Opalinus Claystone Research Articles

Geological analysis of the hydro-mechanical responses of Opalinus claystone to excavation-induced damage via engineering

In the realm of engineering geology, specifically concerning the containment of high-level radioactive waste, this study examines the excavation-induced damage zone (EDZ) in underground drifts and its evolution, which is critical for the effectiveness of underground repositories. The focus is on the self-sealing capability of the EDZ in Opalinus claystone (OPA) under mechanical compression and hydration induced by a bentonite core sealing component post-nuclear waste emplacement. This core applies a swelling pressure of 5 MPa to the drift walls during resaturation, affecting repository integrity. Using numerical modeling, this study pioneers the simulation of real-time self-sealing mechanisms in OPA, a crucial geological barrier in nuclear waste containment. The approach integrates a plastic damage model with an auxiliary deformation model and Biot’s model to accurately depict moisture-dependent OPA deformation. This modeling is essential for understanding claystone geomechanics in nuclear waste repositories. Detailed post-analysis of crack features and unsaturated fractured OPA permeability, following fracture energy regularization principles and the cubic law, provides insights into the geotechnical properties of the EDZ, enhancing the accuracy of geological forecasts in nuclear waste management. The model’s reliability is confirmed through various numerical simulations, including triaxial compression tests, observations of swelling deformations due to moisture variations, and comprehensive water permeability tests assessing the self-sealing efficiency of fractured OPA during water injection. In situ experiments at the Mont Terri Rock Laboratory support these findings, particularly regarding EDZ self-sealing under combined mechanical and hydration conditions. Comparative analysis of numerical results and experimental data validates the model’s ability to replicate fractured OPA self-sealing behavior, crucial for assessing nuclear waste repository safety and effectiveness. Effective self-sealing is underscored by reduced water permeability in the EDZ during resaturation, affirming the model’s relevance in evaluating nuclear waste containment strategies. Moreover, insights from this model extend to assessing CO2 migration, given claystones’ role as caprocks in CO2 geological sequestration schemes, thereby enhancing its importance in long-term environmental safety evaluations for both nuclear waste and CO2 storage scenarios.

Importance of Water‐Clay Interactions for Fault Slip in Clay‐Rich Rocks

AbstractClay‐rich rocks are integral to subduction zone dynamics and of practical importance, for example, as barriers in nuclear waste and CO2 repositories. While the effects of swelling strain on the self‐sealing capabilities of these rocks are relatively well‐established, the implications of polar fluids interacting with charged clay particles on the frictional behavior, and the role of swelling stress in initiating slip in critically stressed faults, remain ambiguous. To address these uncertainties, we conducted triaxial friction experiments using saw‐cut samples, with the upper half composed of Opalinus claystone (OPA) and the lower half of Berea sandstone (BER). The frictional strength of the non‐wetted OPA‐BER interface was estimated based on experiments at confining pressures of 4–25 MPa and constant axial loading rate (0.1 mm/min). Fluid injection friction experiments were performed using decane (non‐polar fluid) or deionized water (polar fluid) at 10 and 25 MPa confining pressures and constant piston displacement control. Macroscopic mechanical data were complemented by distributed strain sensing on the sample surface. Compared to decane, the frictional strength of the OPA‐BER interface tended to decrease when injecting water, which is attributed to phyllosilicate lubrication and the transition of the OPA from a solid rock to an incohesive mud near the saw‐cut surface. When injecting water, slip was initiated during initial hydration of the OPA‐BER interface, although the apparent stress state was below the yield stress. To explain this behavior, we propose that the swelling stress is a crucial factor that should be integrated into the effective stress model.

Chemo‐Mechanical Coupling in Fractured Shale With Water and Hydrocarbon Flow

AbstractThe transport of chemically reactive fluids through fractured clay‐rich rocks is fundamental to many subsurface engineering technologies. Here, we present results of direct‐shear laboratory experiments with simultaneous imaging by X‐ray Computed Tomography in Opalinus claystone with subsequent fluid injection to unravel the interplay between mechanical fracture deformation, fluid sorption, and flow. Under constant radial stress (σc = 1.5 MPa), the average mechanical aperture increases with shear displacement. Upon brine injection, is reduced by 40% relative to initial conditions ( μm) and fluid‐sorption induces a divergent displacement of the two sample halves (Δh = ±50 − 170 μm) quantified by digital image correlation. None of these changes are observed in a control experiment with decane, indicating that creep is subordinate to swelling in sealing the fracture. Swelling‐induced changes in permeability within the fracture are heterogeneous and largely affect the fracture flow field, as computed using numerical simulations.

