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.
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, SFT, also named as Archimedes Bridge, is a concept going back at least 150 years and probably even further back. Historic records show that a rather complete understanding of this idea was brought forward by Sir James Reed, UK in 1886 and later in 1924 by Trygve Olsen Dale, Norway.
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 is 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.
2. SFT CONCEPT
The SFT is a new transportation concept for crossing straits, lakes or waterways in general. It is basically a tube like structure floating at some depth in the water, where the tube is large enough to accommodate road and/or rail traffic. As with any structure floating in water, it must be moored or fixed against excessive movements.
Fig. 2.1 A simple cross section of SFT
Fig. 2.2 A typical cross section of SFT
2.1 BASIC PRINCIPLE
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 of the tunnel and to some extent, 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.
ØPositive 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.
ØNegative 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. In principle, SFT can be considered for all waterway crossings, in practice they are of major interest especially where gentle gradients and low environment impact are important.
2.2 STRUCTURAL COMPONENTS
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:
· Shore connections
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.
There are basically four 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
Fig. 2.3 SFT with pontoons
· 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.
Fig. 2.4 SFT with columns
· SFT with tethers to the bottom:
It is based on tethers being in tension in all future situations, no slack in these tethers may be accepted in any future load cases. The present practical depths for this type of crossing may be several hundred meters, whether the tethers are vertical or a combination of vertical and inclined.
Fig. 2.5 SFT with tethers
· SFT unanchored:
It is interesting as it has no anchoring at all except at landfalls and is then independent of depth. There is obviously a limit to the length but only further development will answer this. Perhaps an alternative for light traffic should be designed, possibly a 100 or 200 meter long.
Fig. 2.6 SFT unsupported
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
6 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.
2.3 PROPOSED PROJECTS
In recent years, as the development of offshore engineering makes available appropriate technology for actual SFT construction, interest in SFT has been growing, especially in Norway, Italy and Japan and a number of projects have been developed, up to quite an advanced state.
Probably the most advanced project is, presently that of the Hogsfjord crossing, carried out by the Norwegian Public Road Administration. The crossing is 1400 metres long, with 150 metres maximum water depth. The tunnel will be placed about 25 metres below the water surface and will be circular in shape with 9.5 m inner diameter, inorder to accommodate a two lane road.
A more difficult and demanding project has been developed for Messina Straits where severe environmental conditions occur, along with a high seismic risk. The crossing is about 3000m long with 350 m maximum depth. Due to high traffic loading, the connection requires a four lane road and two railway lines. The project is still far from the construction stage, to some extent, due to the very challenging problems arising from the extremely adverse environment.
Table 1: Proposed projects
3. DESIGN, CONSTRUCTION AND OPERATION
This chapter contains a brief overview of the main topics related to SFT design, construction, operation and maintenance.
3.1 SFT DESIGN OVERVIEW
The procedures for design of a SFT go through the following main steps:
· Site characterization and definition of operational targets
· Definition of design criteria and loading conditions
· Choice of overall dimension
· Static and dynamic analysis
· Structural safety analysis
· Environmental impact studies
· Detailed design
· Specification of construction and installation procedures
· Specification of operation procedures and maintenance
3.2 DESIGN ISSUES
For design of an SFT the following basic considerations should be taken into account:
· The cross-section must give sufficient space for traffic, evacuation, ventilation, ballast, inspection, maintenance and repair works.
· The alignment must be such that there is no interference with ship traffic passing above the tunnel.
· The joints should have no less strength or integrity than the tube between the joints
· The structure must have a ductile behaviour in the potential failure modes
· The anchoring system should be redundant
· The tunnel must not be unduly susceptible to local damage
· The structural details must be simple & designed to avoid undue stress concentrations
· The tunnel must behave in a satisfactory manner with regard to deformations, settlements and vibration
· The tunnel must have a satisfactory safety against fatigue
· The tunnel should be designed such that the water inflow rate is so limited that people have time to safe evacuation in case of massive water ingress
· Tether lengths must be adjustable to compensate for e.g. possible settlements
· It must be possible to repair or replace parts of the structure that are considered to have a shorter service life than the tunnel tube itself. Such parts can be tethers and other anchoring systems, bearings and moveable joints.
