Design and Construction of Submerged Floating Tunnel

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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).

Structural Components of Submerged Floating Tunnel

A submerged floating tunnel (SFT) consists of various structural components that provide stiffness and strength against the various forces acting under the water surface.

The three main structural components are:

1. Tube

It should accommodate all the traffic lanes and the equipment and is constructed from concrete or steel. The external shape of the tube can be elliptical, circular, or polygonal. As the tube is submerged in water, protection from corrosion is the main issue. The tube is composed of length elements varying from 100 meters to half a kilometer.

2. Anchoring

There are four types of anchoring:

1. SFT with Pontoons

It is independent of the depth of water; the system is sensitive to wind, waves, currents, and possible collision of ships. Therefore, the design should be such that the structure must survive if one pontoon is lost.

2. SFT supported on Columns

It's an "underwater bridge" with foundations on the bottom; in principle, the columns are in compression but may also be a tension-type alternative. Water depth will play a vital role in this case, and a few hundred meters depth is considered the limit. However, much deeper foundations are currently under inspection.

3. SFT with Tethers to the Bottom

It is predicated on the assumption that tethers will be in tension in all future conditions; no sagging in these tethers will be permitted in future load instances. The present practical depth for this type of crossing can be several hundred meters, whether the tethers are vertical, inclined, or both vertical and inclined.

4. SFT Unanchored

The unanchored submerged floating tunnel has no anchoring except at landfalls and is then independent of depth. There is a limit of 100 or 200 m for the length of the submerged unanchored tunnel for light traffic. However, research is underway for determining the ideal length of the submerged floating tunnels.

3. Shore Connection 

The connections of the SFT tube to the shore require appropriate intersection elements to couple the flexible water tube with a much more rigid tunnel bored in the ground. This joint should restrain tube movements without any unsustainable increase in stress. 

The joints must be watertight to prevent entry into the water. Additional care in shore connections is required, especially in seismic areas, due to the risk of submarine landslides.

Optimal Shape of Submerged Floating Tunnel

The shape of the submerged floating tunnel is chosen because it is easier to shrink the concrete tube during installation when the vertical curvature is concentrated in the center of the submerged floating tunnel. The variations in buoyancy in the middle of the tunnel introduce a little bending in the tunnel. Similarly, axial force and bending arise in the tunnel due to an unexpected amount of water in the middle of the tunnel.

Design Principle of Submerged Floating Tunnel

The submerged floating tunnel tube provides buoyancy for carrying different dead and live loads and to accommodate the traffic.

The design load, flow resistance performance, buoyancy-to-weight ratio, durable performance, and other features are considered comprehensively in the tube-design process. 

The optimal design of the SFT should utilize the space to assure the traffic headroom, and meet the demand for ventilation and escape plans. The different design parameters such as cost, technology, and environmental protection shall also be considered during the design stage. The design shall comply with the requirements of safety applicability, economical rationality, advanced technology, and reliable quality.

The principles of submerged floating tube design are as follows:

1. The tube must meet the demand of strength, stiffness, and stability during the construction and operation stages.
2. The ratio of buoyancy to weight is less than 1.0; related research shows that the ratio should be between 0.5 to 0.8.
3. The variation of surface curvature must be gentle to resist the hydrodynamic and to meet the standards for the classification of seismic protection of buildings.

Structural Design of Submerged Floating Tunnel

1. The submerged floating tunnel tube keeps equilibrium under the action of buoyancy, and cable tension bears the wave-current load, vehicle load, temperature load, and so on. 
2. However, in the system transformation during prefabrication, floating, installation and operation, the stress in the tube is complex, so the tube must be designed based on the longitudinal and transverse analysis under the working conditions. 
3. The loads in the SFT tube are divided into permanent, variable, and accidental loads. The permanent load includes buoyancy, structure weight, concrete shrinkage, hydrostatic pressure, etc. 
4. The variable load includes water head load, vehicle load, wave-current load, temperate load, etc. The accidental load has seismic, sunken shipload, leakage, blast load, etc.
5. The submerged floating tunnel tube is designed under serviceability limit state and ultimate limit state just as traditional hydraulic structure design.
6. However, the displacement and stress should be analyzed and checked under the progressive fatigue limit state and damage limit state based on structural reliability theory.

Tube Joint Design

The joint design of the submerged floating tunnel tube must conform to the following four principles:

1. There is no seepage in the construction and operation stage. The tube shall be watertight, and durable.
2. Transferring construction load effectively in the construction stage and convenient construction.
3. Transferring stress and deformation effectively in the construction stage and fine seismic fulfillment.
4. There are two types of SFT tube joints based on deformation and stiffness: flexible joint and rigid joint.

Ventilation Design of Submerged Floating Tunnel

Tube ventilation design is an important part of submerged floating tunnel design. The quality of the ventilation scheme and operation affects directly relates to engineering cost, operation environment, disaster-relieving function, and operation benefit.

Tube ventilation aims to guarantee allowable concentration of harmful gas represented by carbon monoxide, provides suitable visibility for people and vehicles in the tunnel and a healthy environment, and controls the pervasion of smog and heat for evacuation and extinguishment when a fire occurs.

Tube ventilation should meet the following requirements:

1. The design wind speed of a one-way traffic tunnel should not be more than 10 m/s; the design wind speed of a two-way traffic tunnel should not be more than 8 m/s.
2. The noise produced by exhaust emissions and ventilation fans does not exceed the stipulated limits.
3. Ventilation type must be stable when the transportation condition changes or fire occurs. The downstream direction of operation ventilation is stable.

Construction of Submerged Floating Tunnel

The construction of submerged floating tunnels (SFT) is similar to that of offshore structures, floating bridges, and immersed tunnels.

One way is to build the tunnel tube in sections in a dry dock, then float these to the construction site and sink these sections into place while sealed. Unfortunately, when the tunnel sections are fixed, the seals are ruptured. 

Another possibility is to build the tunnel sections unsealed and pump the water out after welding them together. The ballast used here is calculated so that the structure has the required hydrostatic equilibrium (the tunnel is nearly the same as the overall density of water). 

In contrast, more immersed tube tunnels are ballasted to weigh them down to the sea bed. Finally, a submerged floating tunnel must be anchored to the water surface or to the ground to keep it firmly in place depending on which side of the tunnel the equilibrium point is located.

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