Host Rock For Repository Research Articles

Effect of fracture voids on the swelling behaviour of Callovo-Oxfordian claystone

The Callovo-Oxfordian (COx) claystone has been considered as a possible host rock for deep geological repository of high-level radioactive waste in France. When constructing the repository, it is unavoidable to generate a fractured zone in the vicinity of excavated repository walls. In the assessment of the repository performance, it is important to investigate the role of the fractures on the swelling capacity of COx claystone and to further characterize the fractured zone evolution. In this study, the effect of fracture voids on the swelling behaviour of COx claystone was investigated by simulating the fracture voids with a space created by a limited amount of grains sandwiched between claystone disks, and by performing a series of swelling tests in oedometer under a low vertical stress (0.05 MPa) and swelling pressure tests under constant-volume condition. Results showed that, with water infiltrating into the sample with fracture voids, the grains collapsed before the swelling of claystone disks. The whole sample was composed of swelling zones corresponding to the claystone disks with higher dry density than the expected final value, and of compression zones corresponding to the initial fractures with lower dry density. Moreover, the axial strain and swelling pressure decreased as the fracture void increased. The maximum fracture voids that might be fully filled by the swelling of COx claystone (sealing) were found to be 5.14%–7.65% without considering the long-term creep contribution in the field condition.

Retention behaviour of Ba(II) on argillaceous rocks of Cuddapah system

Argillaceous rock formation of Cuddapah System are under evaluation as the potential host rock for deep geological repository in India. Sorption of Ba(II), as a chemical analogue of 90Sr, on argillaceous rock formation has been investigated by batch sorption method. The effect of pH, ionic strength and [Ba(II)] on Ba(II) sorption has been investigated. Ba(II) sorption was found to be independent of pH and decreased with increasing ionic strength, indicating ion exchange as dominant sorption mode for Ba(II). Ba(II) adsorption isotherm was linear up to [Ba(II)] = 3.0 × 10–6 M. Ba(II) sorption was successfully modeled using surface complexation modeling.

Thermo-Poro-Elastic Behaviour of a Transversely Isotropic Shale: Thermal Expansion and Pressurization

The Callovo-Oxfordian (COx) claystone is considered as a candidate host rock for a deep geological radioactive waste repository in France. Due to the exothermic waste packages, the rock is expected to be submitted to temperatures up to 90 {\deg}C. The temperature rise induces deformations of the host rock, together with an increase in pore pressures, involving complex thermo-hydro-mechanical (THM) couplings. This study aims to better characterize the THM response of the COx claystone to temperature changes in the laboratory. To this end, claystone specimens were tested in a temperature controlled, high pressure isotropic compression cell, under stress conditions close to the in-situ ones. Thermal loads were applied on the specimens along different heating and cooling paths. A temperature corrected strain gage system provided precise measurements of the anisotropic strain response of the specimens. Drained and undrained thermal expansion coefficients in both transversely isotropic directions were determined. The measurement of pore pressure changes in undrained condition yielded the thermal pressurization coefficient. All parameters were analysed for their compatibility within the thermo-poro-elastic framework, and their stress and temperature dependency was identified.

Fault reactivation induced by tunneling activity in clay material: Hints from numerical modeling

Seismic events with magnitude 3 and above have been associated with the removal of rock mass in mining environment since long-time. On the contrary, little is known about the possible seismic events induced by tunneling, although it presents similarities with mining. One great example is the case of the 57 km long Gotthard Base Tunnel excavation, which has been associated more than hundred seismic events, with the largest one having magnitude of ML 2.4, damaging the tunnel infrastructures (e.g. gallery floor or portal area).Different underground structures will be built probably up to 1000 m below ground for the construction of future deep geological for the storage of nuclear waste. While seismic risk will probably not constitute a liability for the storage site construction, it is important to understand the potential for reactivation of seismogenic features located nearby the future location of emplacement tunnels.Here we present numerical simulations aimed at understanding the potential for fault reactivation during tunnel construction in clay material, a potential host rock for nuclear waste repository. We evaluate the evolution of the stress changes during the simulation of the excavation with FLAC3D numerical solver. A strain-softening friction model is used to simulate the occurrence of a sudden slip on a fault zone when critical conditions for reactivation are satisfied. This constitutes a worst-case scenario, given the low seismogenic potential of clay rocks. We also present a sensitivity analysis on several critical parameters including fault frictional properties, stress conditions, as well as different tunnel sizes at varying distance from a nearby failure plane, with the final purpose of evaluating safety of a potential nuclear repository site on the short- and long-term.

