Geology Reference
In-Depth Information
be used to prevent debris impacting the portal area. Rock and soil masses
immediately above the portal area are often covered with steel mesh and
shotcrete or similar hard covering and dowelled, nailed or anchored
back using post-tensioned bolts and cable anchors. The requirements for
designing, protecting and maintaining ground anchorages are set out in
national standards and codes of practice such as BS 8081 (BSI, 1989)
and BS EN 1537 (BSI, 2000). Despite such standards, things occasion-
ally go wrong, either because of ground conditions or
flaws in
the anchorage itself, and designers must appreciate the practical dif
cul-
ties that might be associated with maintenance programmes whilst
ensuring safety for the road user. If a major problem is found, then the
tunnel might need to be closed or restricted in usewhilst the problems are
recti
ed. Several cases of the failure of rock anchorages, even in projects
post-dating BS 8081, are discussed in
Chapter 7.
6.5.5.2.2 PERMANENT LINERS
The options for permanent tunnel liner design include:
-
Unlined (ignoring temporary support measures)
-
Unreinforced concrete
-
Reinforced concrete
-
Steel.
Lined tunnels can be designed to be undrained, in which case the
permanent lining must withstand the full groundwater pressure as
well as rock loads. Other tunnels are designed to be drained, whereby
the outer surface (or extrados) of the arch of the liner is lined with a
waterproo
ng membrane laid onto geotextile sheets, which carry
water down to drains and sumps below the tunnel invert. The sumps
may need continual pumping, and the whole drainage system needs
maintenance over the life of the project.
Figure 6.18
shows details of a
design, as used in some recent rail tunnels in Hong Kong. After
shotcreting the tunnel walls, layers of geotextile (outer) and water-
proof membrane (inner) are placed, followed by an inner concrete
liner (250 mm thick). Groundwater is thereby channelled via the
geotextile to an egg box drainage system in the invert. For any
drained lining design, care must be taken that any permanent draw-
down in the water table has no adverse affects on structures above
the tunnel or on water supply from groundwater sources.
Precast concrete segments are commonly erected as part of a TBM
excavation and support process, mainly in soft ground tunnels, but also
in some hard rock applications. The segments aremanufactured externally
and then erectedwithin the shield surrounding the advancingmachine and
bolted together. If required, segments canbe
ttedwithgaskets to formfully
waterproof concrete liners
(Figure 6.19
). As noted earlier, the installed liner
can be designed to provide a reaction to push the TBM forward.
