Immersed-tube tunnels are rare birds. Only a handful are under construction in the world each year. They consist of large tubes, constructed in sections often more than 100 meters in length, which are floated into position in a waterway, sunk into position, and joined with watertight seals. They are always placed in a trench on the bottom of the river or bay. They are constructed of steel or reinforced concrete. [Slideshow: The 10 Longest Immersed-Tube Tunnels]
“While bridges are generally cheaper to build, immersed-tube tunnels are usually located at sites where bridges would not be possible. Either soil conditions are poor, or there is constricted space on the sides of the waterway, or there is a high clearance required for navigation by ships,” says Jonathan Baber, a project director for Mott MacDonald, and co-author of “Immersed Tunnels.”
In certain conditions, immersed-tube tunnels can be a less expensive choice. “If you are crossing a river between a half km to one km wide, an immersed tube will probably be competitive price-wise compared to bore tunnels, because of the procurement cost of a tunnel boring machine,” explains Baber.
The first immersed-tube tunnel was a sewer tunnel built in Boston in 1894. The Michigan Central Railway Tunnel linking Detroit with Windsor, Ontario, Canada, under the Detroit River, was the first immersed-tube vehicular tunnel, when it was completed in 1910. The first immersed-tube tunnel in Europe was the Maastunnel, a road tunnel in Rotterdam that opened in 1942.
The Dutch are probably the most prolific builders of immersed-tube tunnels. “Their flat, delta-type landscape and soft, peaty soils are ideally suited for immersed-tube tunnels,” says Baber, who also serves as animateur (chairman) of the International Tunnelling Association’s working group for immersed tunnels.
The Busan-Geoje tunnel in South Korea is a project whose challenging conditions spurred several new approaches. Strukton Immersion Projects, a Dutch firm, serving as a subcontractor, was responsible for the flotation, transportation, and immersion of the tunnel sections.
“The most challenging conditions were the swell waves. They influenced the immersion,” says Peter van Westendorp, project manager with Strukton, who served as the immersion commander on the project. “We used two purpose-built pontoons and four tugboats to transport the tunnel sections 32 km from the casting yard to the site. We had to travel by night. It took us 10 hours. The second night we would do the immersion, which took 16 hours each time.”
“Our model tests showed that during the positioning of each element on the seabed the elements were still being moved by the currents. So we developed the external positioning system in order to set down the elements at a safe distance from each other in the bottom of the trench. We then jacked up each element, using 800-ton jacks located in the legs of the external positioning system, and moved the element forward by winching.”
Another precedent for Strukton was their decision to use a submarine. “By doing immersion at that great a depth, the exit shaft had to be extremely rigid, which was not practical. We had all kinds of equipment inside the tunnel, which were remotely controlled, such as cameras to watch the bulkheads inside the tunnel, and level gauges to measure ballast tank level. If the systems had failed we would have had to go inside the tunnel sections, so we put technicians in the submarine. But the systems worked, so we did not have to use the submarine for real.”
More recently Strukton has worked on an immersed-tube tunnel in Amsterdam. The project involves building a new metro station beneath the central train station, and a new metro line under the I J river.
A new immersed-tube record breaking tunnel is currently taking shape in China. The Hong Kong-Zhuhai-Macao Bridge is a 50-km long link consisting of three cable-stayed bridges, two artificial islands, and a 6.7 km long immersed tube which will be the world’s longest immersed tube, when it is completed in 2016.
A tunnel between Germany and Denmark that is currently in the design stage is expected to be the new world record holder when it is completed in 2021. The Fehmarn Belt Fixed Link is a proposed 18 km long immersed-tube tunnel between Fehmarn Island in Germany and Lolland Island in Denmark, that will carry both a highway and rail line. It will significantly cut the current travel time between Hamburg and Copenhagen. It is expected to cost $7 billion to build.
