Intrusion process of bentonite-sand mixture and its model in rock fissures with consideration of lateral wall friction

During the construction and long-term operation of a deep geological repository for disposal of high-level radioactive waste, compacted bentonite-sand mixture invades and fills into fissures in the surrounding rock formations. This intrusion reduces the permeability of the rock formations. Meanwhile, it also causes the loss of bentonite with possible leakage of nuclides, resulting in endangering the operation safety of the repository. Aiming at solving the problems related to bentonite intrusion into rock fissures, a test device is developed, and the intrusion tests are conducted on the bentonite-sand mixture specimens (quartz size: 0.075～0.1 mm) with different sand contents (0, 15% and 30%) into artificial fissures with different sizes (0.1, 0.3, 0.5 and 1.0 mm). The images are regularly captured by the high-precision digital camera during the intrusion process of bentonite mixture. Then, with an ImageJ image processing software, the parameters including the intrusion distance and width of the accessory-mineral (quartz, feldspar and cristobalite, etc.) and non-accessory mineral (montmorillonite, etc.) ring are obtained. According to the analyses on the development processes of swelling pressure and friction resistance along the inner side wall of the fissures during the invasion process, an intrusion model for the bentonite intrusion in the surrounding rock fissures considering the friction resistance of the inner wall is proposed and verified. The results show that the bentonite intrusion distance and the widths of non-accessory and accessory mineral rings in the fissures increase significantly with the increase of the fissure size, and decrease obviously with the increasing sand content. After the bentonite intrusion reaches its equilibrium (stability), the dry density at the extrusion outlet decreases with the increase of the fissure width and increases with the increase of sand content. The friction coefficient of the accessory mineral ring area is obviously larger than that of the non-accessory one, indicating that the accessory mineral ring has a stronger inhibitory effect on the bentonite intrusion process. The measured results can be well simulated by the proposed model for the bentonite intrusion with consideration of the inner side wall friction of fissures. The researches on the intrusion tests and model can provide a basis for predicting the intrusion behaviors of bentonite in the actual disposal repository.