Anisotropic swelling pressures of compacted GMZ bentonite infiltrated with salt solutions

Abstract

In the high-level radioactive waste (HLW) deep geological repository, bentonite is compacted uniaxially, and then arranged vertically in engineered barriers. The assembly scheme induces the initial anisotropy, and with hydration, it develops more evidently under chemical conditions. To investigate the anisotropic swelling of compacted Gaomiaozi (GMZ) bentonite and the further response to saline effects, a series of constant-volume swelling pressure tests were performed. Results showed that dry density enhanced the bentonite swelling and raised the final anisotropy, whereas saline inhibited the bentonite swelling but still promoted the final anisotropy. The final anisotropy coefficient (ratio of radial to axial pressure) obeyed the Boltzmann sigmoid attenuation function, decreasing with concentration and dry density, converging to a minimum value of 0.76. The staged evolution of anisotropy coefficient was discovered, that saline inhibited the rise of the anisotropy coefficient (Δδ) in the isotropic process greater than the valley (δ1) in the anisotropic process, leading to the final anisotropy increasing. The isotropic stage amplified the impact of soil structure rearrangement on the macro-swelling pressure values. Thus, a new method for predicting swelling pressures of compacted bentonite was proposed, by expanding the equations of Gouy-Chapman theory with a dissipative wedge term. An evolutionary function was constructed, revealing the correlation between the occurrence time and the pressure value due to the structure rearrangement and the former crystalline swelling. Accordingly, a design reference for dry density was given, based on the chemical conditions around the pre-site in Beishan, China. The anisotropy promoted by saline would cause a greater drop of radial pressure, making the previous threshold on axial swelling fail.