Thermo-Hydro-Mechanical Behaviour of the Argillaceous Rock in the Context of Nuclear Waste Repository

Deep geological disposal is widely recognized as one of the most safe and feasible strategies for isolating radioactive waste from the biosphere and ensuring its long-term management. The fundamental concept involves the emplacement of radioactive waste within a multi-barrier confinement system, wherein clay formations are often selected as host rocks due to their favourable properties, such as low permeability, self-sealing capacity, and the ability to retain radionuclides on the surface of clay minerals. Among their various coupled multi-physical processes that may influence the long-term safety functions of the geological barrier, the increase in temperature resulting from the decay of radioactive waste induces thermal pressurisation both in the excavation damage zone (EDZ) and the surrounding intact far-field. This thermal pressurisation can significantly affect the \textit{in situ} thermo-hydro-mechanical (THM) behaviour of the host rock, thereby potentially compromising the long-term safety performance of the disposal facility.

Thermal effects on the fracturing behaviour within the EDZ surrounding a supported drift are specifically investigated. Fracturing induced by heating is modelled through strain localization in the form of shear bands. A coupled local second gradient model, incorporating a regularisation technique, is employed with thermoelasticity to account for THM couplings. Among the various parameters considered, the discrepancy in thermal dilation coefficients between the solid and fluid phases is identified as a key factor contributing to excess pore pressure generation. The proposed model is applied to simulate a benchmark exercise in the Callovo-Oxfordian (COx) claystone. The results successfully reproduce shear bands oriented along the direction of the minor principal stress, in agreement with experimental observations during excavation. Furthermore, the nature of the contact between the drift wall and its support is shown to significantly influence the development and pattern of the shear bands.

An advanced THM constitutive model is implemented, featuring stress- and strain-dependent permeability and stiffness properties of the clay formation. The evolution of intrinsic water permeability is described using a strain-dependent formulation, which successfully reproduces the significant increase observed in the EDZ, in agreement with experimental measurements. The stiffness of the clay is characterized using the small strain stiffness theory, enabling the model to capture the degradation of stiffness in the EDZ due to large deformations, while preserving the higher stiffness of the intact far-field. The proposed model is applied to simulate both a laboratory-scale experiment from \textit{in situ} sample extraction to laboratory triaxial testing, and the large-scale PRACLAY heater test.

Thermally induced mechanical behaviour of claystone is specifically investigated, with particular attention paid to the temperature dependence of shear strength. Experimental evidence has demonstrated that elevated temperatures can significantly affect both the shear strength and the development of crack networks in claystone. To capture these effects, a thermo-mechanical constitutive law is implemented, which explicitly incorporates the temperature dependence of cohesion. A key feature of the model is that cohesion evolves as a function of mechanical softening, fabric evolution, and thermal softening. The proposed model is applied to the \textit{in situ} large-scale ALC1605 heating experiment, where the evolution of cohesion is shown to play a critical role in the initiation and propagation of shear bands within the EDZ.

The proposed approach has been progressively enhanced from a thermoelastic to a thermoplastic framework, and from assumptions of constant hydraulic properties and homogeneous stiffness to fully coupled THM interactions. The primary objective of this research is to improve the understanding of thermal effects in clay materials, to accurately reproduce THM experimental observations at large-scale, and ultimately to contribute to the design and long-term safety assessment of underground radioactive waste disposal facilities.