Civil Engineering Reference
In-Depth Information
width of a tunnel segment ring, the jacks are retracted leaving enough room
in the tail of the shield to erect the next tunnel segment ring.
There are many possible ways of excavating the ground at the face within
the shield. Manual excavation, i.e. by 'hand', is only considered for very
special applications, e.g. very short advances, due to the low advance rate.
This type of tunnelling is called the manual shield technique. Figure 5.6a
shows a manual shield as used on a pipe jacking project (see section 5.11
for a description of pipe jacking) in a predominantly stiff clay stratum and
Figure 5.6b shows an example of a 9.17 m diameter open face shield used
after the tunnel collapse at Heathrow, UK in 1994, where excavation on
multiple levels was required. The top two levels were excavated manually
and the lower level used mechanical excavation.
As shown in Figure 5.6b, shields can be split into vertical and horizontal
compartments (note the similarities between this shield and Brunel's Thames
tunnel shield described in section 1.3). This shield indicates the use of
working platforms to aid excavation and stability of the face. Open shields
can also have face-breasting plates attached to these working platforms (see
Figure 5.10) to aid stability. The theory is that each area between the
working platforms can be excavated separately, and safely, as the ground
will form a 'plug' in the opening, and hence form a stable face. The stability
of the ground, i.e. whether it collapses into the tunnel shield or not, is
based on its shear strength. The size of the openings is therefore designed
with this in mind, i.e. in weaker, less stable ground the benches must be
Working
platforms
a)
b)
Figure 5.6
a) Manual shield tunnelling as part of a pipe jacking operation,
and b) a 9.17 m diameter open shield (courtesy of Dosco Overseas
Engineering Ltd)
