An immersed tunnel is built from the surface which in this case means from the water. The different phases of constructing an immersed tunnel are detailed hereafter: | - A trench is dredged in the bed of the water channel
| | - Tunnel elements are constructed in the dry, for example in a casting basin, a fabrication yard, on a ship-lift platform or in a factory unit.
| | - The ends of the element are then temporarily sealed with bulkheads.
| | - Each tunnel element is transported to the tunnel site - usually floating, occasionally on a barge, or assisted by cranes.
| | - The tunnel element is lowered to its final place on the bottom of the dredged trench.
| | - The new element is placed against the previous element under water. Water is then pumped out of the space between the bulkheads.
| | - Water pressure on the free end of the new element compresses the rubber seal between the two elements, closing the joint.
| | - Backfill material is placed beside and over the tunnel to fill the trench and permanently bury the tunnel, as illustrated below.
| | - Approach structures can be built on the banks before, after or concurrently with the immersed tunnel, to suit local circumstances.
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