Saturated hydraulic conductivity of compacted bentonite–sand mixtures before and after gas migration in artificial seawater

Abstract

To understand the self-healing property of an engineered barrier for radioactive waste disposal, the hydraulic conductivity of compacted bentonite–sand mixtures saturated with artificial seawater (SW) before and after gas migration was examined. Na- and Ca-bentonites were mixed with fine sand at a ratio of 70% bentonite in dry weight. Two aspects were considered during the experiment: the hydraulic conductivity of the specimen that was resaturated after gas migration and the distribution of water content immediately after gas migration to study gas migration pathways. The gas migrated through the entire cross-section of the specimen, and gas breakthrough occurred in the equilibrium swelling pressure range approximately. Subsequently, the gas flow rate reached a sufficient large value when the gas pressure was approximately twice the equilibrium axial pressure (the sum of swelling and confining pressures), which excluded the back pressure. Although the gas migration pathway was not visible when the specimen was observed immediately after gas migration, the water content distribution showed that several parts of the specimen with lower water content were connected in the direction of gas migration. After resaturation, the change in permeability was within a limited range—two to three times larger than that before gas migration for each type of bentonite in SW. This slight change suggests that gas migration creates a pore structure that cannot be sealed via crystalline swelling of montmorillonite in SW, even if highly compacted bentonite is used under a constant-volume condition.

Keywords

• Bentonite
• Gas migration
• Hydraulic conductivity
• Seawater (SW)
• Self-healing