Extent of oxidation in Boom Clay as a result of excavation and ventilation of the HADES URF: Experimental and modelling assessments

Abstract

For the construction and operation of a geological repository, the excavation and ventilation of galleries is necessary. Oxidation of the anaerobic host rock is unavoidable, and the favourable host rock properties may be affected. In this study, the extent of oxidation as a result of excavation and ventilation of galleries in Boom Clay was studied by experimental and modelling assessments. In situ experiments in the HADES URF aimed at comparing the oxidation around the Test Drift (excavated in 1987) and the Connecting Gallery (excavated in 2002). Pore water sampling and analyses were done from two identical piezometers installed in both galleries. Clay cores were used for detailed mineralogical and pore water analyses close to the gallery. The most striking result is the similarity between both galleries. Only within the first meter of clay around the galleries, the pore water chemistry is affected by oxidation, as indicated by the increased concentrations of sulphate and most cations. The trend of increasing sulphate concentrations towards the gallery, as observed in the piezometer-waters, is further continued in the pore waters obtained from the leaching of the clay cores. The effect of oxidation on the mineralogy is less pronounced and less extended. Gypsum is present within the first 4.5 cm of clay around the galleries. The experimental data and scoping calculations enabled to work out a conceptual model for oxidation around the HADES URF in Boom Clay. The main idea is that oxidation is related to fracturing. After sealing of the fractures, oxidation products are redistributed in the clay matrix by a combined diffusion–advection transport. In addition, a continuous oxygen diffusion from the gallery into the clay is maintained. Both processes seem to have minor effects on the further extent of oxidation, as indicated by the experimental data and modelling assessments. In the case of the Boom Clay, the extent of the oxidised zone is about 1 m, even after 20 years of ventilation.