Femern A/S, the project owner, has prequalified nine large contractor consortia to bid for the four major contracts (northbound tube, southbound tube, portals and ramps, and dredging and land reclamation). Preliminary bids are expected to be submitted in 2014, with construction beginning in 2015.
Longest Immersed-Tube Tunnels
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4. Rotterdam Metro Tunnel, 2.855 km long, the Netherlands. Due to the Netherlands� notoriously soft soils, prior to the 1990s, engineers could not build bored tunnels there. In order to construct the first subway system in the Netherlands, the city of Rotterdam decided an immersed-tube tunnel was the only possible solution. The tunnel runs under the Nieuwe Maas River and carries the subway�s north-south line from Centraal Station in Rotterdam to Zuidplein. The tunnel was designed and built by the firm Christiani & Nielsen, a Danish engineering firm, and built by Hollandsche Beton, a Dutch contractor. The tunnel was completed in 1966, and the metro began operating in 1968.
Map by ENR Art Dept.
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3. Busan-Geoje Fixed Link, 3.2 km long, South Korea. Busan, South Korea's second-largest city with a population of 3.6 million, is separated from Geoje Island, a major shipbuilding hub, by 8 km. The Fixed Link, which consists of two cable-stayed bridges and a tunnel, reduced the driving time between Busan and Geoje from over three hours to only 40 minutes. The immersed-tube tunnel runs from Busan to the small islands of Daejuk and Jungjuk and is considered necessary in order for the shipping channel to be more navigable. Daewoo Engineering & Construction Ltd. led a consortium, called GK Corp., of seven Korean contractors that won the concession to design, build, finance and operate the overall Link project. Halcrow Group Ltd. of Britain and Tunnel Engineering Consultants of the Netherlands served as technical consultants to Daewoo. A joint venture of Danish engineering firm COWI and Daewoo designed the tunnel. A trench was dredged 12.5 meters deep through soft clay, and cement and soil were mixed to form soil-cement columns over areas of weak ground. Strukton Afzinktechnieken, a Dutch firm, serving as subcontractor, was responsible for the flotation, transportation and immersion of the tunnel sections. The tunnel is made up of 18 concrete sections, each of which is 26.5 m wide, 10 m high and 180 m long and weighing 50,000 tonnes. After they were cast, the tunnel segments were floated into position by two large submersible, remote-controlled pontoons. The first tunnel section was placed in position in February 2008. Strukton used huge three-sided, hinged steel frames with hydraulic jacks, called external positioning systems, to precisely position each section. Strukton also used a small submarine to inspect the sections underwater. The Fixed Link opened in December 2010 and cost $1.8 billion to build.
Photo Courtesy Halcrow
6. Parana Tunnel, 2.367 km long, Argentina. The provinces of Entre Rios, Corrientes and Misiones in northeast Argentina were cut off from the rest of the country by the Parana and Uruguay rivers and were reachable only by ferry until the 1960s. This isolation limited trade and communication. The Parana Tunnel helped end the divide by linking the city of Parana in Entre Rios with the city of Santa Fe. Both cities are port towns on the Parana River, and trans-shipment points for wheat, meat, lumber and other products. The tunnel is made up of 37 cylindrical sections of reinforced concrete, 10 m in dia, 65 m long and weighing 4,500 tonnes. At its deepest point, the tunnel is 32 m below the river's average water level. It was built by a consortium headed by Hochtief. Construction began in 1962, and the tunnel opened in 1969. It was the first immersed-tube tunnel built in Latin America.
