The experimental study of bentonite swelling into fissures

Abstract

The geological disposal of radioactive waste, based on a multi-barrier concept wherein the first barrier consists of the metal waste container and the final barrier the host rock, is widely considered the only viable solution to this issue. The bentonite-based seal around the canister forms one of the barriers. The unique swelling and sealing capabilities of bentonite play a major role in repository safety concepts in that they allow the bentonite barrier to withstand serious mechanical damage without its function being compromised.This paper presents experimental research focusing on the dynamics and mechanics of the sealing of cracks and joints using bentonite-based materials. Physical models were used to simulate the contact point of bentonite-based sealants with cracks in the rock mass. The models examined the ability of the tested material to fill the crack thus preventing the creation of a preferential water pathway. The results show that in most cases total bentonite advance (for the same material) into fissures is, primarily, linearly dependent on fissure width. The absolute value of advance could be related to the overall swelling ability of the material characterized by its swell index or swelling pressure.

Bentonite Extrusion into Near-Borehole Fracture

“…We performed the test at room temperature. From Figure 9, we observe that before the intersection of the momentum balance and the volume balance laws (see Appendix E), the maximum bentonite migration length is linearly proportional to the slot width, which also supports Bolchover et al [11] and Svoboda [12]. After the bentonite plugging's effectiveness limiting criterion, the maximum bentonite migration length decreased with respect to the slot width, which also supports Kanno and Wakamatsu [9].…”

“…In support of Kanno and Wakamatsu's [9] findings, who showed the inverse results compared to the equilibrium theory, Islam et al [24] proposed that during bentonite extrusion, two balance laws may work simultaneously. The first one is the momentum balance law, which supports the bentonite extrusion stopping criterion or the equilibrium state (see also Bolchover et al [11]; Svoboda [12]). The second one is the volume balance law (see Islam et al [24] and Appendix E in this manuscript), which is based on the swelling potential (Section 2.1 and Figures 1 and 8).…”

“…Among others, Bolchover et al [11], Svoboda [12], and Islam et al [24] reported that when bentonite extrudes into a fracture (see the swell cell test experiments shown in Figure 2 in Section 2.1), two forces satisfy the equilibrium condition. They are the swelling-induced driving force and the resisting force.…”

“…The bentonite extrusion stopping criterion depends on these forces' equilibrium state, and the details are discussed in Appendix E. The driving force develops when the liquid saturation results in an increase of the swelling pressure (see also Figure 5), which pushes bentonite into the fracture. The driving force is the resultant of the swelling pressure and the fracture width (Svoboda [12]; Islam et al [24] and see Equation (A46)). Additionally, the resisting force is the resultant of the bentonite extrusion length and the bentonite-slot contact shear strength (see Figure 2a, Figure 3a,c and Equation (A47)).…”

“…Inside the fracture, the main source of stress is the swelling-induced stress. Therefore, τ in Equation (A47) can be written as [12]: τ = σ sw tanϕ,…”