Mechanical and hydraulic properties of the excavation damaged zone (EDZ) in the Opalinus Clay of the Mont Terri rock laboratory, Switzerland

Abstract. Construction of cavities in the subsurface is always accompanied by excavation damage. Especially context deep geological nuclear waste disposal, evolving damaged zone (EDZ) near field emplacement tunnels utmost importance concerning safety aspects. As EDZ differs from intact host rock due to enhanced hydraulic transmissivity and altered geomechanical behavior, reasonable location-dependent input data on mechanical properties are crucial. Thus, this study, a hydromechanical characterization an Mont Terri underground laboratory, Switzerland, was performed using three different handheld devices: (1) air permeameter, (2) microscopic camera (3) needle penetrometer. The discrete fracture network (DFN), consisting artificially induced unloading joints reactivated natural discontinuities, investigated portable permeameter combined imaging with automatic evaluation. Geomechanical geophysical claystone conducted based penetrometer testing at exposed surface. Within EDZ, permeable fractures mean aperture 84 ± 23 µm present. Under open conditions, self-sealing suppressed, cyclic long-term oscillations combination closure resulting convergence processes observed. Based measured penetration indices, uniaxial compressive strength 30 13 MPa (normal bedding) 18 8 (parallel determined. Enhanced stiffness directly related near-surface desaturation sharp decrease water content 6.6 wt % 3.7 %. presented methodological approach particularly suitable for time-dependent monitoring EDZs since measurements nondestructive do not change actual state mass. This allows spatially resolved investigation apertures, surface roughness, physico-mechanical parameters their intra-facies variability.