Photo Courtesy Hochtief
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9. Hampton Roads 1, 2.091 km long, Virginia. Hampton Roads is one of the largest natural harbors in the world. It incorporates the mouths of the Elizabeth and James rivers and empties into Chesapeake Bay. It is home to Norfolk Naval Base, the largest U.S. naval base. The Hampton Roads Bridge-Tunnel is a 5.6-km-long crossing for Interstate 64 and U.S. Route 60 that encompasses man-made islands, trestles and an immersed-tube tunnel. A bridge-tunnel was chosen rather than a drawbridge because, if a bridge-tunnel was destroyed in wartime or by a natural disaster, it would not block vital shipping channels. It was designed by the Virginia Dept. of Highways, with Parsons Brinckerhoff Hall & MacDonald serving as a consultant. Merritt-Chapman & Scott of New York was the general contractor for the entire bridge-tunnel, and Baldwin-Lima-Hamilton Corp. of Philadelphia fabricated the tunnel sections of steel. There were 23 tubular sections, each 300 ft long, with double shells: an inner circular section 37 ft in dia and an outer shell, roughly octagonal shape, about 2 ft larger than the core. At the fitting-out, pier workers cast 18-in.-thick concrete layers to the perimeter of the inner tube as well as the 18-inch slab for the roadway. When the casting was completed, the sections were towed to the site, docked between two halves of a screed barge; then, additional concrete was placed on the outer shell, and the sections were sunk into position. The tubes were connected to each other by collars at their ends, aligned by drift pins set by divers. Dredging began in 1954, and the tunnel opened in November 1957.
Photo Courtesy VDOT
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5. Pulau Seraya Utility Tunnel, 2.6 km long, Singapore. The Public Utilities Board of Singapore needed a tunnel to carry electric transmission lines from a powerplant located on Pulau Seraya Island to the Singapore mainland. The PUB selected Mott MacDonald to design and supervise construction of the immersed-tube tunnel. The tunnel comprises 26 sections, each 3.7 m high, 6.5 m wide and 100 m long. The sections were floated out and sunk in a dredged trench, on a gravel bed, and then armored with rock. The tunnel carries seven 230-kV, 500-MVA power lines and accommodates maintenance personnel using battery-powered vehicles. The tunnel was completed in 1988. In the 1990s, Pulau Seraya and many other small neighboring islands were merged through land reclamation to form Jurong Island, which hosts many large refineries and petrochemical plants.
Image Courtesy Christiani & Nielsen PLC
1. Bay Area Rapid Transit Tunnel, 5.825 km long, San Francisco. The world�s longest immersed-tube tunnel, known as the Transbay Tube, carries four lines of the Bay Area Rapid Transit system under San Francisco Bay, between the cities of San Francisco and Oakland. The tube sits in a 60-ft-wide trench that was excavated at varying depths, ranging from 15 ft to 85 ft. It is made up of 57 prefabricated-steel sections, each of them 48 ft wide, 24 ft high, and ranging in length from 323 ft to 350 ft. The sections were fabricated by Bethlehem Steel and towed to an outfitting dock, where Trans-Bay Constructors (TBC), a joint venture of Peter Kiewit Sons� Co., Raymond International, Inc., Tidewater Construction Corp. and Healy-Tibbitts Construction, took command of the 800-ton shells. After placing a concrete lining, they were towed out along the tunnel alignment and sunk by adding 500 tons of gravel ballast to cribs along the tops of each tube section. Hydraulic rams drew together railroad-type couplers built into each section, seating rubber gaskets extending around the abutting ends of each section for a watertight seal. Finally, workers grouted each joint, and barges placed five feet of sand atop each section. The bay bottom consists of soft alluvial deposits, which the designers determined would insulate the tube from severe shaking during earthquakes. It was designed by Parsons-Brinckerhoff-Quade & Douglas. TBC built the tube�s basic structure for $90 million, with an additional $90 million for ventilation structures at either end, 2.8 miles of aerial and subway approaches in Oakland and San Francisco, trackage, final finish work and electrification. Dredging began in 1966, and construction of the tube was completed in late 1969. BART transit service began operating in 1974.
Photo: Wikimedia Commons
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Check out project profiles of the world's ten longest immersed-tube tunnels.
Map by ENR Art Dept.