Assessment of Permian Zubers as the Host Rock for Deep Geological Disposal

Together with renewable energy sources, nuclear power represents an important contribution to a sustainable energy mix in many countries and has an important impact on sustainable development. Nuclear energy production is also a source of high-level radioactive waste (HLW) and spent nuclear fuel (SNF), which require special concern. Disposal in deep geological formations is one of the solutions for the long-term management of HLW and SNF. It requires the development of a concept ensuring long-term safe isolation of waste and its validation applying the safety case methodology, which is a formal compilation of evidence, analyses and arguments that quantify and justify a claim that the repository will be safe. The results of laboratory testing of a potential repository host rock are an important component of the evidence that helps in the safety assessment of the deep geological disposal concept. This paper presents results of research focused on the physical, geomechanical and sorption properties of the Brown and Red Zuber unit rocks from the Kłodawa Salt Mine in Poland, which together with rock salt are an important component of Polish salt domes. The Brown and Red Zubers are typical evaporite lithostratigraphic units for the Polish part of the Zechstein Basin. They consist of halite (15–85%) and water-insoluble minerals, such as anhydrite, clay minerals, carbonates, quartz and feldspar, which occurred in varying proportions in the tested samples. The properties of the zuber rocks have been compared with those of rock salt, which is considered a suitable host rock for deep geological disposal of radioactive waste.

Adsorption behaviors of Eu(III) on granite: batch, electron probe micro-analysis and modeling studies

Although granite has been widely considered as the host rock of the high-level radioactive waste (HLRW) repository in the world, the adsorption behaviors of radionuclides on granite are quite complicated and still unclear, especially at molecular scales. In this study, the adsorption behaviors of europium(III) on Beishan granite (BSG), a preliminary selection of host rock for the HLRW repository in China, were explored under environmental conditions combining batch, electron probe micro-analyzer (EPMA), and modeling approaches. X-ray diffraction (XRD) pattern confirmed that albite, quartz and biotite were the main mineralogical components for the BSG grains. Eu(III) species on BSG grains are predominant as the ion exchange (≡X3Eu0) and the inner sphere complexes of ≡SwOEu(OH)20, ≡SsOEu(OH)2,0 ≡SsOEu2+, and ≡SwOEu2+. EPMA showed that the distribution of Eu(III) on BSG grains was strongly correlated to the biotite, which suggested that biotite is the host phase for retarding trivalent actinides in the BSG grains. The presence of soil humic and fulvic acids could enhance Eu(III) adsorption on BSG grains under low-pH conditions, and inhibit the adsorption under higher pH range.

H2-fuelled microbial metabolism in Opalinus Clay

In Switzerland, the Opalinus Clay formation is considered the most likely host rock for a deep geological repository for nuclear waste. In deep geological repositories, H2 is expected to be the most abundant gas formed from the degradation of waste and from metal corrosion. The microbial community present in Opalinus Clay is capable of utilizing H2 as an electron donor and sulfate as an electron acceptor to produce hydrogen sulfide. This could be problematic due to its potential for increasing the corrosion of metal waste canisters containing radioactive waste, however, the possible impacts of these processes on the clay rock have not been fully investigated. In this study, a series of microcosm experiments were set-up containing Opalinus Clay and porewater from the Mont Terri underground research laboratory (Switzerland) as an inoculum. Uninoculated microcosms were established to investigate abiotic processes.In the presence of clay, a higher aqueous sulfate concentration was detected than in those with only porewater present and this concentration decreased over time in the inoculated experiments. However, there was no evidence of hydrogen sulfide production in the aqueous phase. In all experiments with clay, there was an increase in aqueous Fe2+ concentrations with the highest concentrations found in uninoculated experiments. The sulfur speciation of the Opalinus Clay was analysed and the results of the inoculated sample suggested that hydrogen sulfide reacted with Fe2+, precipitating iron sulfide minerals. After the incubation period, the microbial community was dominated by the sulfate-reducing Desulfobulbaceae family.The study suggests that H2-fuelled, microbially-mediated sulfate reduction can affect the mineral composition within the Opalinus Clay due to the precipitation of iron sulfide minerals. These precipitation reactions may enhance the long-term integrity of the repository by removing corrosive hydrogen sulfide from solution when sufficient Fe2+ is available and so protecting the canisters containing the nuclear waste.

