08-16-2017, 10:46 PM
sub merged floating tunnels
Abstract
Several crossings with a variety of different conditions under which a Submerged Floating Tunnel, SFT or Archimedes Bridge, may be used. However, swell, vortex shedding and slowly varying internal waves due to layers of different salinity presented a hazard of significant dynamic oscillations. In addition to the challenge of these various conditions some common accidental situations have to be solved for all applications including fire, sinking ships, falling anchors as well as sudden massive water ingress into the tube. Combining with the characteristics of submerged floating tunnel (SFT) and surrounding environment, it is of great theoretical and practical significance to develop research in the areas of potential risk and impact factors, risk index system, risk level of SFT. Risk management workflow of SFT was given. Then we focused on discussing the potential risks of SFT in investment, design, and environmental condition during planning and feasibility study stage.
Some measures and suggestions in risk control strategy were given. Based on the design technology of immersed tunnel, bridge and tunnel engineering, combining the current relevant design codes segment is presented according to safety, applicability, economy, fine appearance and environmental protection. The selection of tube cross section type, structural analysis, design load, waterproofing and resistant corrosion, tube joint design and tunnel ventilation of submerged floating tunnel etc are described and explored by comprehensively considering the design load, flow resistance performance, durability and other factors of submerged floating tunnel.
1. INTRODUCTION
1.1 HISTORY
The first underwater tunnel was build over four thousand years ago, but floating tunnels are much more recent. Certainly an engineer and builder of railways, S. Pr ault , proposed but did not build an SFT across the Bosphorus in 1860, an elegant underwater railway viaduct with spans of about 150 m founded on piers, located some 20 m below the surface. Per Hall proposed a deeper SFT for the Bosphorus in 1976, but by 1977 his proposal had become a buried immersed tunnel for environmental reasons (fish habitat). An immersed tunnel is now in place beneath the Bosphorus awaiting the last of the TBMs to reach it. Going back now to 1882, Edward Reed proposed a submerged railway tunnel across the English Channel supported on caissons, but Parliament in England rejected it for fear of invasion. It was patented and since then, many other patents have been taken out for SFT, including some in the UK, USA, Norway, Sweden and Italy. Once the first immersed tunnel had been successfully built in 1893, the way was open also for constructing SFT initially at least those that would be pier supported. Since 1923 [3], the potential of an SFT has been recognized in Norway as a way to create a practical castal highway across fjords that would otherwise be too deep even for bored tunnels to make sense; some of the existing bored tunnel connections even with 10% grades are very, very long. This need for shorter shallower tunnels for a number of fjord crossings has led to detailed investigations and field tests that still continue today. The most wellknown crossing evaluated in some detail in Norway is for Hogsfjord , but the SFT was abandoned for local political reasons. Private investors have examined a number of other locations. Another serious contender is the Sula-Hareld crossing. The first of a series of Strait Crossing Symposia in Norway began in 1986 (the fifth was in 2009) in which SFT have played an increasingly greater part.
1.2 GENERAL
Tunnels in water are by no means new in civil engineering. Since about 1900, more than 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.
1.3 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 cannot 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.
1.4 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.
1.5 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.A submerged floating tunnel (SFT), also called a suspended tunnel or Archimedes bridge, is a tunnel that floats in water, supported by its buoyancy (specifically, by employing the hydrostatic thrust, or Archimedes' principle).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.