A SHEAR-DEPENDENT FRACTURE-ZONE TRANSMISSIVITY MODEL

Shear displacements caused by excavation-through a fracture zone can d ecrease the overall fracture-zone transmissivity. The complex arrangem ent of flow channels found in fracture zones creates a situation where imposed shear displacements will reduce the aperture of critically or iented fractures. This paper presents a simple flow model based on the en echelon structure found in many fracture zones from which a phenom enological relationship between shear displacement and transmissivity change is established. This model is then used to demonstrate the effe ct of shear displacement around a circular opening. The effects of she ar-induced decreases in transmissivity are most dramatic when a contin uous low-transmissivity zone is created around the opening. In this ca se, the steady-state heads can be relatively high and the resulting hy draulic gradient into the excavation can be extremely large. If the lo w-transmissivity zone around the opening is discontinuous because, for example, the shear displacements are nonaxisymmetric, then the inflow becomes channelled and the steady-state heads and hydraulic gradients near the excavation substantially decrease. The concept of shear-indu ced transmissivity reduction in fracture zones provides an alternative mechanism to shear-induced dilation and normal stress induced fractur e closure for interpreting and explaining the observed hydraulic respo nse in fracture zones.