Civil Engineering Reference
In-Depth Information
SELECTION OF BRIDGE OR TUNNEL
New crossings are required for many reasons. They can replace existing
crossings that are life expired, supplement existing crossings that cannot
cope with the volume of traffic, or be part of entirely new routes or develop-
ments. Each will have its own particular set of constraints that will have
to be overcome, whether they are the logistics of building a new crossing
adjacent to an existing one that has to remain in continuous use, or the
environmental and social issues associated with an entirely new crossing.
Whether to choose a bridge or a tunnel often depends on environmental
considerations and the operation of the waterway. A tunnel is usually, but
not always, more expensive than a bridge, so for a tunnel to become the
preferred option, there often has to be another reason where the benefits
outweigh the additional cost. For example, a tunnel rather than a bridge
was chosen at Conwy in the United Kingdom to preserve the views of the
estuary and the setting of the World Heritage Site of the walled town of
Conwy. Another example is the Øresund Tunnel, part of the fixed link
between Denmark and Sweden, which was chosen over a bridge as high
bridge piers that would have been needed to span the navigation channels
were unacceptable so close to the Copenhagen airport. If the crossing is to
carry rail, particularly freight, the long approach spans needed for a bridge
may constitute unacceptable visual intrusion in a valuable landscape or,
in an urban situation, may just be unachievable because there is no space
available, or they would blight the land over which they pass and reduce the
development potential of the land.
On busy waterways, access for shipping is also an important factor. In
general, when crossing a modern major waterway, the air draught required
for a bridge is often 50-60 m, to allow the passage of large freight and cruise
ships. In some major ports this clearance may even rise to 80 m. Authorities
will often not want to compromise any future development upstream of the
crossing, so they will insist on these large clearances under bridges even if
no ships of that size currently use the waterway. This air draught is consid-
erably more than the water draught of such ships, which is generally around
15-20 m. So, in general, a bridge has to be higher above the water level than
a tunnel does below it. The carriageway on a bridge would be about 65 m
above high water level for a 60 m air draught, whereas for a tunnel the car-
riageway would be closer to 30 m below high water level if the channel is
about 20 m deep. This difference makes the approaches to the bridge longer
than those required for a tunnel, which in turn increases the cost and also
makes a bigger intrusion into the adjacent landscape.
This is illustrated in Figure 4.1, which, as well as showing the difference
between a tunnel and a bridge, also includes a comparison between a bored
and an immersed tunnel. Bored tunnels are constructed deeper below
