions of the solid and fluid phases (gas and liquid). In the current modeling approach, the gas pressure remains constant (atmospheric pressure) and during drying, the liquid pressure induces capillary pressure development and decrease of saturation. The laboratory scale simulation is important to evaluate the model of choice and to assess potential numerical problems. Furthermore, it can be used to perform a sensitivity study of material and numerical parameters. This step is necessary during the development or extension of numerical models as well as to evaluate their applicability on new research questions. The simplified in situ scale numerical model is then extended. In this phase the numerical model is evaluated once again, especially with respect to its complexity. Furthermore, specific questions related to this scale are posed: overall behavior of the rock, influence of the excavation, seasonal and long-term effects. In this contribution we deal with the long-term cyclic deformation (CD-A) experiment. The CD-A experiment has been taking place in the Mont Terri Rock Laboratory since October 2019. It consists of twin niches, a closed and an open niche, subjected to either high air humidity or seasonal humidity changes leading to saturation/desaturation during summer/winter in the OPA, respectively. Several parameters are periodically or continuously measured, including relative air humidity, convergence and crack development. We attempt to transfer the knowledge and numerical models developed in the small scale to the large scale and to evaluate the possibilities and limitations of the chosen approaches by comparing the numerical and experimental results.

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