Clamshell dredgers will be used to do parts of the dredging for the tunnel trench. The dredger on the picture - Chicago from the United States - was one of the biggest and most powerful clamshell dredger ever constructed. Unfortunately she capsized in the North Sea some years ago. She is here unloading the soil into a barge.
Dredging and Disposal
The Immersed Tube Tunnel will be embedded in the seabed of the Istanbul Strait. Therefore, it will be required to prepare a trench in the seabed big enough to accommodate the elements and allow for a cover and protection layer to be placed on top of the Tunnel.
The dredging of this trench will have to be done from the surface by heavy dredging equipment. It is calculated that the total amount that has to be taken out will be more than 1,000,000 m3 of soft soil, sand, gravel and rock.
The Istanbul Strait is at the deepest point of the alignment some 44 meters deep The tunnel will need a minimum of 2 meters of protection layer and the cross section of the immersed tunnel will be approximately 9 meters deep. This means that the dredger shall be able to operate at a depth of approximately 58 meters.
There is only a limited number of different types of equipment that will be able to do this job. The Clamshell Dredger (CSD) and the Trailer Suction Hopper Dredger (TSHP) will both most likely be used on the job.
The Clamshell Dredger is a very heavy device mounted on a barge. The device has two or more shells as the name indicates that are open when the device is dropped and hanging in wire systems from the barge. Because of the heavy weight of the clamshell it penetrates the sea bed. The shell automatically closes when lifted from the seabed, and in this way the material is lifted to the surface where the clamshell can off-load on barges.
The most powerful clamshell dredgers can excavate up to some 25 m3 of soil in one cycle. The clamshell dredger is most useful in soft to medium hard materials, and can not be used in hard materials like sandstone and rock. The clamshell dredger is one of the oldest types of dredger but is still extensively used all over the world for this type of dredging.
If contaminated soil is to be dredged, the clamshell can be equipped with some special rubber gaskets that will ensure that none or only very limited amounts of surplus sediment and fines are released in the water column when the shell is lifted from the sea bed.
The advantages of the clamshell are that it is very reliable and able to dredge at great depths. The disadvantages are that the rate of excavation falls dramatically with increasing depth. and the current in the Istanbul Strait will affect the accuracy and the performance in general. It is also not able to dredge in hard materials.
The Trailer Suction Hopper Dredge is a special ship mounted with a submerged trailing cutting device carrying a suction intake tube. The ship sails in the alignment, and the soil mixed with water is then pumped from the seabed into the ship.
The sediment has to settle in the ship. To fill the ship to its maximum capacity it is therefore required that a major amount of surplus water is allowed to flow from the ship when sailing. When the ship is full it goes to the dumping area and off-loads, and the ship is ready for another cycle.
The most powerful TSHD can take some 40,000 tons (some 17,000 m3) of soil in one cycle, and they can dredge up to a depth of some 70 meters. The TSHD can dredge in soft to medium soft materials.
The advantages of a TSHD is that it has a high capacity and it is a mobile system not depending on anchor systems. The disadvantages are that it is not very accurate and there will be areas close to the shore where it can not dredge.
It will be required to dredge some minor amounts of rock close to the shorelines at the terminal joints of the immersed tunnel. There are two different ways to do this. One way is to use traditional drilling and blasting under water, and the other method is to use a special chiselling device that can break the rock without blasting. Both methods are slow and costly, and if drilling and blasting is preferred, special precautions have to be arranged to protect the environment and the near by buildings and structures.
The picture shows a close-up of the clamshell itself. It can easily be seen that we talk about real heavy equipment!
The Trailer Suction Hopper Dredgers have been built in many different configurations. The picture shows one of the biggest TSHD ever build, the HAM318 from Holland built in 2001. The total installed power is in the order of 40,000 HP.
This picture shows an example of the intake "suction head" taking the material from the seabed when dredging with a Trailer Suction Hopper Dredger.