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When a fixed crossing of a waterway is under consideration, if the miracle of Moses cannot be reproduce, which according to the Bible in the book of exodus, he parted the red sea by the power of God using his staff allowing the children of Israel to cross with their feet dry; the first major decision to be made is whether the crossing should be a bridge or a tunnel. The choice of what to construct will depend on the geology of the surrounding and the turbitude that the construction and the building will cause to the habitat as it has to be in respect for the environment and aesthetically pleasant.
In a clayey and silty soil or soft soil environment, where the soil cannot support piers of a long span bridge, even tunnel boring can prove to be very costly; the alternative solution can be to build an immersed road tunnel.
This essay will look at the structural design of immersed tunnels and begins with a basic description of immersed tunnel, the different types of immersed tunnel and construction methodology.
The immersed road tunnel is a system of constructing directly under a waterway whereby very large tube of prefabricated concrete or tube made of steel then filled with concrete elements fabricated a short distance from the site in the dry docks, or in improvised floodable basins sealed with bulkheads at each end and then floated out to be installed by sinking it into place inside the pre-dredged trench. The water between the bulkheads is then pumped out from inside the elements. The elements are then rubber sealed together to ensure a watertight tunnel and crossing. Tunnel cross-sections may have flat sides or curved sides.
TYPES OF IMMERDED TUNNEL
The immersed road tunnel has been constructed around the world for the last 100 years, and there are more than 100 traffic tunnel which have been constructed worldwide and can be classified in two main types which are also related to their method of fabricating them, known as steel and concrete tunnels.
Concrete tunnels are predominantly in rectangular shape and use for highways and combined road and rail tunnels as shown on figure 1.
Steel tunnels which are generally circular curved with a flat bottom and rarely rectangular use structural steel of stiffened plate with the interior encased with concrete. Steel immersed tunnels can be categorized into three sub-types: Single shell (Figure 2), double shell (Figure 3) and sandwich.
There are four stage involve in the construction of the immersed road tunnel: the fabrication of the tube tunnel, the dredging of the seabed of where the tunnel will be installed and the laying of the new foundation of where the tunnel will be resting, and the installation of the tube tunnel.
The site whereby the elements of the tunnel are fabricated differs from the steel immersed tunnel and the concrete immersed tunnel though both have to be in the proximity of the construction site. The elements of the steel are fabricated in ship yards or dry docks similar to ships, then launched into water like a ship before sliding them into the water to be install, see figure 4. Contrary to the concrete immersed tunnel, there are usually cast in specially built basins, then the basin is flooded and the elements are floated out to be install, see figure 5.
4.1. B.TUNNEL ELEMENT FABRICATION
For steel tunnels, fabrication is usually done by modules, each module being in the range of 5m long, spanning between diaphragms. The modules are then connected and welded together to form the completed shell of the tunnel element for an average of 100 m in length.
Concrete tunnel elements are usually cast in full-width segments to reduce the effects of shrinkage with pre-stress reinforcement bar running through them in one batch. Bituminous membrane has been used on the out walls and the roof to keep the tunnel watertight but in recent years reinforce concrete tunnels has been constructed without a membrane just by controlling the concrete temperature during hardening in order to reduce the development of cracks. Their length depend on the capacity of the fabrication facility, restrictions along the waterway used to float the elements to the construction site, restrictions at the tunnel including accommodation of marine traffic during construction, currents, element shape and the availability of space for an outfitting pier, and the capacity of the equipment used to lower the elements into place but it is normally between 100 to 150 m long.
4.1. B. TRENCH EXCAVATION PROCESS
Prior to installing the tube tunnel, the waterbed need to be excavate to provide space for the prefabricated tunnel body and for the foundation under the tunnel body and also for the protective backfill on the sides and on the top of the tunnel.
Excavation process in any waterway is an environmentally sensitive issue as this can be difficult and complex process and can be complicated by contaminated materials, tides, storms and construction restrictions in waterways due to environmental concerns associated with fish migration and mating patterns and with ecology and marine life. The choice of the method of dredging should take all of that in to consideration.
The most common method of excavation for immersed tunnels is the use of a clamshell dredger (Figure 6) except in presence of contaminated materials sealed buckets will be used to reduce turbidity in environmentally sensitive areas. In the presence of silt, clay and other materials than rocks, Dredging methods and equipment to be considering must be designed reduce the dispersal of fine materials in the water. For that Cutter suction dredgers (Figure 7) have been recommended other material than big rocks as Blasting may be required in certain areas to remove rock, though it is highly not recommended as environmentally unfriendly.
4.1. C. LAYING OF THE FOUNDATION
As in every building construction, the foundation methods to be used will vary in consideration of the subsoil conditions and the degree to which different force the building will be subject to like earthquake and dynamic loads in particular such cars, truck and train.
Commonly, for the foundation of immersed tunnels screeded gravel blanket have been used in the bottom of the trench and jetted sand or sand flow methods to prevent the effects of material liquefaction due to earthquake action or dynamic loads in particular. Once all the elements are installed in their acceptable position, a special grout mixture will be pumped under the element through ports spaced every 8 m along the bottom. This material will not penetrate into the gravel foundation but will harden to support the element so that the jacks hold the element can be removed and the element backfilled.
PROCESS OF THE INSTALLING IMMERSED TUBE TUNNEL
Once the construction of the element has been completed, each tube is sealed with a temporary bulkhead allowing them to float with the insides kept dry. The module can be floated out using the purpose built ballast tanks inside the tunnel element and sometime additional ballast tanks are added with the tide which eventually will be broken when in its final place.
The tube is then floated away to its final position and when in position, the tube is anchored to a steel catamaran that is attached to lifting points on the elementââ‚¬â„¢s roof, and then water is added to internal ballast tanks which will weigh the caisson down as illustrated in Figure 9.Each element is sunken into place and lined up next to the element already placed under the waterusing steel jacks attach on each corner of the module which will allow anaccurate vertical and horizontal adjustment of each element.
Figure 9. Illustration of the Installation of immersed tube tunnel
Once the tunnel element is jacked firmly against the element already placed under the water it makes contact with the rubber gasket called GINA, retracting it by compression. Then water is pumped out, by suction, of the space between the bulkheads of the two elements engaging the sealing membranes to lock together, as the gasket compressed to create a perfect seals joint with the help of the hydrostatic pressure on the outside of the tunnel. Once the grout has set the jacks are removed and the tunnel location is fixed, sand is pumped to the river at the base of each element to fill the dredged area and then the rock armour is piled on top of the tubes for protection. After the process, Backfill is then placed over the trench below the existing or the future navigation channel profile to permanently bury the tunnel
The immersed tube tunnel is the only feasible alternative to a bored tunnel for water crossings at some locations as it can be constructed in almost any ground condition, though it has potential disadvantages in term of environmental disturbance to the water body bed which has impact on fish habitats, ecology, current, and turbidity of the water.
But construction, it can be constructed in particular locations that are not always favourable for bored tunnelling such as in soft ground and at a comparable price since they lie only a short distance below water bed level, therefore the overall length of crossing will be shorterunder such conditions will be cost competitive and withstand the movement and forces of an earthquake. See Figure 10 &11
The other benefit is that the cross section is not restricted to a tunnelââ‚¬â„¢s conventional circular shape but can be square or octagonal, making it suitable application for wide road and rail tunnels.