3.3 DESIGN CONSIDERATIONS
3.3.1 Site Characterization and Operational Targets
Since SFT design is strongly dependent on location, a careful characterization of the site and detailed definitions of the operational targets are required, prior to any design development. Operational targets are mainly related to the transportation needs to be satisfied by the connection. These are expressed in terms of traffic volume, and on this basis transportation design (rail, road, number of lanes, and type of traffic) of the connection is performed. Site characterization involves:
· Geographical characteristics of the area
· Geological and geotechnical investigation
· Characterization of the water environment.
· Characterisation of the seismic hazard.
· Investigation of all relevant features (ship traffic, marine operation)
· Data collection for environmental impact assessment.
· Detection of all other constraints to SFT design (land use, water uses).
3.3.2 Design Methods
Like every other structure, SFT must be designed on the basis of expected and possible combination of loading cases. The allowable design methods and general criteria are mainly determined by national codes. Today the design methods are often related to those used in the offshore industry, and are commonly based on the semi probabilistic limit state approach, using partial safety coefficients on both loads and strength of materials.
The semi-probabilistic approach divides the design into the following design limit states:
SLS (Service Limit State):The SLS conditions are set to ensure that the structure meets practical criteria with regards to deflection , crack, widths , factor of safety, acceleration etc.
ULS (Ultimate Limit State): The ULS conditions are set to confirm that the structure has the necessary margin of strength to survive factored load and load combinations , with the factor being set to provide an acceptable risk of failure. The factor must be sufficient to ensure that the structure is capable of continuing to operate satisfactorily after an unfactored event. The accepted risk level differs among countries.
PLS (Progressive Collapse Limit State): the PLS conditions are designed to preserve human lives in the event of certain loads or load combinations at very low probability of occurrence. In this case even if the structure may be severly damaged , loss of lives is still not acceptable. These conditions are often defined at a probability level of approximately 10^-4 per annum.
FLS (Fatigue Limit State): The FLS is required to account for the fact that some materials lose strength due to repeated loading . By computing the accumulated damage in the material and consequently checking the computed life of the structure against the operational life, the sensitivity of certain components of structure can be established. The safety factor of 3 to 10 between computed life and required operational life is often adopted depending on the consequences of failure and the opportunities for repair of the components.
3.3.3 Loading Conditions
Ø Permanent Loads
The permanent loads acting on a SFT are the weight of the various structural and non-structural components, the water buoyancy and the hydrostatic pressure.
Ø Functional Loads
Functional loads are related to the development of the functions for which the SFT is designed for, therefore these loads are associated with the passage of cars, trucks, trains and/or pedestrians, according to the destination of use of the SFT.
Ø Hydrodynamic Loads
Hydrodynamic actions due to the water-structure interaction in presence of waves and currents often represent the most important and onerous environmental actions for a SFT.
Currents in waterways can be of the following types:
· Wind generated currents: water motion is originated by the energy transferred to the water by the wind blowing over the water surface.
· Tidal currents: horizontal water motion resulting from the rise and fall of the water level due to tides (a vertical motion).
· Water waves differ from currents because they are characterized by an oscillating motion of the water particles and can be of two types:
· Wind generated waves: surface waves occurring on the free surface of waterways, due to the wind blowing over a vast enough stretch of fluid surface.
· Internal waves: water particles are kept in motion by the force of gravity acting on small differences in density. A density difference can exist between two fluids or between different parts of the same fluid because of a difference in temperature, salinity, or concentration of suspended sediment.
Strong ground motion occurring to seismic events propagate in the structure by means of the tunnel shore connections and of the SFT anchoring system. However some specific issues have to be considered in the design phase, such as, for instance, the configuration of the shore connections or the behaviour of shorter anchoring elements located close to the shores. More generally, it is necessary to assure that every structural component safely withstand extreme seismic events and that functional performances are met in case of more frequent earthquakes. One safe way of dealing with this challenge is to design the tube and its supports such that the lower fundamental Eigen periods are safely below the periods of these forces. This however may have a big cost penalty since it generally would require a relatively large number of anchoring points, either through tethers or as pontoons. If it had been possible to design reliable artificial damping systems that could limit the resonance phenomena to tolerable levels, the number of such anchoring points and then also the cost could probably be significantly reduced.
