Abstract The influence of temperature on magnetic and elastic properties of the Callovo-Oxfordian argillite formation, host of the French underground research laboratory for studying radioactive waste repository, has been investigated following two types of experiments. In the first experiment, rock samples were heated at 95 °C in an oven for 41 days, and in the second, heating–cooling cycles at increasing maximum temperatures were applied. Magnetic susceptibility and P-wave velocity measurements were conducted to investigate the effect of heating not only on the absolute values but also on the anisotropy. Whereas little variation was observed in the first experiment, the results of the heating–cooling cycles showed that different stages can be identified in the evolution of P-wave velocity and magnetic susceptibility with maximum temperature applied. At temperatures below 200 °C, the velocity increases sharply whereas the magnetic susceptibility remains unchanged: this is interpreted as being the result of mineralogical transformations, probably associated with dewatering of the clays. Above 200 °C, a large increase in susceptibility associated with a levelling off (or even a decreasing trend) of the velocity results from the competition between thermal cracking processes and another kind of mineralogical transformation affecting both properties. A magnetic mineralogical study showed that the main ferromagnetic mineral present is magnetite. An interesting result of the study is that in the temperature range investigated, the elastic properties are enhanced and the anisotropy is increased by raising the temperature, which has important consequences for the seismic monitoring of the storage site.

Abstract Fluid pressures were recorded over 6.5 years in borehole EST207 located at the Andra Meuse/Haute-Marne Underground Research Laboratory in Bure (France). The borehole is equipped with a multipacker system monitoring 11 intervals, including 8 in the Callovo-Oxfordian clay Formation, 2 in the Dogger Formation and 1 at the Oxfordian base. Pressure data were analysed for responses to Earth tides and barometric pressure in the 11intervals. Fourier analyses on the pressure data revealed some of the Earth tidal components. Use of ETERNA software determined a better estimate of Earth tidal wave parameters (amplitudes, phases and their standard deviations) and barometric efficiency. Estimates of the tidal parameters K1, O1, P1, S1 and M2, N2, S2, K2 were made before and after barometric correction. It is suggested that in the Callovo-Oxfordian clay, the greater the clay content of the formation, the greater the barometric efficiency values. A barometric and tidal responses classification using four groups as a function of the 11 borehole intervals is in good agreement with the geological configuration. This preliminary geological interpretation will allow both the estimation of poroelastic and hydrogeological parameters of the clay formation from Earth tidal and barometric responses without classical pump test experiments, and the monitoring of these properties over a long period.

Abstract Moisture-induced swelling and shrinkage deformation is a well-known phenomenon for clayey material. To quantify this deformation a fully coupled thermal-hydro-mechanical (TH2M) model based on the modified effective stress concept has been developed and implemented in the numerical code OGS. Three laboratory tests have been analysed using this concept. All specimens for the tests are argillaceous rocks, from different underground laboratory sites (Tournemire, Bure, France, and Mont Terri, Switzerland). The laboratory tests are performed under different conditions; the first test is free swelling/shrinkage and other two tests are with uniaxial loading. The relative humidity is also controlled. Only one test is carried out under varied temperature conditions. In this study the anisotropic swelling/shrinkage deformation wais simulated and analysed using this modified effective stress concept.

Abstract The investigation of the induced fractures network around seals in drifts or shafts, and in particular its evolution, is a key issue for the performance assessment of an underground waste repository. Within this framework, a specific experiment was designed and implemented in the Meuse/Haute-Marne Underground Research Laboratory (URL). This experiment, called CDZ (Compression of the Damaged Zone), is dedicated to studying the effect of mechanical compression within the induced fractures zone of the Callovo-Oxfordian claystone (COx). An unequalled level of knowledge in the 3D structure of the fractures network has been attained. A multidisciplinary approach was applied to observe not only the initial state of the induced fracture zone but also its evolution during a loading cycle. The investigations show that the fracture network which composed the Excavation Damaged Zone (EDZ) was initially interconnected and open for flow and then partially closed progressively following the increasing mechanical stress applied on the drift wall. Moreover, the evolution of the EDZ after unloading indicates a self-sealing process.

Abstract Deep argillaceous rocks are reducing environments. When exposed to air, reduced minerals of these rocks react with oxygen, modifying the surrounding chemical conditions. Thus, oxidation is an issue in studies about the confining properties of such rocks in the framework of geological disposal projects for radioactive waste. Previous studies in several underground research laboratories (URLs) in argillaceous rocks have shown that oxidation reactions mainly occurred in the excavation-induced fracture network surrounding the drifts. In the Callovian–Oxfordian argillaceous rock, at −490 m in drifts from the Meuse/Haute-Marne URL, oxidized features were systematically looked for in 115 borehole cores. The concerned drifts were of various ages, from a few days to 6.5 years. After 5 months, oxidized features were encountered in numerous excavation-induced extensional fractures. In excavation-induced shear fractures, oxidized features were observed in a few borehole cores after 2 years, and they became frequent after 6 years. In all cases, the oxidized features observed were found on the fracture walls or were connected to them, and were less than 1.8 m from the drift walls. These observations about the oxidation front and its evolution over time provide insights regarding the properties of excavation-induced fractures with respect to oxygen transfer.

