As ENR magazine recently wrote in its cover story on the world’s longest immersed-tube tunnels, these structures, “are rare birds.” Constructing immersed-tube tunnels is no small feat of engineering: large tubes – some 350 feet long – are constructed on land, floated into a body of water, sunk and then connected with watertight seals.
Skanska teams have built two of the world’s ten longest immersed tunnels, both of which play essential roles in U.S. infrastructure: the Second Hampton Roads Bridge-Tunnel in southeastern Virginia and San Francisco’s Bay Area Rapid Transit Tunnel. Along with these existing tunnels, we are currently building another immersed tube tunnel that’s also in southeastern Virginia – Elizabeth River Tunnels.
Here’s a closer look at these three incredible tunnels:
Second Hampton Roads Bridge-Tunnel, Norfolk and Hampton, Va.
This 1.4-mile-long tunnel consists of 21 steel tube sections. These 14,000-ton tubes were submerged 123 feet under water. When a tunnel section was lowered into place, it was connected to the previously placed section by divers using railroad couplers. The sections were drawn together using hydraulic jacks; as the space between these bulkheads of the connecting sections was dewatered, the resulting hydrostatic pressure caused the newly placed tube to move in the direction of the previously placed section. This provided a positive, air-tight seal.
Bay Area Rapid Transit Tunnel, San Francisco and Oakland, Calif.
This tunnel (also known as the Transbay Tube) is a 3.6-mile-long underwater tube carrying Bay Area Rapid Transit‘s four lines under San Francisco Bay. The world’s longest immersed-tube tunnel, it sits 135 feet below sea level. Fifty-seven tunnel sections were placed from a lay barge, which consisted of catamaran hulls connected by two box girders from which the lowering blocks were suspended.
Fun fact: The same lay barge was used at the Second Hampton Roads Bridge-Tunnel to place its 21 sections under the harbor.
photo credit: A History of BART: The Project Begins
Elizabeth River Tunnels, Portsmouth and Norfolk, Va.
Under Construction (Completion 2016)
The centerpiece of the Elizabeth River Tunnels project is a new, one-mile-long tunnel adjacent to the existing Midtown Tunnel under the Elizabeth River. The Midtown Tunnel is the most heavily traveled two-lane road east of the Mississippi, and this project will double its capacity. The new tunnel – being built by SKW Constructors, a partnership of Skanska, Kiewit and Weeks – will be made of 11 hollow concrete segments, each approximately 350 feet long, 54 feet wide, 29 feet tall and weighing 16,000 tons. Instead of a traditional steel-shell immersed tube tunnel, this will be an all-concrete tunnel – only the second one in the U.S.
This project – which also includes rehabilitating the Downtown and Midtown tunnels and improving connecting roadways, as well as operations and maintenance until 2070 – is being done via a public-private partnership (P3) with the Virginia Department of Transportation. Skanska Infrastructure Development and Macquarie Infrastructure and Real Assets are leading the private-sector part of this P3.