Shear strength testing of consolidated claystones: breakpoint detection of shear stress versus shear displacement curves, a statistical approach

The evaluation of shear stress versus shear displacement curves is in the main focus of geotechnical engineering. Such curves, depending on the rock assessed, consist of a quasi-linear section, followed by a “kick” representing the peak shear strength, and a residual part, mostly parallel to the abscissa. The aim of the present study is to facilitate the future automatic detection of these crucial characteristics to take a step towards replacing their visual/analogue determination via modern statistical tools. Breakpoint detection methods (Cross-Entropy, Change Point Model) were applied to curves obtained from laboratory shear tests describing the shearing along discontinuities of nine Mont Terri Opalinus Claystone samples. Smooth and moderately rough claystone surfaces were studied. Results indicated that the end of the rising section and the kick observed on the shear strength curves was effectively approximated with the Change Point Model framework. An additional practical advantage of applying statistical tools such as breakpoint detection to shear strength determination is that it ensures the comparability of the obtained results.

Study of the permeability in the Opalinus clay series (Mont Terri - Switzerland) using the steady state method in Hassler cell

The Mont Terri rock laboratory located in the Northwestern Switzerland is an international platform for applied research in the field of radioactive waste disposal in clayrock formations. A borehole named BDB1 has been drilled in the Opalinus claystone in 2013 to assess its containment properties. Characterization of petrophysical and hydrogeological parameters in the Jurassic shale known as Opalinus Clay were performed by in situ experiments conducted in the rock laboratory of Jura Mountains and on borehole samples. This paper presents the results of laboratory measurements of permeability in a permeameter called Hassler cell by using the steady state method. The study is applied to core samples drilled perpendicular and parallel to the bedding, and involves gas and water permeability measurements. The Hassler cell is an experimental device that allows to put samples under a confining pressure reproducing the in situ one. In all, results were obtained for nine samples over sixteen for gas permeability measurements with values ranging between 6.9 × 10−21 and 2.3 × 10−19 m2 and one sample for water permeability measurements with a value of 1.38 × 10−20 m2. Most of the results are in good agreement with previous studies obtained by in situ experiments and using the Poiseuille type law method. The usage of several methods to determine the permeability of the Opalinus clay unit helps to conclude about the accuracy, trueness and precision of the measurements.

Identification of log units in clay rock formations based on local and spatial statistics of well-log properties: application to the Opalinus claystone in the Benken borehole

Abstract Due to their particularly good mechanical and self-healing properties combined with exceptionally efficient cation adsorbents and exchanger capacities, clay minerals and clay rock formations are considered as suitable geological barriers for radioactive waste disposal. The Middle Jurassic Opalinus Clay Formation has been identified as a potential host rock. Logging data were measured at the Benken borehole drilled through this formation in northern Switzerland. This paper presents a statistical methodology to improve the description of the physical properties of the clay rock based on the well-log data. The methodology involves the classification of a set of local statistics, calculated from a reduced number of principal components computed from well-log properties. The use of a kernel-based method to calculate local statistics allows an analysis of spatial variability to be carried out at different scales, and with different scale effects. The first-order layering was found to be robust and independent of kernel size (i.e. observation scale), while preserving small-scale heterogeneities that are useful for further interpretation. The log units can be more clearly interpreted in terms of stationary or transitional log units, depending on the behaviour of local statistics. Finally, the derived spatial variability of the log-units properties are compared with earlier lithological descriptions and stratigraphic data.

