Coupling of Stress-Strain Behavior and Seepage Flow - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 167

Also, stress changes due to the excavation of a tunnel can lead to permeability changes
in the rock mass. As an example, Fig. 7.3 shows a tunnel with open invert in rock mass
with horizontal bedding-parallel discontinuities and vertical joints striking parallel to
the tunnel axis. The tunnel is equipped with an outside drainage at the level of the
invert, leading to a permanent lowering of the groundwater level. Such solutions are
economical and can be applied if the permeability of the rock mass is low. Shear stresses
underneath the invert in such a case can lead to dilatant displacements on the discon-
tinuities (Section 3.3.4). As a consequence, zones with increased permeability above
and underneath the tunnel arise as depicted in Fig. 7.3. If the increase in permeability
is large enough, the drainage pipes may fall dry and seepage may occur through the
loosened zone parallel to the tunnel. This may lead to uncontrolled erosion and in case
of a soluble rock to the dissolution of chemical components. In such a case the installa-
tion of bulkheads and extensive grouting works may be required to prevent water from
seeping through the rock mass along the tunnel's invert.
For a number of other rock mechanics problems such as the recovery of petroleum and
natural gas, the storage of fluids in caverns, the disposal of nuclear and hazardous waste
in underground openings and the pumping of liquid waste into the ground, hydrome-
chanical interaction needs to be considered as well.
Figure 7.2 Lowering of water table in a rock slope due to stress changes and corresponding
deformations