Dynamic Evolution of Hydration Cracks upon Bentonite Self-sealing with Technological Voids

In the construction of deep geological repository, technological voids are inevitably produced around the compacted bentonite used as the buffer/backfill material. In the operation process, the bentonite absorbs the infiltrated groundwater, gradually expands, and fills the technological voids until completing the self-sealing. Previous studies have verified that the bentonite would crack in the self-sealing process and the cracks would propagate, increasing in number and scale, to form a complex crack network. In this paper, vertical and radial technological voids were simulated to carry out the hydration test of the highly-compacted GMZ bentonite. The dynamic evolution of hydration cracks during the self-sealing process was investigated by time lapse photography. Digital images obtained in the whole duration were analyzed and quantified by using image processing technique. According to the qualitative results, curved cracks first formed near the edge of the sample and then lots of cracks emerged around the sample center. The cracks connected with each other and cut the sample into separated clods. In the complete cracking state, the crack network presented an irregularly grid-like pattern. After that the cracks shrank gradually until they disappeared completely. In the quantitative analysis, each single crack was calculated in the consideration of the geometry parameters including the area, length and width. In terms of the dynamic evolution of crack area, cracks located adjacent to the sample edge displayed a double-peak shape in the area-time curve, and other cracks near the sample center appeared to be single-peak. The former was mainly affected by the crack length, while the latter was caused by the crack width. Essentially, whether the bimodality appeared on the curve or not was related to the re-cracking of the closed crack. Moreover, the early-formed cracks were squeezed by the subsequent cracks to meet the space demand for development. By comparing the dynamic curves of length and width, it can be deduced that cracks increased in length in advance of the width in the crack propagation and decreased in width in advance of the length in the crack closure.