When a fixed crossing of a waterway is under consideration, if the miracle of Moses can non be reproduce, which harmonizing to the Bible in the book of hegira, he parted the ruddy sea by the power of God utilizing his staff leting the kids of Israel to traverse with their pess dry ; the first major determination to be made is whether the crossing should be a span or a tunnel. The pick of what to build will depend on the geology of the surrounding and the turbitude that the building and the edifice will do to the home ground as it has to be in regard for the environment and aesthetically pleasant.
In a clayey and silty dirt or soft dirt environment, where the dirt can non back up wharfs of a long span span, even tunnel tiring can turn out to be really dearly-won ; the alternate solution can be to construct an immersed route 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 building methodological analysis.
The immersed route tunnel is a system of building straight under a waterway whereby really big tubing of prefabricated concrete or tubing made of steel so filled with concrete elements fabricated a short distance from the site in the dry docks, or in jury-rigged floodable basins sealed with bulkheads at each terminal and so floated out to be installed by droping it into topographic point inside the pre-dredged trench. The H2O between the bulkheads is so pumped out from inside the elements. The elements are so rubber sealed together to guarantee a watertight tunnel and crossing. Tunnel cross-sections may hold level sides or curved sides.
TYPES OF IMMERDED TUNNEL
The immersed route tunnel has been constructed around the universe for the last 100 old ages, and there are more than 100 traffic tunnel which have been constructed worldwide and can be classified in two chief types which are besides related to their method of manufacturing them, known as steel and concrete tunnels.
Concrete tunnels are preponderantly in rectangular form and usage for main roads and combined route and rail tunnels as shown on figure 1.
Steel tunnels which are by and large round curved with a level underside and seldom rectangular usage structural steel of stiffened home base with the inside encased with concrete. Steel immersed tunnels can be categorized into three sub-types: Single shell ( Figure 2 ) , dual shell ( Figure 3 ) and sandwich.
There are four phase involve in the building of the immersed route tunnel: the fiction of the tubing tunnel, the dredging of the ocean floor of where the tunnel will be installed and the laying of the new foundation of where the tunnel will be resting, and the installing of the tubing 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 propinquity of the building site. The elements of the steel are fabricated in ship paces or dry docks similar to ships, so launched into H2O like a ship before skiding them into the H2O to be install, see figure 4. Contrary to the concrete immersed tunnel, there are normally cast in specially built basins, so 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, fiction is normally done by faculties, each faculty being in the scope of 5m long, crossing between stop. The faculties are so connected and welded together to organize the completed shell of the tunnel component for an norm of 100 m in length.
Concrete tunnel elements are normally cast in full-width sections to cut down the effects of shrinking with pre-stress reinforcement saloon running through them in one batch. Bituminous membrane has been used on the out walls and the roof to maintain the tunnel watertight but in recent old ages reinforce concrete tunnels has been constructed without a membrane merely by commanding the concrete temperature during indurating in order to cut down the development of clefts. Their length depend on the capacity of the fiction installation, limitations along the waterway used to drift the elements to the building site, limitations at the tunnel including adjustment of Marine traffic during building, currents, element form and the handiness of infinite for an equiping wharf, and the capacity of the equipment used to take down the elements into topographic point but it is usually between 100 to 150 m long.
4.1. B. TRENCH EXCAVATION PROCESS
Prior to put ining the tubing tunnel, the waterbed need to be excavate to supply infinite for the prefabricated tunnel organic structure and for the foundation under the tunnel organic structure and besides for the protective backfill on the sides and on the top of the tunnel.
Excavation procedure in any waterway is an environmentally sensitive issue as this can be hard and complex procedure and can be complicated by contaminated stuffs, tides, storms and building limitations in waterways due to environmental concerns associated with fish migration and coupling forms and with ecology and marine life. The pick of the method of dredging should take all of that in to consideration.
The most common method of digging for immersed tunnels is the usage of a clamshell dredger ( Figure 6 ) except in presence of contaminated stuffs sealed pails will be used to cut down turbidness in environmentally sensitive countries. In the presence of silt, clay and other stuffs than stones, Dredging methods and equipment to be sing must be designed cut down the dispersion of all right stuffs in the H2O. For that Cutter suction dredgers ( Figure 7 ) have been recommended other stuff than large stones as Blasting may be required in certain countries to take stone, though it is extremely non recommended as environmentally unfriendly.
4.1. C. LAYING OF THE FOUNDATION
As in every edifice building, the foundation methods to be used will change in consideration of the undersoil conditions and the grade to which different force the edifice will be capable to wish temblor and dynamic tonss in peculiar such autos, truck and train.
Normally, for the foundation of immersed tunnels screeded crushed rock cover have been used in the underside of the trench and jetted sand or sand flow methods to forestall the effects of stuff liquefaction due to temblor action or dynamic tonss in peculiar. Once all the elements are installed in their acceptable place, a particular grout mixture will be pumped under the component through ports spaced every 8 m along the underside. This stuff will non perforate into the gravel foundation but will indurate to back up the component so that the knuckleboness hold the component can be removed and the component backfilled.
Procedure OF THE INSTALLING IMMERSED TUBE TUNNEL
Once the building of the component has been completed, each tubing is sealed with a impermanent bulkhead leting them to drift with the interiors kept dry. The faculty can be floated out utilizing the intent built ballast armored combat vehicles inside the tunnel component and erstwhile extra ballast armored combat vehicles are added with the tide which finally will be broken when in its concluding topographic point.
The tubing is so floated off to its concluding place and when in place, the tubing is anchored to a steel catamaran that is attached to raising points on the elementaa‚¬a„?s roof, and so H2O is added to internal ballast armored combat vehicles which will weigh the coffer down every bit illustrated in Figure 9.Each component is sunken into topographic point and lined up following to the component already placed under the waterusing steel knuckleboness attach on each corner of the faculty which will let anaccurate perpendicular and horizontal accommodation of each component.
Figure 9. Illustration of the Installation of immersed tubing tunnel
Once the tunnel component is jacked steadfastly against the component already placed under the H2O it makes contact with the gum elastic gasket called GINA, abjuring it by compaction. Then H2O is pumped out, by suction, of the infinite between the bulkheads of the two elements prosecuting the sealing membranes to lock together, as the gasket compressed to make a perfect seals joint with the aid of the hydrostatic force per unit area on the exterior of the tunnel. Once the grout has set the knuckleboness are removed and the tunnel location is fixed, sand is pumped to the river at the base of each component to make full the dredged country and so the stone armor is piled on top of the tubings for protection. After the procedure, Backfill is so placed over the trench below the bing or the future pilotage channel profile to for good bury the tunnel
The immersed tubing tunnel is the lone executable option to a world-weary tunnel for H2O crossings at some locations as it can be constructed in about any land status, though it has possible disadvantages in term of environmental perturbation to the H2O organic structure bed which has impact on fish home grounds, ecology, current, and turbidness of the H2O.
But building, it can be constructed in peculiar locations that are non ever favorable for world-weary tunnelling such as in soft land and at a comparable monetary value since they lie merely a short distance below H2O bed degree, hence the overall length of crossing will be shorterunder such conditions will be cost competitory and defy the motion and forces of an temblor. See Figure 10 & A ; 11
The other benefit is that the cross subdivision is non restricted to a tunnelaa‚¬a„?s conventional round form but can be square or octangular, doing it suited application for broad route and rail tunnels.