Loads Accidental actions mainly include:
· Collisions due to dropping objects, sinking ships or impacts with submarines.
3.3.4 Load Applications
The magnitudes of applied loads and the associated acceptable structural behavior are defined in terms of their probability of occurrence. For example if the structure has a design period of 2000 years, the following levels where adopted:
1. Return period equal to 50 years: Serviceability behavior of both the main structure and the secondary components without any damage or need for inspection after the event.
2. Return period equal to 400 years: Serviceability behavior of the main structure and local damage (plastic behavior) for the secondary components. Limited damage, if any must be reparable without any interruption in the use of tunnel.
3. Return period equal to 2000 years: This was defined as plastic behavior with the complete exploitation of ductility resources of all the structural components. This would be analysed today as PLS. Damage, but not collapse of the main structure is accepted. Damage and collapse are accepted in the secondary component.
The construction of an SFT includes two basic concepts namely, Construction in elements and Incremental launching.
3.4.1 Construction In Elements
The construction in element appears to be more usual in the ongoing projects. The construction procedures developed up to now, mainly refer to SFT placed at sea. SFT in internal water bodies, local conditions, as well as problems are likely to require installation with smaller vessels. SFT elements are constructed in a dock and then towed to the SFT site. Basically there is no constraint to the element length, especially if permanent prestressing is used, the element length is mainly controlled by the features of the construction facility.
At the site the elements are ballasted and lowered to the desired depth, where they are coupled with the elements already in place, by means of specially designed joints. At each joint location, a set of tethers is pre-installed and coupled in a horse shoe shaped support. The element is lowered under the support while temporarily pulling the support aside.
After the element has been fitted to the predetermined tether support system, it is de-ballasted, causing the load to transfer from installation barge to the tether system. During this process, the length of the tethers is adjusted at the support shoe to prevent unacceptable deflection of both tethers, the position of new element and the previous elements. The adjustments can be made by remote control or by diver ROV (Remotely Operated Vehicle) assistance.
Final connection between the tunnel and the tethers depends on the location of the connection, i.e., whether they are external orwithin special chambers added to the tunnel. It also depends on whether the preference is to connect the tethers first to the anchor points on the bed or to the elements before it is lowered.
Fig.3.1 Type of installation at site
3.4.2 Incremental Construction And Launching
As a construction alternative, incremental launching requires that the construction takes place at one of the abutments; this implies that conditions for a large construction facility exist at the site. The tube is constructed in consecutive sections on an inclined skidway in the abutment. After the construction of one section is completed, the tube is moved forward into the water, by one section length through the gate in the abutment. A temporary support system must be provide to keep under control the part of the tube already in water, until the opposite shore is reached and final supports can be installed.
Fig.3.2 Incremental Launching
A combination of these two outline construction methods has also been considered. In this case elements are constructed in a dock and then towed to a specially push-out facility, located at one of the abutments.
3.5 OPERATION AND MAINTANENCE
Operation conditions for SFT will not be known until the first structure of this type has been constructed. In fact all well developed projects include a very extensive monitoring of the tunnel behavior, in order to control SFT performance and to acquire valuable experience for future design. Parameters to be monitored include:
· Environmental parameters: current, wave, temperature, water density, etc;
· Structural parameters: stress, strain, static and dynamic response
· Material behavior: cracking, corrosion marine growth
· Parameters related to environmental impact (inside and outside the tunnel)
However a set of crucial issues, besides monitoring, related to operation and maintenance includes:
· Traffic control
· Corrosion protection
· Surveys and inspection
Traffic control is aimed at ensuring that the crossing SFT fulfils the design assumptions, both in quality and quantity. While control of traffic flows, in order to avoid any traffic congestion in the SFT, can be easily achieved, with the same techniques applied for land based tunnel, more skill is required to prevent access to the SFT for such type of goods (explosives), that must be avoided for safety reasons.
