TBM Tunneling - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 676

The gneiss has a clearly defined schistosity dipping at angles between 0 and 40° in
northern, north-western and western directions. Several mainly steeply dipping joint
sets exist. Two dominant discontinuity directions of strike NW-SE and NE-SW and a
secondary direction NNE-SSW were observed.
In addition, there are joint sets running parallel to the schistosity. The orientation of
the joint sets changes slightly from one part of the horst to the other. Figure 21.15 (left)
shows photographs of outcrops at the south portal where the TBM heading was start-
ed, which clearly show the different discontinuity sets, and the corresponding pole plot.
Figure 21.15 (right) shows a picture of encountered discontinuities at the temporary
face grouped to the specified sets. The degree of jointing varies strongly along the tun-
nel from heavily jointed, such as in the area of fault zones, to only slightly jointed. The
discontinuity spacing predominantly can be described as close to very close.
Figure 21.15 Outcrops at south portal area, pole plot and joints in gneiss at the temporary face
(Lundman et al. 2009)
Along the tunnel a number of fault zones were mapped. The most pronounced of these
are the northern and southern border zones in the portal areas (SRZ and NRZ) as well
as the Möllebäcken zone (MBZ) in the northern area of the tunnel (Fig. 21.14). The
fault zones generally consist of strongly weathered and clay-altered zones alternating
with fractured and crushed zones with high permeability. Weathering may reach down
to the depth of the tunnel as is the case for the Möllebäcken zone so that special solu-
tions were required for tunnel heading.
In the contract documents 10 different rock classes with varying RQD, block size, de-
gree of weathering and rock mechanical parameters were given. Also, the geological
and geotechnical conditions were described, and the results of earlier exploration and
tunnel heading were provided.
The groundwater level in the area of the Hallandsas ridge is located slightly under-
neath the ground surface (Fig. 21.14). Since groundwater lowering was practically not