Experimental study of thermo-hydro-mechanical behaviour of Callovo-Oxfordian Clay-stone

During the different phases of the exothermic radioactive waste deep disposal (excavation, operation) and after permanent closure, the host rock is submitted to various coupled mechanical, hydraulic and thermal phenomena. Hence, a thorough investigation of the thermo-hydro-mechanical behaviour of the rock is necessary to complete existing data and to better understand and model the short and long term behaviour of the Callovo-Oxfordian (COx) clay formation in Bure (Meuse/Haute-Marne - M/HM), considered by ANDRA as a potential host rock in France.In this work, the compression - swelling behaviour of the COx Clay-stone was first investigated by carrying out a series of high-pressure oedometric tests. The results, interpreted in terms of coupling between damage and swelling, showed that the magnitude of swelling was linked to the density of the fissures created during compression. In a second step, the hydro-mechanical and thermo-hydro-mechanical behaviour of the saturated Clay-stone under a mean stress close to the in situ one were investigated by using two devices with short drainage path (10 mm), namely a isotropic cell and a newly designed hollow cylinder triaxial cell with local displacement measurements. These devices helped to solve two majors problems related to testing very low permeability materials: i) a satisfactory previous sample saturation (indicated by good Skempton values) and ii) satisfactory drainage conditions. Some typical constitutive parameters (Skempton and Biot's coefficients, drained and undrained compressibility coefficients) have been determined at ambient temperature through isotropic compression tests that also confirmed the transverse isotropy of the Clay-stone. The consistency of the obtained parameters has been checked in a saturated poro-elastic framework. Two aspects of the thermo-hydro-mechanical behaviour of the COx Clay-stone have then been investigated through different heating tests and through drained and undrained isotropic compression tests at elevated temperature (80 C): the effects of temperature on the behaviour of Clay-stone and thermal pressurization. A drained heating test under in-situ stress conditions evidenced, probably for first time, a plastic contractant response of the Clay-stone (like normally consolidated clays), a feature not considered in the presently conducted numerical modelling of deep disposal systems. Another new important observed feature is the increase in drained compressibility of the COx Clay-stone with temperature, not observed in clays. The investigation of thermal pressurization (caused by the low Clay-stone permeability and by the significant difference in thermal expansion between water and the solid phase) has been carried out by means of undrained heating tests, after a detailed analysis of the major effects of the measurement system (which should perhaps be also analyzed when performing in-situ measurements). The thermal pressurization coefficient appeared to be quite sensitive to changes in temperature and stress, it decreased between 0.14 and 0.1 MPa/C between 25 and 80 C. It is believed that the different thermo-hydro-mechanical volumetric responses obtained here allow a better interpretation and modelling of the behaviour of the Clay-stone formation around the galleries in areas that are mostly saturated, except close to the galleries (a few decimetres). (author)