Effect of Hydration Time on the Hydro-Mechanical Behavior of Compacted GMZ Bentonite With an Artificial Annular Gap

According to the conceptual design of high-level radioactive waste (HLW) disposal repositories, the buffer barrier will be constructed by compacted bentonite blocks, which will inevitably produce technological gaps (voids) in the repository. During the consequent long-term operation of the repository, the technological gaps will be gradually filled up on hydration of the bentonite blocks with infiltration of underground water from the host rock. This hydration process will be accompanied by variations of the hydro-mechanical behavior of the buffer system. Therefore, it is necessary to study the changing process of the hydro-mechanical behavior of bentonite blocks with consideration of influences of the technological gaps. In the present work, a series of hydration tests were carried out on compacted GMZ bentonite specimens with initial annular gaps. During the hydration, variations of swelling pressure and hydraulic conductivity were measured. After each test, distributions of dry density and water content in the specimen were determined and the microstructure at different positions was investigated. Results show that, the swelling pressure of the specimen increased rapidly first and then decreased slightly during the hydration process, while the hydraulic conductivity kept decreasing with time. Moreover, closure of the technological gaps induced heterogeneity of the bentonite blocks, which was characterized by the decrease of dry density and the increase of water content with increasing distance to the center of the specimen. With the hydration time elapsed, the bentonite blocks could be divided into compression zone and swelling zone along the radius. In the compression zone, the dry density of the specimen gradually increased, with a decrease in the total pore void ratio and the macro-pores void ratio. In the swelling zone, the dry density decreased with an increase of the total pore void ratio and the undetectable pore void ratio.