- An immersed tunnel is an underwater tunnel composed of segments made of steel or cast iron tubes lined with concrete using conventional shipbuilding techniques.
- Immersed tunnels are used in conjunction with other forms of tunnel at their ends to continue the tunnel from near the water's edge to the at-grade on-land surface.
- The tunnel is made up of separate, prefabricated segments that are produced offsite and sealed at each end with bulkheads so that they can float like a boat.
- In parallel to segment production, an underwater trench is dredged and graded along the tunnel’s planned route.
- When a trench section and its specific segments are ready, the segments are floated to the appropriate location above the trench, then slowly sunk into place. Once in place, they are aligned and connected to the adjacent segment(s).
- After a segment is connected, a water-tight seal is created between adjacent segments, the tunnel is emptied of water and the underwater trench is then backfilled and any necessary protection is installed.
- Once the immersed tunnel is connected and sealed airtight from end to end, it is linked to its bored or above-ground access points. Once these connections are completed, then the roadway/infrastructure inside the tunnel can be constructed.
- USA: New York City’s 63rd St. 4-bore rail tunnel; San Francisco’s TransBay subway rail tunnel; Oakland’s subway rail tunnel; Baltimore’s 4-tube Fort McHenry tunnel; Detroit-Windsor 2-lane auto tunnel; Boston’s Ted Williams 3-lane auto tunnel.
- Global: Montreal, Denmark, Sweden, Netherlands, Germany, Sydney, UK, Japan.
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