Experimental study on swelling and intrusive behavior of bentonite under various solutions and its simple model construction

Understanding the amount of bentonite intrusion into rock cracks is necessary to evaluate the performance of natural barrier materials for geological disposal. In this study, assuming a possible chemical action in geological disposals, bentonite was immersed in various solutions to accelerate the chemical action, and a half-year swelling and intrusion experiment was conducted. The experimental results showed that the swelling pressure and intrusion distance of bentonite in a solution environment differed depending on the solution type and that both were smaller than those in a purified water environment. The swelling pressure of bentonite in the environments of the KOH, NaOH, MgCl2, and NaCl solutions decreased significantly, and it did not significantly enter into the gaps in these systems. This is undesired in situations where gaps such as the cracks of rock must be filled. Furthermore, a numerical analysis model was constructed that can simply represent the complex intrusion behavior of bentonite in cracks considering the balance of three forces acting on bentonite. The analysis results confirmed that under conditions where the external swelling pressure fluctuates over time, the intruding fluid fills the cracks more slowly than under conditions where the external swelling pressure is constant. It was also confirmed that under conditions where the viscosity of the intruding fluid decreases over time, the intruding fluid fills the cracks faster than under conditions where the upper limit viscosity is constant. The numerical results clarified that the governing equation for bentonite intrusion into closed cracks may express the long-term intrusive behavior of bentonite into cracks during geological disposal.