The submerged floating tunnel is an innovative concept for crossing waterways, utilizing the law of buoyancy to support the structure at a moderate and convenient depth. The Submerged floating Tunnel is a tube like structure made of Steel and Concrete utilizing the law of buoyancy .Its supported on columns or held in place by tethers attached to the sea floor or by pontoons floating on the surface. The tube is placed underwater, deep enough to avoid water traffic and weather, but not so deep that high water pressure needs to be dealt with usually 20–50 m (60–150 ft) is sufficient. Cables either anchored to the Earth or to pontoons at the surface prevent it from floating to the surface or submerging, respectively.
Reasons for choosing submerged floating tunnels:
It can be observed that the depth of bed varies from place to place on a great extent. The maximum depth is up to 8 km. also at certain sections. The average depth is 3.3 km. The two alternatives are available for constructions are:
• Bridge above water level
• Tunnel below ground level
Since the depth is up to 8 km it is impossible to construct concrete columns of such height for a bridge. And also the pressure below 8km from sea surface is nearly about 500 times than atmospheric pressure so one cannot survive in such a high pressure zone. So the immersed tunnels also cannot be used. Therefore, floating tunnel is finalized which is at a depth 30m from the sea level, where there is no problem of high pressure. This is sufficient for any big ship to pass over it without any obstruction.
Structural components of SFT: Submerged floating tunnel consists of many structural components. These components should provide strength and stiffness against the various forces acting under the water surface. The three basic structural components are:
(1) Tube (2) Anchoring (3) Shore connections.
Tube: It should accommodate the traffic lanes and the equipment. External shape can be circular, elliptical or polygonal. It may be constructed of steel or concrete. Corrosion protection is the main issue. Tube is composed of elements of length varying from one hundred meters to half a kilometer.
Anchoring: There are basically three types of anchoring:
(1) SFT with pontoons
(2) SFT supported on columns
(3) SFT with tethers to the bottom. Shown in fig.1, 2, 3
Shore Connections: The connections of the tube to the shore require appropriate interface elements to couple the flexible water tube with the much more rigid tunnel bored in the ground. This joint should be able to restrain tube movements, without any unsustainable increase in stresses. On the other hand, the joints must be water tight to be able to prevent entry of water. Additional care in shore connections is required, especially in seismic areas, due to the risk of submarine landslides.
Competitive features of SFT:
• Invisible: Crossing waterways, whether being from main land to islands in the sea or maybe more important crossing an inland lake, perhaps the one we are at now will in many cases meet protests both from tourist interests and also from the public in general.
Lakes of special beauty or perhaps historical value should be preserved for the future, the crossing of such areas and lakes with SFT may make this possible.
Further, it helps maintain aesthetic importance of the area.
• Very low gradient: An SFT crossing may have a very gentle gradient or being nearly horizontal giving considerable savings in energy used by traffic.
• Easy removal at end of life: All structures will have to be removed or replaced sooner or later and as the cost of structures increases it is important to prepare for these operations already at the planning and design stage.
SFT is in most cases, a floating structure as a whole and may therefore be towed away to some place where parts of the SFT may be reused.