Civil Engineering Reference
In-Depth Information
final element placement to a minimum. Before the foundation course is
constructed, the trench is checked for any sediment accumulation and this
is removed as required (Kuesel and King 1996).
The top of the tunnel should preferably be at least 1.5 m below the
original bottom to allow adequate top protection. If this is not possible,
the tunnel may sit proud of the bottom and be protected by backfilling
extending, for example approximately 30 m on each side of the tunnel or
more, depending on the conditions regarding ship navigation and associ-
ated risks of accidental impact. The fill must be protected against erosion
by currents with a rock blanket or similar means.
Another consideration is that there must be sufficient time during the
tidal cycle when the current is small enough, preferably less that 1 m/s over
a 2 hour period, to permit lowering of the elements.
Immersed tube tunnels are very different from bored tunnels and this is
illustrated by the following design and construction aspects that need to
be considered at the early design stage (after Ingerslev 1990):
•
topography, geology, hydrology and meteorology;
•
alignment criteria;
•
conceptual design as steel or concrete tunnel, including a method of
waterproofing by either watertight concrete or an external membrane;
•
steel fabrication yard in case of a steel tunnel and dry dock location
and lay-out in case of a concrete tunnel (see section 5.9.3);
•
schedule for construction as well as for design, including constraints
on the construction sequence;
•
choice of location with adequate facilities for outfitting of elements;
•
access to the interior after placing in case of very large water depths
where standard solutions are not applicable;
•
towing method and route;
•
survey methods during sinking and placing;
•
navigation, fairway requirements, and the method of sinking (including
plant, equipment, and temporary ballast) and of joining the tunnel
elements. These considerations combined may limit the practical tunnel
element lengths;
•
foundation method and the associated stability for the tunnel elements,
both immediately after sinking and in the longer term. For example,
temporary supports and screeded gravel foundation and sand flowing
(see section 5.9.4);
•
methods, and cost, of excavation and disposal of the tunnel trench
materials which might include gravels, sands, silts, clays or even rock
and boulders. It could be that a longer shallower tunnel on a different
alignment may be cheaper than a shorter deeper tunnel, especially if
rock excavation is involved as this can be expensive below water;
•
source of, type of and method of placing backfill around the tunnel after
positioning. Special backfill may be required for seismic conditions;
