Civil Engineering Reference
In-Depth Information
works required around and over the tunnel at the riverbank. The riverbank
will have to be reinstated to prevent the hinterland being flooded. Often
this will require a flood protection embankment to be reinstated over the
tunnel. Such an embankment places a high vertical load on the tunnel.
Again it is not generally desirable to put such a heavy load on the immersed
section as it increases the bearing pressure and its corresponding settle-
ment. Such high loads are better carried by an in situ structure which, if
necessary, can be of diaphragm wall or piled construction. A balance has to
be struck between all these factors to arrive at the optimum location for the
interface between the immersed and in situ sections of the tunnel.
CONSTRUCTION SEQUENCE
The first immersed tunnel element is normally placed against the in situ
section on one side of the waterway, which means that construction of that
in situ section has to be sufficiently far advanced to receive it. A typical
construction sequence for this is as follows:
1. Construct a temporary earthwork bund or steel pile cofferdam
in the river to reclaim the land required for the end of the in situ
construction.
2. Install cutoff walls to prevent water coming through the earthworks.
Steel pile cofferdams are largely watertight and would not require this.
3. Excavate within the reclaimed area, installing a dewatering system as
excavation progresses to keep the excavation dry.
4. Construct the seaward end of the in situ section and seal it with a
watertight bulkhead.
5. Construct an earthwork bund or piled cofferdam over the completed
tunnel to reinstate the riverbank. This will include a cutoff wall to
prevent water flowing through the bund into the excavation.
6. Remove the outer temporary earthwork bund, or cofferdams, so that
the end of the in situ section protrudes out underwater and is ready to
receive the first immersed element.
7. Place the first element against the exposed end of the in situ section.
This sequence is illustrated in Figure 13.9 in Chapter 13, which describes
the earthworks. At the other end of the tunnel, the approach will be con-
structed similarly to enable the immersed tunnel to connect with it. If the
tunnel elements are placed first, before the cut and cover tunnel is con-
structed, then a bund is constructed over the last tunnel element. This bund
will also connect to the general earthworks around the approach. It then
forms an enclosed area, which can be dewatered to allow construction of
the cut and cover tunnel adjacent to the last immersed tunnel element.
