Gilles Armand

Thermal Conductivity of Argillaceous Rocks: Determination Methodology Using In Situ Heating Tests

This study focuses on the characterisation of thermal conductivity for three potential host rocks for radioactive waste disposal. First, the heat conduction process is reviewed on the basis of an analytical solution and key aspects related to anisotropic conduction are discussed. Then the existing information on the three rocks is summarised and a broad uncertainty range of thermal conductivity is estimated based on the mineralogical composition. Procedures to backanalyse the thermal conductivity on the basis of in situ heating tests are assessed and a methodology is put forward. Finally, this methodology is used to estimate the impact of experimental uncertainties and applied to the four in situ heating tests. In the three potential host rocks, a clear influence of the bedding planes was identified and anisotropic heat conduction was shown to be necessary to interpret the observed temperature field. Experimental uncertainties were also shown to induce a larger uncertainty on the anisotropy ratio than on the equivalent thermal conductivity defined as the geometric mean of the thermal conductivity in the three principal directions.

Velocity survey of an excavation damaged zone: influence of excavation and reloading

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.

Performance of the Opalinus Clay under thermal loading: experimental results from Mont Terri rock laboratory (Switzerland)

The paper presents an overview of the behaviour of Opalinus Clay under thermal loading as observed in three in situ heating tests performed in the Mont Terri rock laboratory: HE-B, HE-D and HE-E. The three tests are summarily described; they encompass a broad range of test layouts and experimental conditions. Afterwards, the following topics are examined: determination of thermal conductivity, thermally-induced pore pressure generation and thermally-induced mechanical effects. The mechanisms underlying pore pressure generation and dissipation are discussed in detail and the relationship between rock damage and thermal loading is examined using an additional in situ test: SE-H. The paper concludes with an evaluation of the various thermo-hydro-mechanical (THM) interactions identified in the heating tests.

Mechanical and SCC Behavior of an API5L Steel Casing Within the Context of Deep Geological Repositories for Radioactive Waste

Andra, the French national radioactive waste management agency, is in charge of studying the possibility of disposal of High Level activity Wastes (HLW) in deep geological repositories. The concept of HLW cells consists of horizontal micro-tunnels of about 0.7 m in diameter, equipped with a steel casing. In order to ensure the reliability of the casing, particularly with respect to Stress Corrosion Cracking (SCC), several in-situ experiments dedicated to the analysis of its short term mechanical and corrosion behavior have been performed at Andra’s Underground Research Laboratory (URL) as well as in surface laboratory.Reduced and full scale in-situ experiments consisting of equipping boreholes parallel to the major horizontal stress (σH) with instrumented steel tubing, have been performed to analyze the mechanisms involved in the casing/rock interface. The main characteristics of the short term mechanical load applied by the rock have been determined from local strain and convergence measurements. Although in-situ stress is isotropic in the section of boreholes parallel to σH, measurements exhibit a strongly anisotropic load.SCC experiments conducted on different steel grades, in contact with the clay host rock containing CO2/H2 revealed that general corrosion is the main type of corrosion expected for steel casings.However, the selected steel casing must have a sufficiently high yield strength (above 400 MPa) to reduce the risk of early stage plasticity due to host rock anisotropic convergence and thus to overcome SCC. API5LX65 steel seems to meet both mechanical and corrosion requirements, therefore being likely the appropriate material for the manufacture of the casing.© 2015 ASME

Convergence analysis of an unsupported micro-tunnel at the Meuse/Haute-Marne Underground Research Laboratory

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.

Specialized Instrumentation for Hydromechanical Measurements in Deep Argillaceous Rock

Within the framework of the feasibility study for a radioactive waste repository in a geological formation, the National Radioactive Waste Management Agency of France is constructing an underground research laboratory within the Callovo-Oxfordian argillite formation located in eastern France. During the sinking of the laboratorys access shaft and during excavation of the galleries, the mechanical and hydromechanical behavior of the argillite was monitored. A vertical mine-by test was carried out at depths between 460 m and 476 m in the main shaft. 12 boreholes were drilled downwards from a niche, where deformations and pore pressures were monitored. Another 11 boreholes were instrumented with mechanical and hydraulic sensors in the galleries. This paper is devoted to the instrumentation, the installation procedures and some first measurement results regarding the mechanical disturbance of the rock around the shaft.