Traditional immersed tunnelling results in a tunnel buried beneath the waterway which it traverses.
Construction procedure
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. | |
Approach structures can be built on the banks before, after or concurrently with the immersed tunnel, to suit local circumstances. |