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

The extent of oxidation as a result of excavation and ventilation of galleries in the Boom Clay was studied by experimental and modelling assessments. It is indicated that the ‘oxidised zone’ within the Boom Clay is related to the ‘fractured zone’ of the excavation damaged zone. Instantaneous oxidation occurs during excavation/fracturing. Because of the sealing of the fractures, oxidation products are trapped. Oxidation products are then re-distributed because of the combined diffusion-advection transport regime that is present around the galleries. Diffusion of the oxidation products further into the undisturbed Boom Clay is extremely slow and hence the majority of the oxidation products are dragged towards the gallery. In addition, a continuous oxygen diffusion from the gallery towards the clay massif is maintained during ventilation. The dissolution of oxygen into the pore water is very slow and hence the extent of the oxidised zone remains about 1 m ahead of the concrete/clay interface, even after 20 years of ventilation. This is an important result for PA since the current concept, namely a reduction of the effective thickness of the clay to account for the perturbed zone around the galleries, is still valid and conservative for the effect of oxidation.