One of the most uncertain issues to the long term behavior of SFT, compared to other civil engineering structures, is the performance of corrosion protection systems and related corrosion control inspections. Experience from ship and offshore structures can be applied, as starting point to define standards and procedures for surveys and inspection. Structural behavior in damaged condition is a crucial for SFT repair. SFT design has to consider that damage can occur, during its lifetime, both to the tube and to the anchoring systems.
Since the concept of an SFT is still innovative, acceptable risk levels may initially need to be higher than for comparable projects because new safety issues are raised. Safety requirements should therefore be quantified to determine acceptable risk levels compared with the expected benefits during the working life of an SFT.
SFT safety criteria may be viewed in two ways:
· As normal rail or road tunnel, emphasizing structural safety.
· As the comprehensive transportation system, emphasizing operational safety.
3.6.1 Critical Structural Components
In order to make a safety assessment those components of a tunnel that are critical to life safety or the environment in the event of collapse, need to be identified, i.e.:
· Tendons or tether system.
· Terminal structures.
· Connection system between tunnel modules.
· Anchorages and foundations.
· Outer and inner shells.
Critical design conditions should include:
a) Extreme design environmental conditions.
b) Operational accidental conditions.
c) Damaged conditions due to deterioration or collapse of some of the above mentioned primary components.
3.6.2 Ship Collision
All of the feasibility studies for SFTs must focus attention on the accidental loading caused by the collision of ship or a submarine. The safety of an SFT is based on avoiding collision with vessels large enough to damage the structure seriously, as has happened many times to various bridges. Collision of surface vessels can be easily avoided as the SFT can be positioned at virtually any deapth beneath the water surface.
In the cases where there is heavy surface traffic the probability of a sinking ship at that particular location, and subsequent consequences must be considered. The energy associated with the impact of a sinking object against the structure must be absorbed by local or global deformation. The magnitude of absorption will depends on the type of ship.
Another form of accidental loading, which is probably more frequent but less sensitive is the impact of fishing equipment. This type of impact can largely be avoided by adherence of marine regulations.
A traffic regulation system has to be provided for submarine traffic. If necessary measures must be taken to ensure that both the tunnel and its support system can be monitored by the submarine navigational equipment. A warning system may be used to ensure the safety of the traffic in the SFT itself.
4. COMPETITIVE FEATURES OF SFT AND BENEFITS
Crossing waterways, whether being from main land to islands in the sea or maybe more important crossing and 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. An illustration of this may be seen in fig.5.
· Length only from shore to shore
Fig.5 also illustrates that the actual SFT structure is only as long as the distance between the shores. If desired the SFT may be connected directly to tunnels and then be completely out of sight for any desired distance.
Fig. 4.1 SFT crossing may be invisible
· An alternative to ferry
If a crossing is very deep and wide, many of the traditional types of bridges, immersed or undersea tunnels may be both technically and economically prohibitive and if a fixed link is wanted an SFT may be the only alternative. Another feature with the SFT is the lower energy use for crossing, driving cars on a fixed link will normally use considerably less energy than ferry operations.
· Very Low Gradient
An SFT crossing may have a very gentle gradient or being nearly horizontal giving considerable savings in energy used by traffic. Crossings with undersea tunnels or bridges will frequently mean longer structures with consequently higher costs and this may offset the higher cost per meter for an alternative SFT.
· Access to underground service parking space at end
As SFT may continue in tunnels having crossed the waterway, it is possible to arrange parking places or service areas under ground and provide access to the surface by lifts directly into cities or recreational areas as the case may be. These possibilities may be one of big advantage in future, in fact for all types of tunnel.
Fig. 4.2 Parking and service areas
· Constructed away from densely populated areas
One very interesting feature with SFT is that the actual construction may be done away from the densely or highly populated areas, a feature also for immersed tunnel construction. After the sections of the tunnel are finished they may be towed to the actual site and there joined together and installed at the desired depth. In some instances the whole length of the SFT may be assembled at the construction site and the complete structure towed to the actual site and installed. This would ensure minimum disturbances to the local area and perhaps the whole operation may only take months instead of years.
