Modeling of water retention behavior of densely compacted Gaomiaozi bentonite based on pore structure evolution

Abstract

Compacted bentonite has a dual-porosity structure characterized by intra- and inter-aggregate pores. The density of the water preserved in these two types of pores differs, and the interactions between these two types of pores lead to a complicated water retention behavior. In this study, this issue was addressed by establishing a dual-porosity water retention curve (WRC) based on the porewater density and pore structure evolution. The water retention behavior of densely compacted Gaomiaozi bentonite was tested. WRCs at three dry densities were obtained and unified using the water ratio ew (void ratio times the degree of saturation, eSr) as a state variable. Subsequently, mercury intrusion porosimetry (MIP) tests of samples taken from the specimens after the water retention tests were conducted. The intra- and inter-aggregate void ratios were determined based on the results of the MIP tests, and the pore structure evolution was described in terms of the intra-aggregate void ratio (em). The evolution of the microstructure state variable ξm (em/e) along the confined wetting path and its boundary state after water saturation were also determined. Moreover, water contents of the intra- and inter-aggregate pores were differentiated using the boundary water contents at which the intra-aggregate pores reached saturation, and the degrees of saturation of the intra- and inter-aggregate pores were calculated based on their porewater density. Finally, van Genuchten type WRCs were introduced for the intra- and inter-aggregate pores, and the overall WRC was derived by integrating ξm. The WRCs based on the porewater density and pore structure evolution are in better agreement with the testing data than those directly fitted from the experimental calculations, especially in the transition zone where the intra-aggregate pores approach water saturation. This dual-porosity WRC would help in modeling the hydromechanical behaviors of compacted bentonite in deep geological repositories for the disposal of high-level radioactive waste.