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.