Numerical modelling of moisture controlled laboratory swelling/shrinkage experiments on argillaceous rocks

Document available in extended abstract form only. Moisture induced swelling/shrinkage is a well-known and very important issue for the final disposal of nuclear waste in argillaceous formation. After underground excavation fluid saturated rock is contacted with relatively dry air. Due to de-saturation process shrinkage takes place and as a result fractures may occur. During the post-closure phase of a repository the near field of the tunnel will be re-saturated and with the result of swelling and the closure of fractures. Opening and closing of fractures due to shrinkage and swelling have large influence on the hydro-mechanical behaviour of argillaceous rock and the safety of a repository. Laboratory experiments have been carried out to investigate the swelling/shrinkage mechanism of argillaceous rocks. Different models have been developed to understand this coupled hydro-mechanical process. This work focuses on the numerical modelling of swelling and shrinkage induced deformation process. A fully coupled thermal-hydro-mechanical (TH2M) model is developed and implemented in the numerical code OpenGeoSys (OGS) to simulate mechanical processes during de- and re-saturation experiments on the argillaceous rock samples. In this model a water saturation dependent effective stress concept takes material suction property into consideration and is applied to the mechanical process using gas pressure, capillary pressure and the Bishop coefficient, which can be expressed as a power function of the effective saturation. The transverse isotropic elastic behaviour of the medium is described by the generalized Hooke's law. The hydraulic process of de- and re-saturation is simulated by a two-phase-flow model. Gas pressure and capillary pressure are defined as the primary variables. The relationship between capillary pressure and water saturation is described by the von Genuchten model, in which the parameters (gas entry pressure and shape factor) are fitted with laboratory suction experiment. The relative permeabilities to water and gas are calculated by the Mualem approach. The flow process is described by the Darcy law. Similar to the mechanical model the intrinsic permeability is transversely isotropic to consider the bedding plane structure of argillaceous rocks. With the coupling of thermal process water vapour in gas phase is involved. Gas phase is assumed to be a mixture of two perfect ideal gases. Diffusion of water vapour in the gas phase is described by the Fick law. Three laboratory experiments on the specimens taken from the Tournemire site (France), the Bure site (France) and the Mont Terri Rock Laboratory (Switzerland) have been recalculated. For the specimen from Tournemire Site a free swelling/shrinkage experiment was performed with controlled moisture. Deformations in parallel and perpendicular direction of the bedding plane were recorded. The experiments of the specimens from Bure site were under a uniaxial load condition and moisture was not controlled. Two specimens were tested simultaneously with loading parallel and perpendicular to the bedding plane. During the experiments of the specimen from Mont Terri Rock Laboratory both moisture and temperature were controlled. The specimen was also under uniaxial load condition. Strong swelling/shrinkage capacity of all specimens was observed when moisture changed. The calculated results considering this approach with the swelling and shrinkage induced deformation process agree very well with the laboratory data of deformation and released water mass of the specimens. The hydro-mechanical behaviour during swelling/shrinkage induced by change of moisture can be well interpreted by the effective stress concept considering the fluid saturation dependent Bishop coefficient. During the de-saturation process effective stress is decreased due to the increase of capillary pressure (suction). As a result shrinkage takes place. Conversely, during the re-saturation process swelling deformation occurs or swelling pressure can be recorded if the specimen under confined conditions. (authors)