Swelling pressures and hydration times in a clay seal

Abstract

Underground repositories for nuclear fuel waste will eventually become filled, and access shafts will have to be sealed. In many international studies, the preferred sealant is a compacted sand–bentonite mixture in which montmorillonite in the bentonite swells as it absorbs groundwater and develops swelling pressure. Lowered porosities in the bentonite lead to reduced hydraulic conductivity. Sealing is most effective when the seal is saturated and swelling pressure has been maximised. The time required to reach complete hydration is important. This paper describes the development of swelling pressures during hydration of a field-scale shaft-sealing experiment at Canada’s underground research laboratory. Numerical modelling used the finite-element program Code_Bright and hydro-mechanical parameters from laboratory tests to determine time-dependent porosities, total pressures and degrees of saturation in the full-scale field project. Data from laboratory-scale ‘wetting’ tests on specimens of seal material were then used in models by Liu and Dueck and Börgesson to evaluate changes in swelling pressures with time. Six years of data from field instruments indicate that the seal was functioning as designed. In the future, when the seal becomes fully hydrated and reaches stress equilibrium, the modelling suggests that swelling pressures in the seal will be in the range of 0.43–0.86 MPa. This is broadly similar to the value of 0.8 MPa suggested by a separate series of laboratory swelling tests. Full saturation and maximum swelling pressure can be expected in 17–19 years after the seal has been completed.