· Easy removal at end of life
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.Some possibilities of reuse or recycling SFT. Sections of a tunnel may be used for many purposes, depending on its size and condition. One obvious possibility is for various types of storage facilities, whether in the sea or on dry land, a section of tunnel
· Minimize the traffic congestion
· No impact on the landscape and no interference to sea surface activities.
· Better control of air pollution by providing proper ventilation system.
· SFT cost per unit length is approximately constant, while, for bridges, it significantly increases with the span length.
5. AREAS FOR IMPROVING SFT COMPETITIVENESS
In an effort to establish the present status of SFT and suggest areas for strengthening the competitiveness a short discussion of some possibilities are presented.
· The basic SFT design should be presented in a simple and understandable way to everybody; the importance of clear and good drawings cannot be overemphasized, clear 3-D presentations may be more important than pages of text.
· There are quite a number of structures similar to SFT around the world; especially immersed tunnels have many similarities with SFT. A large part of immersed tunnel technology is used for SFT construction and one of the important centers for immersed and floating tunnels may be found in the International Tunneling and Underground Space Association, ITA. Working Group 11 is dealing with Immersed and Floating tunnels and this is a forum for people interested in SFT and new members are very welcome at the yearly meetings. The structures similar to SFT should be published in SFT literature and reports. This would inform the engineering community about the present experience with these structures and also be a valuable reference of the behaviour of these structures in practice.
· Some of the more complex technical elements and procedures should be explained, for instance the construction methods of the tunnel, towing procedures, installation, anchoring methods and so on. This would familiarize the public with this structure and make it more acceptable to be used for great benefit to both local and regional areas.
· The structure itself should be studied for technical simplification in all areas. The simpler and clearer the methods and procedures, the greater the probability for a good end result.
· The structure should be robust and have some reserve capacity in all important areas; refinery should come at a later stage when experience is gathered.
6. CHALLENGES TO BE FACED
Because submerged in the water, SFT as a new type of structure, is confronting with different public safety risks during its construction and operation compared with common bridges. SFT is also facing many challenges because there is still no mature specification or criterion for the relevant design and construction technology of SFT. Therefore, systematical risk analysis and assessment are needed according to local environment and structural characters.
The natural hazard risks of SFT are as follows:
· Typhoon: The water waves caused by the typhoon make just tiny effects on SFT because SFT is usually placed at 10m or more than 10m under water surface, and the wave force is decreasing exponentially with depth. Although typhoon action on SFT structure is tiny, we should also consider secondary disasters caused by typhoon, such as landslide and slope failure.
· Earthquake: The main effect of earthquake to SFT includes forced
vibrations caused by subsoil’s vertical and horizontal shake. These may lead to local damages in SFT such as foundation damage and anchor cable failure, which can threaten the whole SFT structure in its underwater environment. So earthquake will bring about big losses once it happens.
· Landslide: Landslide would have great impact on the connection between tube and land.
The operation risks for SFT are following:
· Fire: Just like tunnels, SFT also has many similar problems such as long distance, humidity, poor ventilation and so on. The air pollution caused by vehicles is very serious in SFT. When fire happens in the SFT, the heavy smoke and heat caused by burning are hard to discharge and finally threat trapped persons’ life. Meanwhile, it also increases the safety risk if there are no special escape ways in the structure.
· Traffic accident: SFT is an enclosure space under water; it would be very difficult to take rescue work when traffic accident occurs. It may cause great casualties and economic losses. What’s more, traffic accident would also trigger other problems, such as fire and dangerous goods leakage.
· Water leak: Waterproofing is also the key point for safety of SFT, because water leak could lead to the public a tremendous psychological fear. Once the leakage occurs, the great pressure water would make leakage further increase. The reliability of waterproof is very important. Connection of tube and construction detail of waterproof should be carefully treated.
· Overload: The risk of overload is relatively small, but overloading can lead to structural damage.
· Environmental impact
·Cost: Due to lots of material and machinery involved in project, estimated cost may be high.
