Procedure for construction
Tunnel elements are built in a casting basin or fabrication yard or on a ship lift platform. It is quite the usual thing to fabricate the outer shell of the element in steel. The section is then floated out to sea and when roughly in position, the steel forms are filled with concrete, which gives the tunnel body and weight. Tunnels have also been built using only concrete walls cast in a casting yard. Once the elements are ready, the two ends are fitted with temporary bulkheads that will not allow water to enter. Often the weight of the element is such that separate floatation arrangements have to be made to enable it to be shipped to the final spot. Rubber seals are also a part of the ends of each element.
While these tunnel elements are being made or cast, a trench is dug into the water channel where the tunnel will finally rest. This is done by dredging. This by itself is a laborious job, as excavated material has to be carefully removed so as not to disturb marine life or the surrounding ecology. This may even involve rock breaking and blasting where rocky layers are in the way of the alignment. It is also the norm to lay foundations for the tunnel element that may involve piling and concreting for the base.
Extremely high load-carrying capacity floating cranes are the norm nowadays, and these are mainly available on hire from suppliers all around the world. Once the element is in place at the water level, it is lowered into the trench and placed against the previous element that is already under water. Water between the bulkheads of the old and new element is then removed which causes the rubber seals to press against each other and close the joint.
Backfill material, obtained from the dredged material, or sand and gravel is then placed over the tunnel segments, permanently burying them at their final resting place.
Image source: Wikipedia: tunnel