EFFECT OF TUNNEL EXCAVATION ON TRANSMISSIVITY DISTRIBUTIONS AND FLOW IN A FRACTURE-ZONE

During an excavation-response experiment performed at the Underground Research Laboratory (Atomic Energy of Canada Limited, Pinawa, Man.) a decrease in fracture-zone transmissivity was measured as a tunnel inte rsected the room 209 fracture zone. The decrease in transmissivity was greatest as the pilot and the slash faces passed the fracture zone. T he transmissivities increased towards their preexcavation values as th e faces proceeded past the fracture zone. This response suggested that shear stresses or displacements controlled the hydraulic behaviour of the fractures. The hydraulic response in the fracture zone was analyz ed using finite element models. Predictions of shear-displacement dist ributions in the fracture zone as a function of face position were obt ained from a three-dimensional finite element model using joint elemen ts to represent the fracture zone. A phenomenological relationship bet ween shear displacement and transmissivity change was used to modify t he transmissivity distributions in the fracture zone based on shear di splacements for different excavation stages. Seepage analyses with the se transmissivities provided predictions that matched closely the fiel d measurements obtained from the room 209 fracture zone. These results and the inability of conventional, normal stress dependent, fracture closure to predict consistently the hydraulic response support the con cept of shear causing a reduction in fracture-zone transmissivity. Exc avation-dependent, shear-induced reduction in transmissivity provides an alternate mechanism for interpreting and understanding the hydrauli c response of disturbed fracture zones.