Abstract This paper describes a field-scale experiment on gas transport mechanisms performed at Andra’s Underground Research Laboratory (URL) in a clay rock. The experimental layout consists of two parallel boreholes that are equipped with multiple packer completions isolating three intervals each, which have been continuously monitoring the pore pressure evolution of the clay rock. Nitrogen gas was injected in the middle test interval of one of the boreholes at increasing rates. The entire gas test comprised six periods of controlled gas injections, each followed by a shut-in pressure recovery phase. The experimental data are presented along with their interpretation by means of numerical modelling of two-phase flow of gas and water using different numerical codes and different geometrical approaches that include axisymmetric, half-space and full 3D models. An iterative modelling process was used to show step-by-step how an accurate description of each component of the experiment system produced a satisfactory reproduction of the experimental data and an improved understanding of the relevant phenomena. For instance, the initial volume of remaining water in the test interval, and the presence of a damaged zone around the boreholes, was important for the models to obtain good agreement with the field data.

Abstract Hydrogen gas was injected, together with helium and neon, into a borehole in the low-diffusivity Opalinus Clay rock. The hydrogen partial pressure was at most 60 mbar. A water production flow rate from the surrounding rock of c. 15 ml/day had been obtained previously, indicating that the test interval wall was presumably saturated with water. Helium and neon concentrations decreased as expected while taking into account dissolution and diffusion processes in the porewater. In contrast, the disappearance rate of hydrogen observed (2×10 −4 to 3×10 −4 mol/day/m 2 ) was c. 20 times larger than the calculated rate considering only dissolution and diffusion. The same rate was observed following a new hydrogen injection and over a six-month semi-continuous injection phase. Simultaneously, sulphate and iron concentrations decreased in the water, whereas sulphide became detectable. These evolutions may be due to biotic processes involving hydrogen oxidation, sulphate reduction and Fe(III) reduction.

Abstract We describe and compare the morphology and activity of two types of gullies with different orientations collocated on the Kaiser dune field in the southern hemisphere of Mars: large apron gullies and linear dune gullies. The activity of large apron gullies follows an annual cycle: (i) material collapse into the alcove (mid-autumn/late winter) as CO 2 condenses; (ii) remobilization by mass flows (late winter); and (iii) continuous appearance of hundreds of ‘digitate flows’ on the fan (autumn/winter). We find that large apron gullies could form in hundreds of Martian years. In contrast, linear dune gullies are active briefly in late winter, when the CO 2 frost disappears. Their activity is characterized by the extension of channels, the creation of pits and the darkening of the surface. Linear dune gullies are likely to form within one to tens of Martian years. We infer that insolation, which influences the depth to ground ice and the amount of volatile deposited, may be the factor differentiating large apron gullies and linear dune gullies. Sediment transport by CO 2 sublimation is a good candidate for the activity observed in all of these features. However, linear gullies could also be formed by brine release when the temperature rises abruptly after the removal of the CO 2 ice.

Abstract This paper describes how four scientific and safety relevant issues have been addressed in special-purpose research laboratories focusing on the geological disposal of high level and long-lived radioactive waste. These are: (a) the effects of heat on the engineered barriers and the geological environment; (b) the geochemical characterization of pore-water in argillaceous rocks; (c) the diffusion and retention of radionuclides; and (d) the full-size sealing of a waste emplacement. They are illustrated by experiments conducted in five underground research laboratories (URLs), three of which are in clay formations (Mol in Belgium, Centre de Meuse–Haute-Marne in France, and Mont Terri Rock Laboratory in Switzerland) and two in granite (Aspö Hard Rock Laboratory in Sweden and Grimsel Test Site in Switzerland). This paper highlights how the various types of experiments are related and how their results have been applied to foster progress. The most complex experiments have revealed artefacts and technical or methodological difficulties associated with interactions among multiple phenomena, the occurrence or intensity of which cannot be analysed by simple models. In turn, these difficulties have prompted experiments targeted at elementary phenomena, thereby encouraging the development of new investigation protocols and monitoring tools. More than 30 years of investigations in special-purpose URLs show the benefits of in-situ experimental programmes in the context of radioactive waste management. The laboratories have opened up avenues for research and advanced knowledge and technology. Thanks to a large component of international cooperation, they have made it possible to mobilize the financial and human resources required for this type of research. They have, above all, shared thoughts and promoted interdisciplinary studies around the same subject. They make common strategies possible at international level.

