Submerged Floating Tunnel

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.


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