Immersed Tube Tunnels (ITT)
| There are over 150 ITT’s in the world. The diagram below shows the diverse shapes used. The top right design is the configuration used in the Oresund Crossing between Denmark and Sweden. The photo below shows the Oresund crossing as it exists today. | |
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Large scale ITTs are a recent innovation in tunnelling although smaller ITTs were built in the USA in the early 1900’s and these are in use today. The diagrams and photos below show the construction sequence of the immersed tubes that were built as part of the Oresund Crossing.
The crossing proposed by Forthtag is very similar to this. This ITT has two boxes for three lane vehicular traffic with an access tube between for pedestrian access. It also has two running tubes for heavy rail.
The consultants who designed the Oresund Crossing, (a 3 km long ITT), estimate the cost for a similar crossing under the Forth would be £450m in today’s prices. This price does not include the railway work or the Rosyth bypass but it does include the cost of a new dry dock. If the abandoned Trident dock at Rosyth was used, there would be a corresponding saving.This photograph below shows the precast reinforced concrete units used in Oresund being built in the dry dock. These units were 8m high, 40m wide and 175m long and they weighed 55,000 metric tons. Heavy water-proof bulk heads are installed prior to flooding the dock. These reinforced concrete boxes were produced at one per month. The next photo shows how the dry dock at Rosyth could be adapted to perform the same function as that of the one at Oresund at considerably less cost than building a new one. The skills need to build these units already exist in Fife and the Lothians and would employ local construction workers. This photo actually shows the units floating in the flooded dry dock at Oresund, prior to being towed out.
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The photo below shows the units under tow on their way to their final placing. Prior to placing, additional buoyancy is attached using purpose made pontoons. The next diagram shows the unit suspended over the dredged trench with the tugs holding the unit in position, ready for lowering to the sea bed.
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| Once the units are in place, divers are sent down to remove the water
tight bulkheads. The units are position to a tolerance of + or - 1cm and are jointed together to form a continuous immersed tube. The tubes are then pumped dry and up to 1m of concrete is placed in the invert of the tube to insure that it never floats again. Armour stones are laid on top of the units to give additional dead weight and then the sea bed is reinstated using the natural material that exists in the location.. This photo shows a view of the inside of a completed tunnel. |
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Bored Tunnel Solution.
Bore Tube Tunnels are an alternative to ITTs, They use Bored Tunnel Machines (TBM).
A solution for this crossing would use a 11.0m internal diameter TBM. As the diagram shows this would give twin tunnels,
each with 2 lanes of traffic (7.3m-wide carriageway plus walkways, all to Highway Agency standards), and sufficient space
for a light rail system in the invert of the tunnels. A bored tunnel costs less as the crossing length increases.
The geology in the area of the tunnel is well known, but the extent of any mine working would have to be established
by a very detailed site investigation. Using up-to-date prices, the current day price including approach roads but
excluding the Dunfermline by-pass would be in the region of £350m. This price would need to be adjusted for any unknown
ground conditions. This would give a £100m difference between this and an ITT.
Environment The environmental impact of the various types of construction should be the primary basis on which to found a decision about a second crossing. Here again our consultants are very clear that a bored tunnel solution would be the least damaging. Their reason for this in the main is that this form of tunnel uses entry an exit portals on the foreshore with no disturbance to the sea bed of the Firth of Forth, thus minimising any disturbance to marine wild life. They rank an immersed tube as the second least damaging. In this form of construction the foreshore and the sea bed would only have to be disturbed temporarily but would eventually revert back to nature. In the case of a bridge, there would be a permanent scar from the main mast foundations, shadowing and further interruption to the migratory paths of birds. There are severe restrictions, covered by EU regulations, on the damage to wildlife. These virtually prohibit construction in certain areas. To meet current environmental standard any construction project of this size will need a comprehensive study done of these issues. There are also other issues of “Whole Life Costing” which should be taken into account. This calls for all the costs associated with the planning, building and maintenance to be calculated. FETA calculated that the maintenance of a tunnel would be 3.25m over a twenty year period. Currently they spend £10m a year maintaining the existing FRB. A considerable amount of rubbish is blown off the bridge such as metal and asphalt, petrol and oil based particles which pollute the air and the sea. Issues such as suicide risks should also be factored into the equation.
Time Scales A suspension Bridge would take up to 12 years to plan and build. This would give an earliest completion date of 2019. A tunnel could be built in 8 years from start to finish, allowing the same period of 5 years for design and legislation. This would mean a tunnel could be in place for the 2015. FETA are now on record as saying that the existing bridge will close to vehicles over three and one half tons by 2013 and will have to close completely by 2018.