Chemo‐mechanical behavior of clay‐rich fault gouges affected by CO2‐brine‐rock interactions

AbstractThe impact of long‐term CO2‐brine‐rock interactions on the frictional properties of faults is one of the main concerns when ensuring safe geological CO2 storage. Mineralogical changes may alter the frictional strength and seismogenic potential of pre‐existing faults bounding a storage complex. However, most of these reactions are too slow to be reproduced on laboratory timescales and can only be assessed using geochemical modeling. We combined modeling of CO2‐charged formation water and fault gouges (1–1000 years residence time, i.e. 10–106 pore volume flushes) with friction experiments on simulated fault gouges (T = 22–150°C; σneff = 50 MPa; Pf = 25 MPa; V = 0.2‐100 μm/s), having mineralogical compositions as predicted by the models. As an analogue for clay‐rich caprocks overlying potential CO2 storage sites in Europe, we used the Opalinus claystone. Our experiments showed that, although significant mineralogical changes occurred, they did not significantly change the frictional behavior of faults. Instead, initial fault‐gouge mineralogy imposed a stronger control on clay‐rich fault behavior than the extent of CO2‐brine‐rock interactions, even under chemical conditions allowing for significant reaction. We demonstrated that the impact of mineralogical changes due to CO2‐brine‐rock interactions on the frictional behavior and seismogenic potential of faults could be assessed using our combination of geochemical modeling and friction experiments. Note that a complete understanding requires evaluation of additional effects, such as that of shear velocity, effective normal stress, and other fault characteristics (maturity, shear strain). © 2018 The Authors. Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons, Ltd.

Testing High-Voltage Electrical Discharges in Disintegrating Claystone for Isotopic and Mineralogical Studies: An Example Using Opalinus Claystone

AbstractThe radiogenic isotope systematics of clay minerals are complex because of the intimate mixture of minerals from different origins such as detrital and authigenic sources. An important aspect of dating clays is the primary sample preparation and disintegration method. In the present study, a sample of weakly deformed Opalinus claystone from the Mont Terri underground site (Switzerland) was investigated after disintegration by three different methods. The Opalinus Clay was sedimented in the late Toarcian and early Aalenian and reached maximum temperatures of ~85°C during burial in the Cretaceous. The present study reports data from a comprehensive investigation comparing the effects of disintegration by: (1) disc milling; (2) repeated freezing and thawing; and (3) high-voltage discharges. K-Ar age values of the finest clay (<0.1 µm) released by the different disintegration methods are indistinguishable, indicating that the high-voltage liberation method does not influence grains as small as 100 nm. The K-Ar age values of particle-size separates decreased with decreasing particle size. The age values of the 2–6 µm separates correspond to the Carboniferous Period, which reflects the dominance of Paleozoic detritus in that size range. The age values of the smallest separates (<0.1 µm), on average 213 ± 4 Ma, exceed the numerical age of the formation (~177−172 Ma), which show predominance of detrital grains over authigenic grains even in the finest illite. In summary, isotope geochronology data suggest that the high-voltage method can be applied reliably for disintegrating claystones.

Rare metals on shatter cone surfaces from the Steinheim Basin (SW Germany) – remnants of the impacting body?

AbstractThe ~3.8 km Steinheim Basin in SW Germany is a well-preserved complex impact structure characterized by a prominent central uplift and well-developed shatter cones that occur in different shocked target lithologies. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy and electron probe microanalysis have revealed, for the first time, the occurrence of rare metals on the Steinheim shatter cone surfaces. Shatter cones produced from the Middle Jurassic (Aalenian) Opalinus Claystone (‘Opalinuston’), temporarily exposed in the central uplift in spring 2010, and shatter cones in Upper Jurassic (Oxfordian) limestones from the southeastern crater rim domain are commonly covered by faint coatings. The Opalinus Claystone shatter cone surfaces carry coatings dominated by Fe, Ca, P, S and Al, and are covered by abundant small, finely dispersed microparticles and aggregates of native gold, as well as locally elevated concentrations of Pt. On several surfaces of the claystone shatter cones, additional Fe, Ni and Co was detected. The Ca–Mn-rich coatings on the limestone shatter cone surfaces locally include patches of Fe, Ni, Co, Cu and Au in variable amounts and proportions. The intriguing coatings on the Steinheim shatter cones could either stem from the impacted Lower Jurassic to Palaeogene sedimentary target rocks; from the crystalline-metamorphic Variscan crater basement; or, alternatively, these coatings might represent altered meteoritic matter from the Steinheim impactor, possibly an iron meteorite, which may have been remobilized during post-impact hydrothermal activity. We here discuss the most plausible source for the rare metals found adherent to the shatter cone surfaces.