Site screening of clay formations in NW China as host rocks for a high-level radioactive waste disposal repository

Abstract Deep geological disposal of high-level radioactive waste (HLW) in a repository with a system of engineered and natural barriers has been recognized as an appropriate disposal concept by Chinese authorities since 2003, and both crystalline rocks and argillaceous rocks are considered as the candidate host rocks for HLW disposal repository. The 1:200 000 regional survey indicated that there are potential clay formations in Mesozoic–Cenozoic sedimentary basins in NW China. Five candidate areas have been suggested with potential clay formations including the Tamusu and Suhongtu areas with upper K 1 Bayingebi clay formations in the east Bayingebi Basin, in the Inner Mongolia Autonomous Region. On the basis of a detailed ground geological, hydrological and geophysical surveys, two test boreholes drilled to a depth of 800 m in the Tamusu area revealed that there are three lacustrine-facies clay formations (K 1 b 2-3 , K 1 b 2-2 and K 1 b 2-1 ). The thickness of the K 1 b 2-3 and K 1 b 2-2 clay formations is about 300–600 m with sandstone and siltstone interbeds, while the thickness of the K 1 b 2-1 homogeneous clay formation is more than 200 m with the depth of 450 m below the surface. The spatial extension of the clay formations could meet the fundamental criteria to ensure the long-term safety of the repository. Initial mineralogical studies on core samples indicated that the mineral assemblage is dominated by analcite, kaolinite, illite and dolomite. The homogeneous argillaceous rocks rich in analcite in Tamusu area could be a new type of host rock for a HLW disposal repository.

Investigation into water retention and gas permeability of Opalinus clay

Opalinus clay (OPA) is currently being investigated as a potential host rock for radioactive waste repository. The construction of this repository will lead to an excavation-damaged zone (EDZ) in the surrounding. Its sealing ability is crucial for the safety assessment of the geological repository. The sealing ability of OPA has a close relationship with its water retention and gas permeability behaviours. For this purpose, the water retention and gas permeability of OPA and its comparison with the artificial barrier (i.e. bentonite) were investigated in this study. The results show that OPA absorbed less water than granular bentonite material with equal suction. Compared with the other two similar materials (Boom clay and COx argillite), which were selected as natural engineering barriers for nuclear waste storage in Belgium and France, the suction behaviours of OPA are similar to those of COx argillite but notably different from those of Boom clay. The gas permeability tests show that OPA sample is quite sensitive to the changes in confining pressures but is less sensitive to applied gas pressures. Further, it is found that the OPA is more permeable than bentonite barrier after 10.5 years of hydration. Then, the OPA samples were artificially water-saturated. We find that the sealing ability can be recovered after long-time water saturation. However, when comparing with bentonite barrier, it is still a weak zone. This indicates that the EDZ should be paid more attention for the assessment the performance of the nuclear repository.

Monte Carlo based study of radiation field in a deep geological repository for high-level nuclear waste with different host rock types

Disposal in deep geological formations with arrangements for monitoring and retrieval has been considered as one of the most promising management concepts of heat-generating, high-level nuclear waste. Typical design of a deep geological repository requires a shielding cask containing the waste, which is in turn located underground in an emplacement drift of a host rock formation. Various rock types, such as rock salt, clay stone and granite, are possible candidates as host rock for a deep geological repository. The massive host rock formation around the emplacement drift is the most important barrier against radionuclide release into the biosphere. However, the host rock layers have also influence on the radiation field in the emplacement drift, which should be taken into account when assessing occupational exposure in the repository. In the current study, impact of two different materials, i.e. rock salt and concrete (building material of the supporting liner for a drift in clay stone or granite), on radiation field around a nuclear waste package disposed in an emplacement drift was investigated with Monte Carlo method. The high-level nuclear waste was simulated with a 252Cf neutron source. Both neutron and gamma dose rates and spectra around the waste package were calculated. It was found out that neutrons dominate the radiation field in the drift and the dose rates in the drift are enhanced due to backscattered radiation by the surrounding material layers. Furthermore, due to different material compositions of rock salt and concrete, the resulted neutron spectra have also different characteristics. In general, concrete moderates neutrons better than rock salt, which leads to a lower dose rates in the emplacement drift.