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7. Hampton Roads 2, 2.229 km long, Virginia. To accommodate increased traffic, a second Hampton Roads Bridge-Tunnel was built in the 1970s (for information about the first one, see entry No. 9 on this list). The second tunnel is also two lanes wide. Made up of 21 sections, it was designed by Parsons, Brinckerhoff, Quade & Douglas. It was built by a joint venture of Tidewater Construction Corp., Peter Kiewit Sons' Co. and Raymond International Inc. After the second bridge-tunnel was completed, the original tunnel carried the westbound traffic, while the new tunnel carried the eastbound traffic. It opened in 1976.
Photos Courtesy VDOT
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2. �resund Link Tunnel, 3.5 km long, Denmark. The �resund Link is a massive combined road-and- rail crossing of the �resund Strait, connecting Denmark and Sweden. It includes a 7.8-km-long bridge and a 4-km-long tunnel. The link connects the road and rail networks of Scandinavia with those of cenral and western Europe. The Drogden Tunnel consists of a 3.5-km-long immersed-tube tunnel and two 270-m-long approach tunnels, running from the artificial island of Peberholm to Kastrup on Amager Island. It was built by �resund Tunnel Contractors, a design-build joint venture, comprising NCC AB (Sweden), E. Pihl & Son A.S. (Denmark), Dumez-GTM S.A. (France), John Laing Construction Ltd. (U.K.) and Boskalis Westminster Dredging BV (the Netherlands), under a $670- million contract. OTC's designer was Symonds Traverse Morgan Ltd. (UK). The tunnel consists of 20 precast-concrete sections, each 176 m long, 40 m wide and 8.6 m high and weighing 55,000 tonnes. After casting, each section was pushed into a shallow basin, where ballast was added. The adjacent sea lock was then opened, and the section was towed to the site with pontoons attached; then, it was lowered into the rock-lined trench. The tunnel's five galleries hold two rail lines, two two-lane roadways and a service passage. Casting began in 1996, and the link opened to traffic in 2000.
Photo Courtesy Oresundsbron
10. Blayais Power Station Outfall, 1.935 km long, France. The Blayais Nuclear Power Plant is located on the Gironde estuary near the city of Bordeaux in southwest France. Its four reactors have a total capacity of 3,804 MW and came on line between 1981 and 1983. It is owned and operated by �lectricit� de France S.A. (EDF), the largest electricity producer in the world. The powerplant's outfall tunnel, for dispersing out to sea used cooling water, was built by Campenon Bernard Cetra-Dodin. The dredging subcontractor was Atlantique Dragage, the steel work was done by Welbond, and the prestressing was carried out by STUP Freyssinet. The tunnel was completed in 1978.
Map by ENR Art Dept.
8. Tuas Bay Cable Tunnel, 2.1 km long, Singapore. This tunnel was built to transmit power from two powerplants located on Singapore's Tuas Peninsula to electrical substations on the other side of Tuas Bay. Given Singapore's population density, utility PowerGrid Ltd. decided it would be advantageous to route the power lines through an underwater tunnel rather than face the challenges of an overland route. The tunnel was designed by Parsons Brinckerhoff, with concept work by Development Resources Pte. and Mott MacDonald. Obayashi Corp. was the design-builder. The tunnel is made up of 21 reinforced-concrete sections, 17 straight sections 100 m long and four shorter, curved sections. The sections are 11.8 m wide and 4.4 m high. The tunnel sections were placed in a dredged trench and covered with rockfill and concrete panels, calculated to protect the tunnel against possible damage from sinking ships and falling or dragging anchors. The top of the tunnel is 13 m below sea level at its lowest point. Inside the tunnel, cable troughs hold together ten 400-kV and 230-kV power lines with cooling water pipes and thermal sand, which is necessary for temperature control. The tunnel accommodates a battery-powered maintenance vehicle that runs on rails. Terminal buildings at both ends of the tunnel house ventilation and electrical equipment. Obayashi was awarded the job in 1966, and the tunnel went on line in early 1999. It cost 65 million British pounds to build.
Image Courtesy of Mott MacDonald
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