The Angle Between the Sample Surface and the Shear Plane; Its Influence on the Direct Shear Strength of Jointed Granitic Rocks and Opalinus Claystone

Shear strength along rock discontinuities by means of direct shear strength tests were performed on granitic and clayey rocks. Both rock types represent potential host rocks of radioactive waste. The paper focuses on the effect of one parameter in particular influencing the shear strength: the angle enclosed by the plane of the sample surface and the shear plane, in the direction of shear. 3D surface roughness measurements were carried out on 18 Opalinus Claystone sample (Switzerland) and 6 on granitic rock sample (Hungary) surfaces. An interval of angle was analyzed in the direction of shear and against the direction of shear. Direct shear strength tests were carried out under constant normal loading (CNL) conditions, the influence of the angle on the shear strength values were determined. Additionally, a percentage value was calculated for how much the range of both peak and residual shear strength values change, with taking into consideration the effect of upslope and downslope shear.

Shear Strength Behavior of Mont Terri Opalinus Claystone in Fault Zone

The paper summarizes the results of direct shear strength tests carried out on Opalinus Claystone deriving from a fault zone, at the site of Mont Terri (North-Western Switzerland). In Mont Terri an Underground Rock Mechanical Laboratory is conducting studies aimed at specifying the rock mechanical parameters of the rock surrounding, which is at the same time the potential host rock of a highly active radioactive waste disposal facility. The samples for the tests were derived from blocks and cores collected from the main fault zone, during the excavation of a tunnel, named: Full scale Emplacement demonstration experiment Gallery (FE Gallery). Direct shear strength tests were carried out on 50x50 mm samples undergoing a 1 MPa constant normal load, where the natural shear plane was parallel to the direction of the shear. The orientation of the bedding planes in the samples was recorded and the maximal shear strength and residual shear stress were determined. The maximal shear strength values obtained from the tests fell in the interval 0.34-0.78 MPa, while the residual values fell between 0.31 and 0.72 MPa. The effect on the shear strength of different angles defined by the bedding and the shear plane of the samples was also examined. The bedding of the samples enclosed different angles with the shear plane, ranging from 0 to 49 degrees.

Shear strength behavior of Mont Terri Opalinus Claystone in fault zone

The paper summarizes the results of direct shear strength tests carried out on Opalinus Claystone deriving from a fault zone, at the site of Mont Terri (North-Western Switzerland). In Mont Terri an Underground Rock Mechanical Laboratory is conducting studies aimed at specifying the rock mechanical parameters of the rock surrounding, which is at the same time the potential host rock of a highly active radioactive waste disposal facility. The samples for the tests were derived from blocks and cores collected from the main fault zone, during the excavation of a tunnel, named: Full scale Emplacement demonstration experiment Gallery (FE Gallery). Direct shear strength tests were carried out on 50x50 mm samples undergoing a 1 MPa constant normal load, where the natural shear plane was parallel to the direction of the shear. The orientation of the bedding planes in the samples was recorded and the maximal shear strength and residual shear stress were determined. The maximal shear strength values obtained from th...