Experiments on thermo-hydro-mechanical behaviour of Opalinus Clay at Mont Terri rock laboratory, Switzerland

Repositories for deep geological disposal of radioactive waste rely on multi-barrier systems to isolate waste from the biosphere. A multi-barrier system typically comprises the natural geological barrier provided by the repository host rock – in our case the Opalinus Clay – and an engineered barrier system (EBS). The Swiss repository concept for spent fuel and vitrified high-level waste (HLW) consists of waste canisters, which are emplaced horizontally in the middle of an emplacement gallery and are separated from the gallery wall by granular backfill material (GBM). We describe here a selection of five in-situ experiments where characteristic hydro-mechanical (HM) and thermo-hydro-mechanical (THM) processes have been observed. The first example is a coupled HM and mine-by test where the evolution of the excavation damaged zone (EDZ) was monitored around a gallery in the Opalinus Clay (ED-B experiment). Measurements of pore-water pressures and convergences due to stress redistribution during excavation highlighted the HM behaviour. The same measurements were subsequently carried out in a heater test (HE-D) where we were able to characterise the Opalinus Clay in terms of its THM behaviour. These yielded detailed data to better understand the THM behaviours of the granular backfill and the natural host rock. For a presentation of the Swiss concept for HLW storage, we designed three demonstration experiments that were subsequently implemented in the Mont Terri rock laboratory: (1) the engineered barrier (EB) experiment, (2) the in-situ heater test on key-THM processes and parameters (HE-E) experiment, and (3) the full-scale emplacement (FE) experiment. The first demonstration experiment has been dismantled, but the last two ones are on-going.

Fifteen years of microbiological investigation in Opalinus Clay at the Mont Terri rock laboratory (Switzerland)

Microbiological studies related to the geological disposal of radioactive waste have been conducted at the Mont Terri rock laboratory in Opalinus Clay, a potential host rock for a deep geologic repository, since 2002. The metabolic potential of microorganisms and their response to excavation-induced effects have been investigated in undisturbed and disturbed claystone cores and in pore- (borehole) water. Results from nearly 15 years of research at the Mont Terri rock laboratory have shown that microorganisms can potentially affect the environment of a repository by influencing redox conditions, metal corrosion and gas production and consumption under favourable conditions. However, the activity of microorganisms in undisturbed Opalinus Clay is limited by the very low porosity, the low water activity, and the largely recalcitrant nature of organic matter in the claystone formation. The presence of microorganisms in numerous experiments at the Mont Terri rock laboratory has suggested that excavation activities and perturbation of the host rock combined with additional contamination during the installation of experiments in boreholes create favourable conditions for microbial activity by providing increased space, water and substrates. Thus effects resulting from microbial activity might be expected in the proximity of a geological repository i.e., in the excavation damaged zone, the engineered barriers, and first containments (the containers).

Implementation of the full-scale emplacement (FE) experiment at the Mont Terri rock laboratory

Opalinus Clay is currently being assessed as the host rock for a deep geological repository for high-level and low- and intermediate-level radioactive wastes in Switzerland. Within this framework, the ‘Full-Scale Emplacement’ (FE) experiment was initiated at the Mont Terri rock laboratory close to the small town of St-Ursanne in Switzerland. The FE experiment simulates, as realistically as possible, the construction, waste emplacement, backfilling and early post-closure evolution of a spent fuel/vitrified high-level waste disposal tunnel according to the Swiss repository concept. The main aim of this multiple heater test is the investigation of repository-induced thermo-hydro-mechanical (THM) coupled effects on the host rock at this scale and the validation of existing coupled THM models. For this, several hundred sensors were installed in the rock, the tunnel lining, the bentonite buffer, the heaters and the plug. This paper is structured according to the implementation timeline of the FE experiment. It documents relevant details about the instrumentation, the tunnel construction, the production of the bentonite blocks and the highly compacted ‘granulated bentonite mixture’ (GBM), the development and construction of the prototype ‘backfilling machine’ (BFM) and its testing for horizontal GBM emplacement. Finally, the plug construction and the start of all 3 heaters (with a thermal output of 1350 Watt each) in February 2015 are briefly described. In this paper, measurement results representative of the different experimental steps are also presented. Tunnel construction aspects are discussed on the basis of tunnel wall displacements, permeability testing and relative humidity measurements around the tunnel. GBM densities achieved with the BFM in the different off-site mock-up tests and, finally, in the FE tunnel are presented. Finally, in situ thermal conductivity and temperature measurements recorded during the first heating months are presented.

