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Complex coupled thermo-hydromechanical (THM) loading paths are expected tooccur in clay rocks which serve as host formations for geological radioactivewaste repositories. Exothermic waste packages heat the rock, causing thermalstrains and temperature induced pore pressure build-up. The drifts are designedin such a way as to limit these effects. One has to anticipate failure andfracturing of the material, should pore pressures exceed the tensile resistanceof the rock. To characterise the behaviour of the Callovo-Oxfordian claystone(COx) under effective tension and to quantify the tensile failure criterion, alaboratory program is carried out in this work. THM loading paths whichcorrespond to the expected in situ conditions are recreated in the laboratory.To this end, a special triaxial system was developed, which allows theindependent control of radial and axial stresses, as well as of pore pressureand temperature of rock specimens. More importantly, the device allows one tomaintain axial effective tension on a specimen. Saturated cylindrical claystonespecimens were tested in undrained conditions under constrained lateraldeformation and under nearly constant axial stress. The specimens were heateduntil the induced pore pressures created effective tensile stresses andultimately fractured the material. The failure happened at average axialeffective tensile stresses around 3.0 MPa. Fracturing under different lateraltotal stresses allows one to describe the failure with a Hoek-Brown orFairhurst's generalized Griffith criterion. Measured axial extension strainsare analysed based on a transversely isotropic thermo-poroelastic constitutivemodel, which is able to satisfactorily reproduce the observed behaviour.

Inducing tensile failure of claystone through thermal pressurization in a novel triaxial device