17-01-2013, 02:42 PM
SUBMERGED FLOATING TUNNELS
SUBMERGED FLOATING TUNNELS.docx (Size: 250.11 KB / Downloads: 411)
INTRODUCTION
Tunnels in water are by no means new in civil engineering. Since about 1900, more then 100 immersed tunnels have been constructed. Bridges are the most common structures used for crossing water bodies. In some cases immersed tunnels also used which run beneath the sea or river bed. But when the bed is too rocky ,too deep or too undulating submerged floating tunnels are used .
The Submerged Floating Tunnel concept was first conceived at the beginning of the century, but no actual project was undertaken until recently. As the needs of society for regional growth and the protection of the environment have assumed increased importance, in this wider context the submerged floating tunnel offers new opportunities. 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 .It supported on columns or held in place by tethers attached to the sea floor or by pontoons floating on the surface. The Submerged floating tunnel utilizes lakes and waterways to carry traffic under water and on to the other side, where it can be conveniently linked to the rural network or to the underground infrastructure of modern cities.
REASON FOR CHOOSING FLOATING TUNNEL
Floating tunnel is the totally new concept and never used before even for very small length. 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 or 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 can not survive in such a high pressure zone. So the immersed tunnels also cannot be used. Therefore, floating tunnel is finalised 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.
.BASIC PRINCIPLE OF SFT
SFT is a buoyant structure which moves in water. The relation between buoyancy and self weight is very important, since it controls the static behaviour f the tunnel and to some extend, also the response to dynamic forces. Minimum internal dimension often result in a near optimum design. There are two ways in which SFT can be floated. That is positive and negative buoyancy.
Positive buoyancy : In this the SFT is fixed in position by anchoring either by means of tension legs to the bottom or by means of pontoons on the surface. Here SFT is mainly 30 metres below the water surface.
Negative buoyancy: Here the foundations would be piers or columns to the sea or lake. This method is limited to 100 meters water depth
SFT is subjected to all environmental actions typical in the water environment: wave ,current , vibration of water level, earthquake, corrosion, ice and marine growth. It should be designed to with stand all actions, operational and accidental loads, with enough strength and stiffness. Transverse stiffness is provided by bottom anchoring.
OPTIMAL SHAPE OF SFT
The shape of the SFT in Fig. 1 has been chosen for the following reasons: When the vertical curvature is concentrated in the middle of the SFT, it is easier to shorten the concrete tube during installation, variations in the buoyancy in the middle of the tunnel introduce little bending in the tunnel. Similarly an unusual amount of water in the middle of the tunnel gives little bending and axial force.
Optimal shape of an SFT
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:
• Tube
• Anchoring
• Shore connections
Tube : It should accommodate the traffic lanes and the equipments. 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 kilometre.
Anchoring:
There are basically fours types of anchoring:
• SFT with pontoons
• SFT supported on columns
• SFT with tethers to the bottom
• SFT unanchored
SFT with pontoons: It is independent of water depth, the system is sensitive to wind, waves, currents and possible ships collision. Design should be such that if one pontoon is lost ,then also the structure will survive.
SFT supported on columns: It is an “underwater bridge” with foundations on the bottom, in principle the columns are in compression but they may also be a tension type alternative. Water depth will play an important role in this case and a few hundred meters depth is considered a limit at the present time. However, much deeper foundations are at present under investigation.