Porewater Chemistry in Claystones in the Context of Radioactive Waste Disposal

The development of a combined experimental-modelling approach has enabled to constrain the porewater chemistry of different low-permeability clay formations (Boom Clay, Callovo-Oxfordian Fm., Opalinus Clay) foreseen as host rocks for nuclear waste repositories. A variety of methods are available to directly sample porewater or to derive information on the solute concentrations. These include analysis from seepage waters in boreholes, aqueous extractions, high-pressure squeezing, and advective displacement from core samples. Geochemical equilibrium modelling is used for data integration and calculation of internally consistent reference water compositions. The paper provides an overview of current achievements in experimental developments, modelling approaches and open questions.

Interaction of Ions with Hydrated Clay Surfaces: Computational Molecular Modeling for Nuclear Waste Disposal Applications

The Callovo-Oxfordian clay formation (COx) is the potential host rock for long term nuclear waste repository in France. The clayey component of COx consists mostly of illite, smectite and interstratified illite/smectite (I/S) clay minerals. We performed a series of molecular dynamics (MD) computer simulations in order to quantify the molecular scale mechanisms responsible for the adsorption and transport of ions at the hydrated surfaces of illite, smectite, and I/S clays. New structural models of illite, smectite, and I/S allowed us to identify several structurally different adsorption sites at the basal surfaces of all three clay substrates. Adsorption free energy profiles above each individual adsorption site on each clay surface for a wide range of metal cations were then calculated and the metal sorption properties for the three clay surfaces are compared in terms of the preferable sorption sites and their surface distributions, most stable adsorption distances, and free energies of adsorption. The resulting equilibrium constants for surface adsorption and ion exchange were calculated and found in general agreement with available literature data. The observed discrepancies between the exchange energies obtained in the current MD simulations and the values obtained through the interpretation of recent X-ray reflectivity measurements can be attributed to the differences in the description of the exchange reaction equilibria between the experimental conditions and the simplified conditions of our simulations.

Use of petrophysical data for siting of deep geological repository of radioactive waste

The paper is devoted to analyzing the petrophysical properties and petrographical characteristics of Volyn region with the view to choosing the least permeable and so the most suitable geological formation for the radioactive waste disposal. On a basis of the petrophysical estimations of the granitoids properties the argumentation of permeability has been developed for the petrotypes of Volyn region. Also method of classification of the petrotypes with their relative rate of suitability for radioactive waste disposal was developed. As a result of studying the perspectives were shown of the zhytomyr and korosten types of the granitoids as host rock for the radioactive waste disposal. According to the results of investigations performed by Swedish researchers a comparative analysis of rocks based on the age of formation, composition, structural features and some petrophysical properties of granitoids as host rocks for repository of radioactive waste was performed. Detail comparison the data of the granitoids of the Forsmark site in Sweden and the data of the granitoids of the Volyn megablock can be one of the next steps in researching the host rocks for the development of the RW disposal system in Ukraine.

Poroelasticity of the Callovo–Oxfordian Claystone

This work is devoted to an experimental investigation of the poroelastic behavior of the Callovo–Oxfordian claystone, a potential host rock for the deep underground repository of high-level radioactive waste in France. Drained, undrained, pore pressure loading and unjacketed tests were carried out in a specially designed isotropic compression cell to determine the poroelastic parameters of fully saturated specimens. Great care was devoted to the saturation procedure, and small loading rates were used to ensure full drainage conditions in drained and pore pressure tests (0.5 kPa/min) and in the unjacketed test (2 kPa/min). High-precision strain measurements were performed by ensuring direct contact between the LVDT stems and the specimen. An analysis in the framework of transverse isotropic poroelasticity provided the Biot effective stress coefficients b1 (perpendicular to bedding) between 0.85 and 0.87 and b2 (parallel to bedding) between 0.90 and 0.98 under different stress conditions (pore pressure 4 MPa, total isotropic stresses of 14 and 12 MPa, respectively). A set of equivalent isotropic poroelastic parameters was also determined and a very good compatibility between the results of different tests was found, giving confidence in the parameters determined. The unjacketed test provided a directly reliable measurement of the unjacketed modulus (Ks = 21.7 GPa) that was afterward confirmed by an indirect evaluation that showed the non-dependency of Ks with respect to the stress level. These parameters were obtained for specimens cored and trimmed in the laboratory. A parametric study was then conducted so as to provide an estimation of the parameters in situ, i.e., not submitted to the damage supported by laboratory specimens. A minimal value b = 0.77 seems to be a reasonable lower bound for the equivalent isotropic Biot parameter.

