09-11-2012, 06:20 PM
Immersed Tunnels
Immersed Tunnels.ppt (Size: 2.56 MB / Downloads: 216)
Typically, an immersed tunnel is made by
sinking precast concrete boxes into a dredged channel and joining them up under water.
Tunnel sections in convenient lengths, usually 90 to 150 meters, are placed into a pre-dredged trench,
joined, connected and protected by backfilling the excavation.
The sections may be fabricated in shipyards, in dry docks, or in temporary construction basin serving as dry docks.
Immersed tunnels are,
more advantegous as a subaquous solution in soft soils
increasingly used alternative to traditionally used shield tunnelling, without having the risks associated with pressure chambers and inrush of water.
also suitable in water deeper than it is possible with the shield method, which essentially is restricted to less than 30 m of water (concerning the maximum air pressure at which workers can safely work).
Advantegous as there is less loss in height than with tunnelling deeply under the riverbed, and the tunnel may therefore be shorter overall.
The most profound effect of an immersed tunnel on the environment concerns the element it is meant to bypass—water.
The influence of the tunnel on the groundwater and the surface water in the area plays a predominant role in the tunnel design and construction methods.
An aspect of more recent concern affecting construction is the possible presence of contaminated soils that must be removed for the tunnel trench. Ways of removing these soils and transporting them to depositories that are especially equipped to receive them are environmental problems requiring novel techniques and quality control procedures.
The more traditional environmental aspects are those encountered on any construction job: noise, dirt, and traffic hindrance.
Tunnel Trench Dredging
The dredging works required for the construction of an immersed tunnel will normally comprise some, or all, of the following items:
Dredging of a casting or launching basin.
Dredging of test pits in the waterway for evaluation of siltation of tunnel trench.
Widening of the existing navigation channel in order to provide temporary navigation channels outside the marine working area.
Compensation grouting to make up for the reduction of the waterway cross section caused by the permanent tunnel works, and thereby avoiding changes in hydrographical and biological conditions in the waterway.
Dredging of the tunnel trench for the immersed tunnel section.
Dredging of an access channel between the casting/launching basin and the tunnel trench.
Maintenance dredging.