Swelling pressure and permeability of laminated bentonite-sand mixtures with different exchangeable cation composition

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4. Swelling pressure and permeability of laminated bentonite-sand mixtures with different exchangeable cation composition

Changes in the swelling pressure and permeability of laminated compacted bentonitesand mixtures with different exchangeable cation compositions were investigated using a permeating salt solution. A laminated specimen was produced by stacking two compacted bentonitesand mixtures: a 7% Na-bentonitesand mixture on the upper side and a 30% Ca-bentonitesand mixture on the lower side. The laminated specimen was infiltrated using 0.1 and 1.0 mol/L Na₂SO₄ solutions. The vertical pressure, hydraulic conductivity, and ion concentration in each effluent solution were measured. Ca-bentonite (upstream) discharged Ca ions by ion exchange, whereas Na-bentonite (downstream) exhibited ion exchange from Na to Ca ions. The equivalent hydraulic conductivity slightly differed from that of the laminated specimen at 0.1 mol/L. As lower concentrations cause osmotic swelling, it is essential to estimate the hydraulic conductivity of the downstream material using a complex ion-exchange process to obtain precise equivalent hydraulic conductivity. After changing the solution to deionized water, the vertical pressure increased, whereas the hydraulic conductivity decreased. The estimation of swelling pressure is expected to be useful in setting the overburden pressure to prevent swelling deformation and maintain the low permeability of the engineered barrier in radioactive waste disposal.

9th International Congress on Environmental Geotechnics (ICEG2023)

Engineered Barriers