Civil Engineering Reference
In-Depth Information
Figure 25.1
Methods of construction of tunnels in soft ground.
shown in Fig. 25.2(a). The top of the tunnel is its crown, the bottom its invert and the
dimensions are as shown where D is the tunnel diameter and C is known as the cover.
As the tunnel is excavated at the face the permanent lining cannot be constructed
immediately and there is a short length P called the heading which requires temporary
support. The temporary support is usually provided by a shield which is essentially
a stiff and strong steel tube. In the front of the shield is an excavator: this may be a
simple digger separate from the shield or a 'cheese-grater' rotating cutter which is part
of the shield. The shield is pushed forward from the completed permanent lining and
the face is excavated until there is enough space to build more permanent lining, often
inside the tail-skin of the shield.
There are different types of shield. Figure 25.3(a) shows an open shield with an inte-
gral cutter head in which excavated ground falls directly onto a conveyor. The total
Figure 25.2
Dimensions of shallow tunnels.
