Unsaturated permeability of Gaomiaozi bentonite under partially free-swelling conditions

Abstract

In the deep geological disposal of high-level radioactive wastes (HLW), Na-bentonite produced from Gaomiaozi (GMZ) deposit has been selected as a possible buffer and backfill material for the Beishan repository in China. A sealing buffer is manufactured with bentonite at its hygroscopic water content (initially unsaturated). After taking water from the surrounding geological medium, bentonite under totally free condition is used to seal the technological voids, and then the outer surrounding bentonite (affected by the water mostly) compresses the innermost bentonite under partially free-swelling condition. However, permeability of GMZ bentonite under the partially free-swelling condition is still unknown. In this article, using the instantaneous profile method, we present a laboratory test of the GMZ bentonite unsaturated hydraulic conductivity under partially free-swelling condition. Changes in the cross-sectional area and motion of the sensors (to measure temperature and relative humidity) were considered in computing the unsaturated permeability. Our experimental results show that before the finial hydration stage, although permeability increases locally at some suctions (due to saturation degree increase and bentonite swelling to some extent), unsaturated permeabilities decrease and become constant in general for the reasons as follows. Initially, water is tightly adsorbed to the clay sheet surface and then water film is adsorbed to the tightly adsorbed water which leads to the initial highest permeability, and then diffuse double-layer forces lead to water uptake between the hydrated montmorillonite unit layers which is partially restricted by the confining stress, while capillary water flows on the water adsorbed on the internal surface of the bentonite large pores. For the capillary water flow due to surface tension, complex microstructure evolution (e.g., mesoscopic swelling compacts the inter-aggregate pores due to hydration under the confining stress from the rubber tube) before the finial hydration stage leads to flux decrease if under a certain hydraulic gradient (i.e., permeability decreases). Results show that in the final hydration stage, permeability increases rapidly with decreasing suction, because large pore compacting is complete while saturation degree increases and bentonite swells to some extent. Results show that at the same suctions, the unsaturated permeabilities located at the sensor closest to the water injection device are the largest among the four sensors, which could be explained with “critical gradient” concept for unsaturated expansive clay or effect of hydraulic gradient from boundary water pressure.