Effect of heating on clay formations: results of a THM modelling benchmark exercise on two heater tests in indurated and poorly indurated clays

In the framework of the European Joint Programme EURAD, the HITEC work package aimed at investigating the influence of thermal loading on the behaviour of clay-based materials. Three clay formations considered to host potential radioactive waste repositories were studied, the Callovo-Oxfordian (COx) claystone, the Boom Clay and the Opalinus clay as well as different types of bentonite used as engineered barrier. Through HITEC, a series of benchmarking exercises focusing on the clayey host formations were proposed to seven European modelling teams. The benchmark consisted in three consecutive steps, starting with 2D generic models to study the near-field and far-field effects of heating on the behaviour of clay rocks and then considering triaxial compression tests under elevated temperature on COx samples. The last step of the benchmark consisted in modelling two in-situ heating experiments: ALC1605 in the COx claystone and the large scale PRACLAY Heater test in the Boom Clay. In both cases, the teams successfully managed to reproduce the anisotropic response of the clay host rocks to excavation and heating. The evolutions of temperature and pore pressure were well modelled in the far-field with an anisotropic poro-elastic approach but this article highlights particularly how advanced models allowed to capture the processes occurring around the tunnels. In order to match best the measurements, these models required a dependence of the stiffness and permeability with the deformation. One of these advanced models was tested on both in-situ experiments and gave remarkably good results, even though the two clayey host formations have different hydro-mechanical properties. It illustrates a common understanding of THM processes in poorly indurated clay and in claystone and provides some keys for transferring knowledge from one clay formation to another.