Hydraulic response of a fracture zone to excavation-induced shear

The hydro-mechanical response of a fracture zone to tunnel excavation was investigated. Field data from the Room 209 tunnel at the Underground Research Laboratory of Atomic Energy of Canada Limited showed an unexpected transmissivity reduction as a tunnel face intersected a fracture zone. The observed hydraulic response could not be interpreted with conventional normal stress-fracture closure models. However, the similarity between shear stress development near a tunnel face and the measured hydraulic behaviour in the Room 209 fracture zone lead to the hypothesis that shear stresses or displacements were responsible for the excavation-induced transmissivity reductions.;The primary object of this thesis is to assess and validate of the mechanism or concept of shear-induced transmissivity reduction in fracture zones by comparison of field data from Room 209 with finite element predictions. A phenomenological relationship between shear displacement and transmissivity change was introduced based on the geometric structure of flow paths in en echelon fracture zones. Extensive numerical modelling was undertaken to simulate the hydro-mechanical response in the Room 209 fracture zone. Shear displacements were generated at different excavation stages as the tunnel face passed through the fracture zone. The shear displacements were determined with a three-dimensional finite element model using linear-elastic joint elements. The shear displacements were generally less than 1 mm with the largest displacements present during fracture zone intersection. The shear displacements and the shear displacement-transmissivity relationship were then used to generate transmissivity distributions in the fracture zone. The transmissivity distributions were transferred to a two-dimensional finite element seepage model to predict the hydraulic response.;Seepage analyses were made for various tunnel-face positions and provided predictions of steady-state heads, inflows, and ratios of flow to head drop at packer locations. The predicted response varied depending on the shear displacement distribution. The excavation-dependent variation in hydraulic response closely matched field measurements, showing a significant transmissivity reduction as the fracture zone was intersected.;The correspondence between measured and predicted hydraulic response supports the hypothesis of fracture zone transmissivity reduction due to excavation-induced shear displacements and provides a new mechanism for the interpretation of hydraulic responses in fracture zones.