Abstract This paper describes a large-scale experiment on gas transport and hydromechanical processes around underground structures as part of a long-term geoscientific research programme at the Mont Terri Underground Rock Laboratory in the Jura Mountains of Switzerland. A horizontal microtunnel with a diameter of 1 m and a length of 13 m was drilled in an overconsolidated claystone formation. After installing monitoring instruments in the open tunnel, the end of the tunnel was backfilled with sand (test section) and a large hydraulic packer was emplaced in the seal section. The packer was inflated and subsequently the test interval was saturated with a synthetic pore-water. Following saturation an extended programme of hydraulic testing was performed over a two year period. A series of gas injection tests was then performed over a period of approximately 1.5 years. Following this first series of gas injections, a long post-gas hydraulic test has been initiated. The paper presents data and interpretation of the gas injections and subsequent hydraulic testing. The ability of the excavation damage zone to transport gas at pressures below fracturing is demonstrated. The post-gas hydraulic performance is considered and related to the self-sealing of the damage zone observed during saturation and hydraulic testing.

Abstract In the underground laboratory of Meuse/Haute-Marne (Bure) in France, different fracture types have been intensively investigated. Within a co-operative project between BGR and ANDRA, geophysical measurements and borehole gas tests were conducted in three well-designed boreholes in the gallery GRM to characterize the fracture structure and to determine the gas tracer velocity within fracture networks. On the basis of seismic measurements and nitrogen gas interference tests, helium was injected into an interval of borehole OHZ3003, where an unloading joint has been identified. The injection took place in the form of a 10 min ‘pulse’ with an injection pressure of 2 bar. The other two boreholes, OHZ3002, where an upper part of the shear-mode fractures (‘chevron’ pattern) dominate, and OHZ3001, where shear-mode fractures (subvertical ‘oblique’ fractures and ‘chevron’ pattern) exist, served as observation holes. To maintain the pressure gradient between the injection hole and the observation holes, nitrogen gas was subsequently flushed into the injection hole. Two breakthrough curves of helium concentration and pressure developments in the two observation holes were continuously monitored using two helium leakage detectors and pressure gauges. To interpret the measured pressure and concentration data, numerical models were constructed. A 3D model was used to simulate nitrogen gas flow and 2D models were applied to simulate a helium transport process. The real volume of the injection interval was considered in the model and the experimental process was simulated. Using the calibrated transport parameter data for a helium tracer from previous studies in the Mont Terri Rock Laboratory, the calculated breakthrough curves agreed well with those obtained from measuring the variation in permeability. The permeability derived from the helium tracer test agrees well with the estimation obtained from the nitrogen gas tests.

Abstract Convergence measurements recorded for one representative micro-tunnel (diameter c. 0.7 m) in Callovo-Oxfordian claystone were analysed. The micro-tunnel was excavated in the direction of the horizontal principal major stress. In situ observations showed anisotropic convergence with the maximum and minimum values in the horizontal and vertical directions, respectively. The horizontal closure of walls was fitted on the basis of a semi-empirical convergence law. This law is a predictive model reflecting the global response of the ground to excavation works. As the convergence measurements were performed after the end of excavation, their evolution in time can only be related to the time-dependent behaviour of the ground and the effect of the face advance cannot be captured. It is shown that some parameters of the semi-empirical law did not change along the micro-tunnel. An easy and efficient method is thus proposed for the long-term prediction of wall closure by the fitting of a single parameter on recorded data. Comparison with a drift (diameter c. 5 m) highlighted the influence of the support installation and the rate of excavation on the variation in the parameter values of the semi-empirical law. The vertical closure of the micro-tunnel walls, which showed a very weak evolution over time, was analysed based on the rate of convergence.

Abstract During gallery excavation, regardless of the method chosen, the surrounding rock is mechanically disturbed in the case of underground disposal of chemical or radioactive wastes, and such mechanical changes to the rock state can create preferential pathways for the release of material from the waste inside the excavation (e.g. chemical waste gases, brine or dissolved radionuclide) up to the biosphere. The mechanical characterization of this disturbed zone is thus essential in assessing the rock capacity necessary to form an impermeable geological rock barrier. The key to this effort lies in determining the nature, extent and change to this disturbed zone. A survey by ultrasonic wave analysis is particularly appropriate for these purposes, as it provides information on the zone left undisturbed by the boreholes. The main aim of this paper is to describe a recent in situ experiment (called EZ-A) conducted in Opalinus clay at the Mont Terri Rock Laboratory in Switzerland. This experimental campaign successfully studied the possibility of improving the properties of rocks forming the excavation damaged zone (EDZ). The experiments consisted of three stages: the first entailed locating the EDZ caused by the construction of gallery EZ-A at the Mont Terri laboratory; the second traced the evolution of the EDZ arising from a 20 cm thick and 150 cm deep slot excavation in the EZ-A gallery floor; and the third stage focused on the P-wave velocity evolution surrounding the slot during a pressure loading at the slot walls, which serves to characterize the EDZ evolution in the slot. These three stages were carried out using seismic tomography and then recording the wave propagations during the excavation and reloading stages. The measurement of velocity before and after slot excavation reloading, together with the survey during slot excavation, showed a decrease in P-wave velocity underneath the slot floor down to a 0.2 m depth. Reloading clearly improves P-wave propagation along the sidewall and slightly decreases it under the slot.