Abstract
Hydraulic behaviour of compacted smectite-rich clays is of practical interest in geological disposal of high-level nuclear waste, as the buffer component of the engineered barrier system. Despite several decades of research, a sound theoretical description of the unsaturated hydraulic conductivity of clay buffer (compacted bentonite) and its performance remains challenging. This is evidenced by experimental data on unsaturated hydraulic conductivity, which, albeit limited, show that the hydraulic behaviour of compacted bentonite during confined wetting is considerably different from that typically observed in non-expansive clays. This work addresses the challenge by proposing a predictive model for the saturated and unsaturated hydraulic conductivity of compacted smectite under confined wetting. By considering the microstructure evolution of compacted smectite, a theoretical description of its pore system variations with relative humidity is presented based on a geochemical modelling approach. The Kozeny–Carman (KC) relationship for hydraulic conductivity of compacted smectites is revisited as a basis to derive a new model, which also incorporates more accurately the effects of key properties, such as porosity, specific surface area and tortuosity. The model predictions for saturated hydraulic conductivity for four types of bentonite clays (GMZ, Kunigel-V1, MX-80 and FEBEX) show close correlations with the experimental data. The results of model prediction for unsaturated hydraulic conductivity of compacted bentonite are in close agreement with the experimental data over a large range of suction values.
Highlights
Despite significant efforts in the last few decades, developing a sound theoretical description of the hydraulic properties of compacted highly swelling clays such as smectite-rich clays has remained a challenge
2 2n Atotal where the total specific surface area of smectite, Atotal, can be measured experimentally, and n is the average number of stacked unit layers per particle, which can vary from several layers up to hundreds of layers and can be measured by X-ray diffraction (XRD) analysis or calculated by the structural formula of smectite (Dixon et al, 1999; Saiyouri et al, 2004)
The results demonstrate that the simulation using the revised relative permeability model (equation (15)) provides much closer correlations with the measured data, while modelling using the conventional relative permeability model (equation (14)) has overestimated the rate of water flow, leading to larger values of relative humidity, especially at low water contents
Summary
Despite significant efforts in the last few decades, developing a sound theoretical description of the hydraulic properties of compacted highly swelling clays such as smectite-rich clays has remained a challenge. Observations of Komine & Ogata (1999) on the re-saturation of a smectite–sand mixture system under constrained swelling by scanning electron microscope indicate that, upon water supply, the clay particles are hydrated, resulting in the swelling of the interlayer distance of bentonite (micro pores) and consequent reduction of macro pores in the sand–bentonite system This would result in reduced water flow through the macro porosity. The formulation presented by Sedighi & Thomas (2014), which is based on a solid solution geochemical model of interlayer hydration by Ransom & Helgeson (1994) to develop an understanding of the micro pore evolution during saturation, is extended. The interlayer water hydration/dehydration reaction is analogous to a regular solid solution reaction, which yields the mass action law in an expanded form as
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