Modeling intrusion of bentonite buffer into fractures under the BExM framework: incorporation of wall friction effects

During the long-term operation of a repository, bentonite materials are inevitably extruded into fractures in the surrounding rock while groundwater infiltrates. This process induces the escape of radionuclides adsorbed on bentonite colloid particles at the intrusion front into the biosphere with groundwater seepage. Existing models developed for simulating the intrusion behavior are founded on empirical relationships using axial swelling pressure at full hydration under constant volume conditions. Meanwhile, the simulation of intrusion ignores the inhibition effects of the friction between the extruded bentonite and fracture walls. This leads to a significant discrepancy in assessing intrusion behavior. In this study, the equilibrium between swelling pressure at the fracture entrance and fracture wall friction was first analyzed, and a friction model was developed based on the dependence of the friction angle on average effective swelling pressure. Then, an improved hydraulic model associated with capillary water retention was established. Meanwhile, the BExM constitutive model was employed for simulating the time-dependent profiles of swelling pressure under the influence of mass loss of bentonite induced by the intrusion behavior. Finally, with the incorporation of wall friction effects, a BExM-based hydro-mechanical model was proposed to predict the evolution of intrusion distance with time. Validations were made by reproducing the intrusion behavior of five tested cases, including varying fracture widths (0.2, 0.4, and 0.6 mm) and dry densities (1.6, 1.7, and 1.8 Mg/m 3 ). Sensitivity analyses of the parameters involved in wall friction were conducted along with a comparative analysis with that of the existing models.