Mechanical load applied on a steel casing in a 500 m deep clay formation - small scale and full scale in-situ experiments

Document available in extended abstract form only. Andra, the French national radioactive waste management agency, is in charge of the study on the possibility of disposal for High Level activity Wastes (HLW) in deep geological repositories. The concept of HLW cells consists of horizontal micro-tunnels (at least) 40 m long and almost 0.7 m in diameter, equipped with a non-alloy steel casing to allow the retrieval of the waste overpacks during the reversibility period and to contribute to the mechanical integrity of the cell during the thermal phase. Several full scale experiments dedicated to the analysis of the Thermo-Hydro-Mechanical (THM) behavior of the cell and of the surrounding rock have been designed and set up in Andra's Underground Research Laboratory. Additional 1:5 scale in-situ experiments have been performed on steel casing inserted in horizontal 10 m long boreholes, to analyze more precisely the mechanisms involved in the casing/rock interface evolution and their influence on the mechanical behavior of the casing. The interest in studying small scale casing is to be sure that a significant mechanical load will be applied by the rock in a reasonable time, the initial annular space being always less than 0.02 diameter. The main characteristics of this load are determined from local strain and convergence measurements on different sections of the casing. Four experiments have been performed at constant temperature (22 deg. C) with different casing thicknesses and tightness specifications. The test results have led to the definition of the radial load evolution for boreholes drilled along the major horizontal stress. In accordance with convergence measured on full scale HLW cells demonstrators with the same orientation, the initial radial load is localized and mainly horizontal. Once the rock is in contact all around the periphery of the casing, the radial load is anisotropic and evolves with no impact on the casing deformation. Some measurements show a decrease in the anisotropy ratio after more than 150 days. All the experiments show that the load rate increases with the borehole depth. For all the tests, the yield stress of the steel (more than 450 MPa) has never been reached. The influence of a thermal load has been determined by heating a casing more than 100 days after its installation, so that a significant mechanical load is already applied by the rock. A heating of 15 deg. C shows an acceleration of the radial load (which seems to be related with the convergence rate of the borehole at the moment of the application of the thermal load) and an axial compression of the casing due to the thermal elongation of the steel in this direction, which is limited by the rock load. On a qualitative point of view, an experimental procedure consisting in equipping a borehole with a glass tubing, has been developed to observe the evolution of a borehole wall and to determine the nature of the material which will progressively fill the annular space. No significant evolution of the wall damage and no water flow in the interface have been observed. This experiment has been stopped after 8 months due to the fracture of the tubing. A full scale 40 m long HLW cell has been excavated in September 2011. This cell has been equipped with a 20 mm thick steel casing itself equipped with several sensors (relative humidity sensors, displacement sensors, strain gauges) to determine the kinetic of saturation of the annular space and to estimate the mechanical load applied by the rock. Two optical fibers have been installed all along the casing to estimate its overall thermo-mechanical behavior and to test the feasibility of this kind of instrumentation within the context of Andra's environmental survey and disposal monitoring program. The first results show that the annular space is not yet saturated (97% of relative humidity has been reached) and a radial load is already applied by the rock in spite of the important initial annular space (20 mm). This load is mainly applied in the horizontal direction which is consistent with the results obtained from small scale experiments. (authors)