Influence of tectonic shears on tunnel-induced fracturing

The Opalinus Clay is currently under investigation as a potential host rock for deep geological disposal of nuclear waste at the Mont Terri Rock Laboratory in Switzerland. Bedding in the Opalinus Clay at Mont Terri is ubiquitous and highly persistent leading to mechanical transverse isotropy. Adding to the complexity at the Rock Laboratory is the frequent occurrence of small-scale tectonic shears.This paper explores the influences of millimetre-thick tectonic shears and bedding on the development of induced fractures mapped in the EZ-B field experiment at the research facility. A series of numerical analyses were carried out by increasing the geological complexity of the host rock and comparing the redistributed stress field with geological maps of the induced fractures. The analyses show that if tectonic shears are not kinematically constrained, mobilisation of the shears can play a key role in the development of the induced fracture network and therefore, be a primary factor in the development of the excavation damaged zone. This illustrates that under certain conditions rock mass heterogeneity (in this case, resulting from the tectonic shears) may dominate over rock matrix anisotropy (in this case, resulting from bedding) and must be considered when predicting the induced fracture network of the excavation damaged zone.