Characterization and Simulation of Swelling Pressure Kinetics for Compacted Bentonite Hydrated by Alkaline Solutions

The swelling pressure (SP) of bentonite material is of great interest for geological disposal of high-level radioactive waste (HLW). Once emplaced as engineering barrier in a HLW repository, the bentonite material would be subjected to infiltration of aggressive alkaline groundwater, resulting in distinctive SP kinetics dramatically different from those hydrated by deionized water. In this study, fifteen SP kinetics of compacted bentonite were collected from literature and divided into three types. For bentonite hydrated by deionized water, the SP ascends continuously with time to a final steady value (type I), or undergoes a mid-stage collapse before reaching a final stable value (type II). While for bentonite hydrated by alkaline solutions, the SP develops following a trend similar to type II and then turn to long-term degradation (type III). It was identified that the SP kinetics was co-governed by the temporal evolutions of accumulated and dissipated wedging pressures (AWP and DWP) within the sample. The AWP resulted from the hydration of montmorillonite and increased continuously with infiltration time. The DWP comprised two components, namely the RDWP attributed to the rearrangement of fabric and the CDWP related to the chemical effects of alkaline solutions. Over time, the RDWP first increased then decreased while the CDWP increased continuously. Accordingly, three equations in forms of exponential, Weibull and logistic functions of time were proposed for describing the AWP, RDWP and CDWP, respectively. A simple and practical SP kinetics model was then established as superposition of the three equations and validated against experimental data in literature. Results indicated that the proposed model was capable of satisfactorily reproducing all the three types of SP kinetics (R2 > 0.96) and could be a useful tool for estimating the temporal evolution of swelling pressure during the long-term operation of a HLW repository.