Microfabric of fractured Boom Clay at depth: a case study of brittle–ductile transitional clay behaviour

Recent Underground Research Facility (URF) gallery excavation works in the Boom Clay at 224 m below the surface revealed intense fracturing of this weak claystone. Large fault planes (meter-scale) and joints were observed during tunnelling and found to belong to the excavation-disturbed zone (EDZ). Samples taken from the new gallery have been investigated using scanning electron microscope (SEM) imagery and microstructural analysis. This revealed a brittle–ductile transitional behaviour of the mudstone sediment regarding deformation, implying dilatant shear and hybrid fractures in high strain zones and ductile, compacted plastic shear bands in the lower strain zones, following an elasto-plastic constitutive model. Microfabric analysis contributed in this way to a better understanding of clay behaviour and effective fracturing. This information is primordial for understanding the rheology of the clay and assessing the influence of fracturing on permeability, critical issues in radioactive waste management.