TBM Tunneling - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 681

along the Hallandsas tunnel. Assuming schistosity-parallel discontinuities dipping
into the tunnel, rock wedges may slide into the tunnel due to self-weight alone. If,
however, the schistosity dips in the direction of tunnel heading, rock wedges can-
not slide due to self-weight only, and other effects are necessary to cause instabili-
ties. To prove that instabilities occur even under such conditions, all considerations
and analyses were carried out assuming the schistosity striking perpendicularly to
the tunnel axis and dipping at angles of 30 - 40° in the direction of tunnel heading.
Furthermore, two steeply dipping joint sets J1 and J2, which are striking parallel
and normal to the tunnel axis, were taken into account.
2. Owing to the tunnel excavation, the horizontal in-situ stress acting parallel to the
tunnel axis is redistributed, leading to a compression of the rock mass along the
tunnel contour (Fig. 21.18). At the same time, the area in front of the temporary
face is unloaded, which causes destabilization (Fig. 21.18). Assuming the schistosi-
ty dipping into the tunnel, this unloading may cause rock wedges to slide into the
tunnel.
Figure 21.18 Stress redistributions due to excavation (Lundman et al. 2009)
3. Given the restricted groundwater lowering due to tunnel heading, the groundwater
table in the tunnel sections in question was located approx. 80 - 100 m above the
tunnel roof. Tunnel heading below the groundwater table leads to seepage flow
towards the temporary face. Based on the results of FE analyses, independently
of the permeability of the ground, a rather high hydraulic gradient in the order of
10 - 20 was to be expected in the area of the temporary face for open mode hea-
ding. These high gradients led to high seepage forces F S acting on the rock mass
adjacent to the temporary face and thus also on rock wedges (Fig. 21.19, upper).
Assuming a block with volume V at the temporary face, the seepage force acting
on this block in the horizontal direction towards the tunnel is higher by a factor of
6 - 13 than the weight W of the block (Fig. 21.19, below).