Civil Engineering Reference
In-Depth Information
Table 5.1
Possible excavation and face support methods for shields
(after Stein 2005)
Shield with
Working face with
Completely unsupported
partial excavation
natural support
open face
(described later in
Working face with
Face divided into sections to
this section)
partial support
aid stability using horizontal
benches and/or breast plates
Working face with
Face is pressurized with
compressed air support
compressed air to help prevent
water ingress and aid stability
Shield with full
Working face with
Very open cutterhead
face excavation
natural support
(Tunnel boring
Working face with
Cutterhead has limited
machines, TBMs,
mechanical support
openings (can be closed by
see section 5.5)
face-breasting plates)
Working face with
Face is pressurized with
compressed air support
compressed air to help prevent
water ingress and aid stability
Working face with fluid
Slurry shield machines
support
Working face with earth
Earth pressure balance
pressure support
machines
respectively). In order to avoid these settlements, the gap is generally injected
with mortar. At the same time the injection results in a direct bond between
the lining and the ground. There are two different methods of injecting the
mortar: through the tunnel lining and through the tail of the tunnel shield.
The supply of the injection lines through the tail of the tunnel shield has,
in contrast to the supply of the injection mortar through the tunnel lining,
the advantage that the ring gap can be injected without any time delay and
is continuous over the whole circumference. In the case of a supply through
the tunnel lining segments there is always a delay until the shield has moved
forwards and the injection sockets are outside the shield tail.
Tunnelling shields do not have an 'engine' to propel themselves forwards,
but push themselves forward using hydraulic jacks. In order to create the
necessary force to push the tunnel shield forwards, jacks are placed around
the circumference of the shield. These jacks push against the last erected
tunnel segment ring and also push the shield against the tunnel face in
the direction of the tunnel construction (Figure 4.24). The tunnel segments
transfer the jacking forces into the ground using friction. Of course this
principle does not work at the start of the tunnel construction and therefore
in the starting shaft a reaction frame is necessary to take the jacking forces
(Figure 5.11). The jacks can be operated individually or in groups, allowing
the shield to be steered in order to make adjustments in line and level and
to be driven in a curve if required. When the shield has advanced by the