Microbial sulphate-reducing activity over load pressure and density in water saturated Boom Clay

The Boom Clay formation in Mol, Belgium, is studied as a reference host rock for the future Belgian repository for high-level and long-lived radioactive wastes. An apparently dormant sulphate-reducing bacteria (SRB) population in Boom Clay can be activated during repository construction and reduce sulphate to sulphide which may enhance the corrosion of metallic components of the engineered barriers. Thirteen test cells constructed of titanium were installed with saturated Boom Clay cores at three different wet densities, 1800, 1900 and 2000kgm−3. For the purpose of analysing microbial sulphate-reduction to sulphide, a previously developed method utilizing the radiotracer 35SO42− was applied. Copper discs were installed towards which produced sulphide diffused and reacted to form CuxS. The amounts of radioactive sulphide on the copper disc surfaces were analysed and the sulphide production rates in the clay were modelled using a diffusion coefficient for sulphate that was determined to 2.2×10−12m2s−1 for a fully saturated Boom Clay at a wet density of 2000kgm−3. The diffusion coefficient for sulphide was set to 4.4×10−12m2s−1. Heat treated clay (120°C, 48h) was included as negative controls in 4 test cells. The analysis for SRB reported from 107 up to 109cellsL−1 pore water for 34 sample positions and 5 positions in the negative control clay cores were below detection limit. These numbers were within the range of cultivable bacteria observed previously in Boom Clay. There was no clear cut-off in density with respect to presence of cultivable SRB and sulphide production, but it appeared as if sulphide production was increasingly possible at or below 1800kgm−3. At higher densities, numbers were lower, but the SRB were still cultivable and active which suggests that SRB can be active and produce sulphide in a Boom Clay repository for as long as sulphate is available.

Crystalline Rock as a Repository for Swedish Spent Nuclear Fuel

The granitic bedrock at Forsmark (Sweden) provides a well suited host rock for a geological repository in which to safely dispose of spent nuclear fuel. The properties of the host rock have been thoroughly investigated through boreholes from the surface. This repository will be at a depth of approximately 500 m where the spent nuclear fuel will be contained in 6,000 copper canisters able to withstand potential earthquakes and glaciation events. The canisters will be surrounded by a bentonite clay buffer to prevent canister corrosion by groundwater. The safety assessment in support of the site's license application suggests that almost all of the canisters will remain tight even one million years into the future.

Gas permeability evolution mechanism during creep of a low permeable claystone

Clayey rocks, due to its low permeability and self-sealing ability, have been selected as a privilege candidate host rock for underground radioactive waste repository in many countries. The gas permeability evolution is very sensitive to the deformation process in such rocks and can be used as a good indicator of microstructure changes such as the growth of micro-cracks. In this work, the mechanism of gas permeability evolution in the Callovo-Oxfordian (Cox) claystone during a creep deformation is investigated. Firstly, multi-step creep tests with different confining pressures are carried out to characterize gas permeability evolutions under different levels of the deviatoric stress. Secondly, in order to minimize effects of multiple deviatoric loading steps, one-step creep tests are also realized. Throughout all creep tests, the gas permeability is measured by a transient pulse decay method in together with the axial and radial strains. It is first found that the gas permeability of the claystone significantly decreases with the confining pressure or hydrostatic stress. The gas permeability in multi-step creep tests can exhibit a four-stage evolution with the progressive increase of deviatoric stress level, composed of a rapid decrease, a gentle decrease, a gentle increase and a rapid increase. However, the gas permeability is continuously decreasing during one-step creep tests. The permeability change in a one-step creep test can be well correlated with the volumetric strain variation by a logarithmic function. Effects of loading orientation with respect to claystone bedding planes are also investigated.