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A submerged floating tunnel is also called a submerged floating tube bridge (SFTB), a suspended tunnel, or an Archimedes bridge, is a proposed design for a tunnel that floats in water, supported by its buoyancy (specifically, by employing the hydrostatic thrust, or Archimedes’ principle).
1. Submerged Floating Tunnel
The tube would be kept underwater, deep sufficient to neglect water traffic and weather, but not at a higher depth that high water pressure requires to be given out with; normally, a depth of 20 to 50 m (66 to 164 ft) is enough. Cables either anchored to the seabed or to pontoons on the surface would save it from floating to the surface or submerging, respectively.
The idea of submerged floating tunnels is carried out from well-known technology used in floating bridges and offshore construction, but the structure is mostly the same as that of immersed tunnels:
After the tube is precast in the segment in a dry dock and the segments are transported to the site, one way is to first cover the segments; descend them into location, while covered; and, when the areas are attached to each other, smash the seals. Another chance is to leave the segment uncovered, and after welding them together at the site, pump the water out.
The ballast is measured so that the structure has exact hydrostatic equilibrium (that is, the tunnel is abruptly the same whole density as water), whereas immersed tube tunnels are ballasted to attain negative buoyancy so they lean to stay on the sea bed.
This, of course, defines that a submerged floating tunnel must be attached to the ground or to the water surface to retain it in place, based on the buoyancy of the submerged floating tunnel: moderately positive or negative, respectively.
2. Reason for Using Floating Tunnel
The floating tunnel is a modern idea and has never been constructed, but many plans have been floated by various entities. It is seen that the depth of the bed ranges from one location to another to a considerable area.
The peak depth can be up to 8 km and the mean depth is 3.3 km. The two alternatives found for preparation are a bridge over water level or a tunnel beneath ground level. However, since the depth is 8 km, it is impossible to build concrete pillars of such height for a bridge.
The pressure beneath 8 km from the sea surface is almost about 500 times atmospheric, so no one cannot live in such a high-pressure zone. Therefore, the maximum depth of the floating tunnel is 30 m from sea level, where there is no problem due to more pressure. This will be enough for any big ship to move over the SFT without blockage.
3. Problems of Submerged Floating Tunnel
1. Cost
As a lot of matter and machinery are used in this project, the estimated cost is almost double that of a simple tunnel.
2. Fire
It results to be hard to save people if a fire breaks out in the train when inside a tunnel.
3. Collision
In this case of two trains collision, it is very hard to save the passengers.
4. Discomfort to train passengers
When a train moves through a tunnel, pressure waves are induced which propagate along with the tunnel extract at sonic speed. These pressure changes may move into the interior of the trains unless they are pressure-covered, and can cause discomfort to train customers.
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