Dd. Tannant et al., EFFECT OF TUNNEL EXCAVATION ON TRANSMISSIVITY DISTRIBUTIONS AND FLOW IN A FRACTURE-ZONE, Canadian geotechnical journal, 30(1), 1993, pp. 155-169
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.