·No Stoppage: It is very difficult to stop the train travelling on such a high speed.
The construction risks of SFT can be separated into foundation construction, tube construction, anchor cable construction and ancillary facility construction. When constructing SFT structure, first, the tube section should be prefabricated at the boatyards near the construction site, and then sealed at both ends by bulkhead. Next; the section can be towed to assigned position by tugboat. The anchor cable installation and deep water pile foundation construction should have in advance been carried out. Once the segment is at assigned position, the floating crane fixed by four wire ropes which are anchored at the lakebed controls the segment of SFT and lets it down to the preset position by special buoyancy balance system. After the sinking, this segment can be connected to previous one. Finally, the bulkhead is removed and waterproof at the connection should be done, and the other ancillary facilities are installed.
7. RISK CONTROL OF SFT
The meaning of risk control is to minimize the risk loss through prior treatment and process control according to the result of risk assessment. The risk control measures can also be taken by three aspects as follows:
· Corresponding to the natural hazard risk of SFT, we should put forward reinforcement and protection methods against hazard under the construction of SFT to improve capacity of disaster prevention. In the one hand, we should establish the system of hazard monitoring and hazard early warning as well as hazard database of SFT project site, which includes hazard’s type, duration, destructive degree and repair measures. On the other hand, the mechanism research of structure damage caused by hazard, nonlinear elastic-plastic analysis by using computer simulation technology and some control technologies and methods of SFT should be carried out.
· Corresponding to the operational risk of SFT, not only should we improve the SFT disaster prevention and relief system, but also ensure SFT facility integrity including smooth line shape, explicit traffic indicator sign and adequate ventilation as well as lighting system. Meanwhile, we may also research the influence of longitudinal ventilation on working fire and smoke emission. The comprehensive set emergency evacuation system are considered and designed, such as special evacuation channel.
· Corresponding to the constructional risk of SFT, we should consider the combined action between structure and environment in each construction
stage. The control section’s structure parameters during the construction of SFT, such as strains and stresses should be monitored in time in order to guide construction and guarantee the constructional reliability of SFT structure.
8. CASE STUDY ON A SFT: TRANSATLANTIC TUNNEL
A Transatlantic tunnel is a theoretical submerged floating tunnel which would span the Atlantic Ocean between North America and Europe. The transatlantic tunnel would be built of 54000 prefabricated sections connected by watertight and vacuum-tight gaskets. Each section of tunnel is to be attached to tethers which are to be affixed to an anchor at the sea floor, which is, in some places almost 8 km deep. The tunnel would hover at about 45 meter below the see surface, ideal to avoid ships and still minimize pressure and also to sway a bit under pressure. A high-speed train could theoretically run from New York to London in 54 minutes. But the train would have to go at a speed of 8000 km/h through a 5000 km long tunnel, which is itself floating in the Atlantic Ocean. To reach this speed, almost a perfect vacuum would have to be maintained in the tunnel and the train would have to be magnetically levitated. There will be 3 rails. Two are bidirectional and one reserve, to be used during accidents and repairs.
Fig. 8.1 Location of Transatlantic Tunnel
8.1 COMPONENTS OF TRANSATLANTIC TUNNEL
Transatlantic tunnel consists of many components. The main components of this Tunnel are listed below.
· Sea anchors
· Utility conduits and service port
· Vacuum pumps
· Maglev train
· Guide ways
As the tunnel is situated at a depth of 30m, it should be perfectly water tight and secondly it should resist the salty sea water and thirdly it should be withstand against hydrostatic forces coming on it. It is made of 4 layers. Outermost layer is constructed of aluminium to resist the salty sea water. Second and third layer is made of the foam to float the tunnel easily in water. Fourth layer is of concrete which gives strength to the tunnel. As the length of tunnel is very large, it is not possible to construct the tunnel at situ. Therefore it is made up of 54000 precast units. These units are casted on shore and transported to place where they have to fix the units with two large floating platforms. A diagram of shell is given in fig.
Fig. 8.2 Shell
8.1.2 Sea Anchors