Investigation on intrusion of bentonite–sand mixtures in fractures with consideration of sand content and seepage effects

During the long-term operation of a repository, bentonite–sand mixture could invade into fractures in the surrounding rock with infiltration of groundwater, resulting in loss of bentonite from the barrier with possible leakage of nuclides and consequently endangers the safety of the repository. In this work, an apparatus was designed with an artificial joint (fracture) being made between two parallel stacked transparent acrylic plates with a stainless-steel gasket inserted. Intrusion tests were conducted on the mixture specimens with different sand contents into artificial fractures with two sizes. A flow with constant velocity was applied to simulate seepage in the surrounding rock formations. Images were regularly captured with a camera, and parameters including the intrusion distance and widths of the non-accessory and accessory mineral rings were determined using a commercial software. Simultaneously, fluid was collected at the outlet and the erosion rate bentonite was determined by a turbidimeter. Finally, based on the force equilibrium, an intrusion model for describing bentonite–sand mixture intrusion into fractures with consideration of sand content and seepage effects was established and verified. Results show that the intrusion distance, two-region widths, and erosion mass decrease significantly with increasing sand content. After intrusion, the dry density of the bentonite specimen nearby the extrusion port decreases with increasing sand content and fracture size. Meanwhile, results of XRD tests indicate that the ratio of accessory minerals in the accessory mineral ring was higher than that in the pure Gaomiaozi bentonite specimen. Verifications confirmed that the intrusion model proposed in this work can well simulate the data measured.