Rock mass response ahead of an advancing face in faulted shale

Abstract

In this study, the rock mass response ahead of an advancing test tunnel in the Opalinus Clay at the Mont Terri Rock Laboratory (Switzerland) was investigated. Characterisation of the excavation-induced damage zone at Mont Terri is a challenging task due to the anisotropic and heterogeneous nature of the shale: pronounced bedding leads to intact rock anisotropy and prevalent small-scale tectonic shears lead to rock mass heterogeneity. Rock mass damage ahead of an experimental tunnel or niche was characterised through single-hole seismic wave velocity logging, borehole digital optical televiewer imaging, and geological drillcore mapping. Three-dimensional elastic stress analyses were completed and showed that rock mass degradation can be correlated to changes in the maximum to minimum principal stress ratio (i.e., spalling limit). Numerical results showed that close to the niche boundary, unloading lowers stress ratios, which correspond with decreasing seismic wave amplitudes and velocities; thus, indicating that strength degradation resulted from increasing crack-induced damage. Considerations of tectonic shears and distance from a previously stressed volume of rock were necessary in understanding both the damage state and extent ahead of the face. By integrating field and numerical data, the investigation showed that geological structures (i.e., bedding and bedding-parallel tectonic shears) were most influential near the entrance but played a lesser role as the niche deepened. Additionally, a portion of the niche is located in the perturbed zone of the intersecting Gallery04.