Influence of bedding planes to EDZ-evolution and the coupled HM properties of Opalinus Clay

Field observations at the Mont Terri site demonstrate an excavation damage zone (EDZ) around tunnels consisting of a complex crack network related to the bedding and the existing stress field. To separate the different impacts of the rock mass and bedding planes laboratory investigations were performed on Opalinus Clay samples including triaxial strength and direct shear tests. During the deformation tests monitoring of ultrasonic wave velocities, permeability and volumetric strain measurements facilitates the detection of stress induced onset of damage. Two stress dependent criteria were estimated, referring to various damage states, i.e. (1) to initial onset of damage and (2) occurrence of dilatancy. Based on the experimental results a new modelling approach for a prognosis of the evolution of the EDZ is developed consisting of two parts: (1) a (visco-)elasto-plastic constitutive model, comprising the hardening/softening behaviour and dilatancy effects of the rock mass, and (2) a specific friction model, which described displacement- and velocity-dependent shear strength softening for the bedding planes. The capability of the new approach is demonstrated by recalculating the spatial development of EDZ around a drift at the Mont Terri site.