On the evolution of hydration cracks of compacted bentonite under different boundary conditions

Abstract

Bentonite is widely used as a buffer/backfill material to dispose of the high-level radioactive wastes (HLWs). The precast bentonite blocks can cause the technological voids to be widespread in the repositories, acting as the primary channels for water migration. Luckily, bentonite will swell considerably when exposed to water, making it possible to seal the voids. The self-sealing process intensifies the uneven expansive deformation of bentonite and induces the cracking phenomenon. To be close to the actual situation, various boundary conditions were considered in this paper to carry out a series of hydration tests on compacted Gaomiaozi (GMZ) bentonite. The dynamic evolution of hydration cracks was investigated in both qualitative and quantitative way. Experimental results showed that the cracks basically followed a tendency of emerging first, then extending and finally closing. The bentonite sample with radial and vertical technological voids was characterized with a greater cracking degree than the case where only a radial void existed. According to the time history curves of geometric parameters, there was a close correspondence between the increase and the decrease stages from different cracks, indicating that the existing cracks provided deformation space for the initiation and propagation of subsequent cracks by sacrificing themselves. Due to stress or suction change, a re-cracking phenomenon was reflected on some bimodal curves, which demonstrated a significant memorability of the hydration cracks. Generally, the discrepancy in cracking behavior under different boundary conditions was related to the friction effect. The existence of vertical technological voids decreased the swelling pressure, thus reducing the friction force on the boundary. The suppression effect on the crack propagation was consequently weakened, which was conducive to forming wide cracks and strengthening the mutual connection between cracks.