Shear Strength Behavior of Mont Terri Opalinus Claystone in Fault Zone

Shear Strength Behavior of Mont Terri Opalinus Claystone in Fault Zone

A Laboratory Investigation on Thermal Properties of the Opalinus Claystone

Some aspects of the thermal behavior of the Opalinus claystone are investigated through laboratory tests conducted on a new hollow cylinder triaxial apparatus specially designed for studying the thermo-hydro-mechanical behavior of very low permeable materials. Two hollow cylinder samples are first resaturated under isotropic stress state equal to the mean effective in situ one in order to minimize swelling and induced damage during the resaturation phase. Two drained heating–cooling cycles are performed on the first sample of Opalinus claystone. During the first cycle, a thermo-elasto-plastic response similar to that of plastic clays with low overconsolidation ratio is obtained. The thermal hardening of the sample is demonstrated by the quasi-reversible behavior of the sample during the second heating–cooling cycle. An undrained heating test performed on the second sample of Opalinus claystone induces an excess pore pressure in this sample. This induced pore pressure is attributed to the higher thermal expansion coefficient of pore water compared to that of the solid phase. It is shown that the excess pore pressure generated in the sample by undrained heating cannot be modeled by considering the free water thermal expansion coefficient. The thermal expansion coefficient of the Opalinus claystone water is back-analyzed from the experimental results which show a higher value than free water.

Study of the micro-mechanical behaviour of the Opalinus Clay: an example of co-operation across the ground engineering disciplines

The high degree of scientific cross-fertilisation possible between the three geo-engineering disciplines soil mechanics, rock mechanics and engineering geology, is demonstrated by means of a micro-mechanical model of the Opalinus Clay. After a brief review of Terzaghi’s effective stress principle and the importance of micro-mechanical models in general, a conceptual study of a micro-mechanical model of a claystone is presented in some detail. The model is based on the Particle Flow Code (PFC) developed by Itasca Corp. It introduces into the model the pertinent composition and structure of the Opalinus Claystone established in the local engineering geology of Switzerland and SW Germany. This includes elongated clay platelets, various layers of densified water around the platelets, free water in the pores and a specific texture of the platelets after consolidation. The model is numerically subjected to a series of loading stages. It is shown that the micro-mechanical model reproduces a number of features which have been known for a long time in soil and rock mechanics but which are often intractable in conventional generic models. The features include non-linear stress–strain curves with pre-failure damage and post-failure strain softening, a non-linear increase of the particle contacts with loading, distinct clustering of deformations, clustering of micro cracks leading to the development of shear bands and hysteresis in cyclic loading. It is concluded that micro-mechanical models are promising tools for further development of our understanding of the mechanical behaviour of geological materials. They offer an excellent opportunity for scientific co-operation between engineering geologists and soil and rock mechanics engineers.

Detection and characterization of the disturbed rock zone in claystone with the complex resistivity method

Underground excavations provoke in their vicinity a region, where the rock is disturbed, i.e., loosened due to micro as well as macro fractures. Shape, dimension and properties of this so-called ‘excavation damaged zone (EDZ)’ are of increasing importance for the planning and construction of geotechnical barriers in underground repositories for toxic and problematic wastes. One way of assessing the EDZ around a drift is the investigation of the geoelectrical complex resistivity. In addition to resistivity, the phase measurements yield information about the pore spaces and not only about the water content. Measurements of resistivity and phase are conducted in a newly built tunnel in the opalinus claystone of the Mont Terri underground rock laboratory. Special ring profiles, as well as a geometry-adapted inversion scheme, are used. Significant local and temporal changes are found in the resistivity as well in the phase, which are directly associated with the disturbed rock zone. Laboratory measurements allow the correlation and quantification of resistivity and water saturation of the rock. A geotechnical modelling and an analysis of the stress pattern around the drift show that the geoelectrical properties agree remarkable with the mechanical state of the rock.

Regulatory objectives and safety requirements for the disposal of radioactive wastes in Switzerland

Abstract According to Swiss Law the producers of radioactive waste are responsible for management and safe disposal of all types of radioactive waste. Therefore, in 1972, the electricity utilities involved in nuclear power production formed the National Cooperative for the Disposal of Radioactive Waste (NAGRA) which is responsible for final disposal and related work. The general disposal strategy includes two repository siting programmes, a deep repository for high-level waste in Northern Switzerland (crystalline basement or Opalinus claystone) and a repository with horizontal access for low- and intermediate-level waste in the Alps (marls and shales of the Wellenberg-site). The safety of waste disposal is assessed on the basis of specific protection objectives formulated by the Swiss regulatory authority. The protection objectives limit the annual dose equivalent or the radiological risk to an individual and they require that post-closure safety must not depend on long-term monitoring or any other interventions to ensure safety. After a final repository has been sealed, it must be possible to dispense with safety and surveillance measures.