A submerged floating tunnel (SFT), also called Archimedes bridge or suspended tunnel, is a tunnel that floats in water supported by its buoyancy (specifically, by employing the hydrostatic thrust, or Archimedes' principle).
Tunnels in water are not new in civil engineering. Since about the 1900s, several immersed tunnels have been constructed around the world. Bridges and tunnels are the most common structures used to cross water bodies. In some cases, immersed tunnels run beneath the sea or river bed. But submerged floating tunnels are used when the bed is too rocky, deep, or undulating.
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Reason for Choosing Floating Tunnel
The floating tunnel is a new concept and has never been built, but several proposals have been floated by different entities. It is observed that the depth of the bed varies from one place to another to a greater extent. The maximum depth can be up to 8 km and the average depth is 3.3 km.
The two alternatives available for construction are a bridge above water level or a tunnel below ground level. However, since the depth is 8 km, it is impossible to construct concrete columns of such height for a bridge.
And also, the pressure below 8 km from the sea surface is nearly about 500 times atmospheric, so one cannot survive in such a high-pressure zone.
Therefore, the optimum depth of the floating tunnel is 30 m from sea level, where there is no issue due to high pressure. This will be sufficient for any big ship to pass over the SFT without obstruction.
Basic Principles of Submerged Floating Tunnel
The submerged floating tunnel is a buoyant structure that moves in the water. The relation between buoyancy and self-weight is very important since it controls the static behavior of the tunnel and, to some extent, the response to dynamic forces.
The submerged floating tunnel can be floated in two ways:
1. Positive Buoyancy
In this type, the submerged floating tunnel is fixed by anchoring either tension legs to the bottom or pontoons on the surface. Here, the submerged floating tunnel is mainly 30 m below the water surface.
2. Negative Buoyancy
The foundations would be piers or columns to the sea bed or lake bed in this type. This method is limited to 100 meters of water depth.
The submerged floating tunnel is subjected to all the environmental actions typical in the water environment: current, wave, the vibration of water level, corrosion, earthquake, ice, and marine growth.
Therefore, it should be designed to withstand all actions, operational and accidental loads, with enough strength and stiffness. Transverse stiffness is provided by bottom anchoring.
Features of Submerged Floating Tunnel
1. Clear Sight
Crossing waterways, whether being from the mainland to islands in the sea or crossing an inland lake, perhaps, in many cases, invite objections both from environmentalists and from the public in general.
But, on the other hand, lakes of special beauty or historical value should be preserved for the future; crossing such areas and lakes by SFT may make this possible without affecting the natural surroundings.
2. Fixed Length
The submerged floating tunnel structure is as long as the distance between the two shores. The submerged floating tunnel may even be connected directly to the tunnels and then be completely out of sight for any desired length.
3. Very Low Gradient
Crossings with bridges or undersea tunnels will frequently mean longer structures with consequently higher costs, which results in the higher price per meter for an alternative submerged floating tunnel. In addition, a submerged floating tunnel crossing may have a very gentle gradient or be nearly horizontal, providing considerable savings in energy for the traffic.
4. Access to Underground Parking-Service Space at Ends
It is possible to arrange service areas or parking places underground and access the surface by lifts directly into the town or city. These possibilities may be one of the biggest advantages in the future, in fact, for all types of tunnels.
5. Surface just above the shoreline
The submerged floating tunnel can be placed at any depth below the surface, and arrangements can be made in a way that it surfaces very near the shoreline. This arrangement shall be advantageous for connections to new or existing road systems and allow planners to establish links flexibly.
6. Constructed away from densely populated areas
The major problem many cities are facing these days is piling up of traffic, and the conversion of two-way streets to one-way streets. Therefore, the construction of a submerged floating tunnel is a viable option that can be carried out for densely populated cities to decongest the traffic.
After the tunnel sections are finished, they are towed to the actual site and installed at the required depth.
This would complete the whole operation in months instead of years and ensure minimum disturbances to the local area.
7. Easy removal at the end of lifespan
All structures will have to be replaced or removed sooner or later once the lifespan of the structure is completed. It becomes important to consider the removal/dismantling operations at the planning and design stage. In addition, removing, recycling, or reusing materials or parts of the structures will become increasingly necessary for environmental and economic reasons.
In most cases, SFT is a floating structure as a whole and may therefore be towed away to someplace where parts of the SFT may be reused.
8. Possibilities of recycling/reusing submerged floating tunnel
Depending on their size and conditions, the tunnel sections may be used for many purposes. One apparent possibility is for various types of storage facilities, whether on the dry land or in the sea, a section of the tunnel having a diameter of 10 meters cut to a length of 10 to 12 meters, would not pose any difficulty to get up on dry land.
Challenges faced in Adoption of Submerged Floating Tunnel
1. Cost
As a lot of material and machinery are involved in the project, the estimated cost is nearly twice that of a normal tunnel.
2. Fire
It becomes difficult to rescue people if a fire breaks out in the train when in the tunnel.
3. Collision
In case of two trains or vehicles collision, it isn't easy to rescue the passengers.
4. Discomfort to train passengers
When a train passes through a tunnel, pressure waves are generated which propagate along the tunnel approximately at sonic speed. These pressure variations may pass into the interior of the trains, unless they are pressure sealed, and may cause discomfort to train passengers.
FAQs
A submerged floating tunnel (SFT), also called Archimedes bridge or suspended tunnel, is a tunnel that floats in the water supported by its buoyancy (specifically, by employing the hydrostatic thrust, or Archimedes' principle).
The submerged floating tunnel is a buoyant structure that moves in the water. The relation between buoyancy and self-weight is very important since it controls the static behavior of the tunnel and, to some extent, the response to dynamic forces.
1. As a lot of material and machinery are involved in the project, the estimated cost is near twice the normal tunnel.
2. It becomes difficult to rescue people if a fire breaks out in the train when in the tunnel and they face problems due to the smoke.
3. In case of two trains or vehicles collision, it isn't easy to rescue the people.
4. When a train passes through a tunnel, pressure waves are generated which propagate along the tunnel approximately at sonic speed. These pressure variations may pass into the interior of the trains, unless they are pressure sealed, and may cause discomfort to train passengers.
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