Mechanical properties and constitutive modeling of compacted bentonite under effects of solution concentration
摘要
In deep geological repositories, permeation of saline groundwater into bentonite buffers and backfills can significantly alter their mechanical behavior, impacting repository safety. This study investigates the influence of NaCl solution concentration on the swelling behavior, shear strength, and hydraulic conductivity of sodium bentonite. A series of laboratory tests were conducted under distilled water and various NaCl solutions, with microstructural changes characterized using scanning electron microscopy and mercury intrusion porosimetry. Results demonstrate that increasing salt concentration suppresses swelling due to compression of the diffuse double layer, which reduces montmorillonite interlayer spacing. Elevated salinity also promotes the agglomeration of montmorillonite layers, thereby enhancing the soil’s shear strength. This particle agglomeration forms larger clusters, creating macroscopic pores that serve as dominant seepage channels and increase hydraulic conductivity. By incorporating a modified effective stress (pe) that accounts for pore fluid salinity, the deformation and strength characteristics at salt saturation were found to be uniquely governed by effective stress. The relationships between void ratio (em) and pe, and between shear strength (τf) and pe, were unified across different salt concentrations. A predictive model for hydraulic conductivity (k) versus pe was developed by integrating the em-pe relationship with particle agglomeration effects. The model shows strong agreement with experimental data, confirming that the influence of salinity on hydraulic conductivity arises not only from a reduced void ratio but, more importantly, from particle agglomeration that enlarges the macroscopic pore structure. These findings provide a new constitutive framework for predicting long-term hydraulic conductivity in the engineering of disposal repositories.