Anisotropic hydration-induced cracking behavior of compacted bentonite

Abstract

In deep geological repositories, compacted bentonite subjected to hydration in the self-sealing process is prone to cracking due to uneven swelling deformation. In this paper, a series of hydration tests were carried out on compacted bentonite samples to investigate whether the anisotropy has an impact on the cracking behavior. The evolutionary process of hydration cracks was recorded by real-time image acquisition during the tests, while scanning electron microscopy (SEM) was performed after the tests to characterize the microstructure of compacted bentonite. Experiments show that there existed two distinctly different crack networks in the directions perpendicular and parallel to the compaction, respectively following a radial and a grid pattern of distribution. The complexity of the latter was obviously higher than that of the former, which was derived from the analysis of various quantitative parameters. SEM results indicate that the micromorphology of compacted bentonite was loose and rough in the perpendicular direction, but flat and smooth in the parallel direction. Conclusions thus can be drawn that the anisotropy exerted a significant effect on both the cracking pattern and cracking degree, which was the response of macroscopic cracking behavior to the microscopic structure characteristic. The stratified structure caused by uniaxial compaction highly affected the crack initiation and propagation, which determined the distribution and development of crack networks by influencing the cracking path and the energy consumption thereof. This paper provides a new insight into the anisotropy of compacted bentonite and has a practical significance to the barrier design of repositories in controlling the incidence of hydration cracks.