Civil Engineering Reference
In-Depth Information
is generally prohibited by the river authorities for this reason. Such material
would also generally be unacceptable for engineering reasons. Granular
material can be placed carefully by fall-pipe so that it is contained rather
than being simply bottom- or side-dumped into the river.
The tunnel trench does not have to be backfilled completely for engineer-
ing reasons, although it may be a requirement of the river authority for
safety or other reasons. Once the tunnel has been covered and scour protec-
tion is placed to protect the backfill, the rest of the trench can be left to fill
in naturally. This generally applies only to the hidden part of the excavation
that is permanently underwater. The visible excavations through intertidal
areas usually have to be returned to their original states.
FISHERIES
Constructing an immersed tunnel can impact the fish and shellfish in the
river. The very act of carrying out construction in the river can disturb
migrating fish as they are sensitive to the noise and vibration caused by
marine plants. For piling works, it is preferable to use push techniques
rather than percussive techniques, if ground conditions permit. Mitigation
measures are also available and bubble jackets and bubble curtains to pre-
vent the noise arising from piling operations spreading have been developed
in the United States. This technique is quite simple and uses perforated
pipes laid individually around a pile being driven or around the zone of
piling. Air is pumped through the pipework and the curtain of air bubbles
that rises through the water column is sufficient to dampen the noise travel-
ling through the water.
In addition, high sediment content introduced into the water by dredging
can cause significant oxygen depletion in the water and lead to large-scale
fatalities of fish. There is inevitably some short-term disruption during con-
struction, but by careful planning of the operations, this can be minimized
so that there is no long-term effect on the fish. For example, if there are
significant fish migrations along the waterway, then marine operations can
be suspended during the migration season. This practice has been adopted
on many immersed tunnels, including those recently built beneath the River
Tyne in the United Kingdom and the Bosphorus in Istanbul. For the latter,
marine operations were suspended during two fish migrating seasons, both
northward and southward each year. Such restrictions will have an impact
on the contractor's planning, but generally, the dredging is not on the criti-
cal path, so such restrictions can be accommodated within the overall con-
struction program, provided they are clearly defined at the outset.
Other examples include the Bjørvika Tunnel, which crossed the mouth
of the Akerselva River that flows into Oslo Harbor. The river is important
for the migration of Atlantic salmon and trout. In this instance, no seasonal
