Tunneling in Swelling Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 754

can be explained by a natural sealing of the rock mass as a result of swelling (Wittke &
Wittke-Gattermann 2010, Section 8.7, Fig. 8.18). Because of this natural sealing, a tight
zone is formed at the anhydrite surface and a tunnel can be excavated in dry rock mass
conditions provided that it is located with sufficient distance underneath the anhydrite
surface and that no water is brought into the tunnel during construction.
Fig. 23.1 shows a longitudinal section through a tunnel located deep underneath the
anhydrite surface. Around the tunnel a disturbed rock zone exists which is the result of
rock excavation and stress redistribution. Rock loosening due to excavation, or in other
words the disturbed rock zone, amounts to only a few decimeters while larger areas are
affected by stress redistributions.
Figure 23.1 Tunnel located deep underneath the anhydrite surface (Wittke 2012a)
Figure 23.2 shows the FE-mesh and the boundary conditions for the stability analy-
sis of an unsupported tunnel with an overburden of 263 m. The tunnel has a circular
cross-section with a diameter of 10 m and is located in unleached Gypsum Keuper, well
below the anhydrite surface. The calculated principal normal stresses, and the area in
which the strength of the horizontal bedding-parallel discontinuities and vertical joints
(cf. Fig. 8.1) is exceeded, are represented in Fig. 23.3 (left). The excavation of the tunnel
leads to dilatant displacements and thus to an increase in rock mass permeability. In
this example, this zone reaches up to about one tunnel diameter above the roof. As a
consequence the vertical permeability coefficient k V above and underneath the sidewalls
increases from 10 -9 m/s in undisturbed condition to >10 -3 m/s in the immediate vicinity
of the tunnel (Fig. 23.3, right).