Early and post-stage piping erosion in bentonite buffer materials exposed to groundwater inflow

Abstract

Piping erosion presents a significant concern in engineered barrier system (EBS), contributing to performance uncertainties. However, the early-stage hydration and piping erosion characteristics of calcium-type bentonite under concentrated water inflow conditions are not fully understood. To address this gap, we examined early- and post-stage piping erosion in bentonite buffer materials. Specifically, we focused on the onset and evolution of piping channels with changes in the inflow pressure and their impact on buffer material integrity. Piping experiments were conducted using bentonite in block, granule, and powder forms under constant flow rate conditions. We analyzed the hydraulic-mechanical responses at the bentonite-cell interface, fluctuations in inflow water pressure, and eroded soil mass. Additionally, X-ray computed tomography imaging was utilized to assess the deterioration of buffer materials after piping. The results revealed that early-stage hydration-induced erosion behaviors are contingent upon the state of the bentonite buffer, with compacted blocks exhibiting predominant piping erosion. The rapid pressure buildup and breakthrough is essential in triggering piping erosion in the blocks, while the evolution of piping channels is influenced by the flow rate. Furthermore, severe cracks occurred along with the piping channels under reduced flow rate conditions, creating voids in the buffer amounting to 1.5–3.1 % of its initial volume. These findings provide insights into buffer-rock interfacial interactions in EBS, serving as the basis for in situ disposal experiments and the safe design of disposal repositories.