Immersed Tunnels

WSP has pushed the limits of tunneling technology with landmark projects that shaped the course of tunnel engineering and defined industry practice. Our achievements over more than 130 years cover every aspect of tunnels from program management to design and construction management. We serve clients in the rail, highway, offshore/maritime, utilities, and power and energy industries. Our designs have resulted in record-setting advances with tunnel boring machines (TBMs), and we have employed innovations such as earth pressure-balanced (EPB) and slurry-face tunneling techniques, and fiber-reinforced sprayed concrete liners (SCL).

Immersed tube tunnels are used in situations where building a bridge is undesirable, unsightly or environmentally unacceptable, to provide road, rail or utilities across bodies of water. As such, these tunnels are extremely rare, with just over a hundred having been constructed worldwide. We have been involved with multiple immersed tunnel projects on all continents, in a variety of ground conditions, and we have been at the forefront of this technology since its earliest years, including the design and construction of the Detroit-Windsor Tunnel between the U.S. and Canada in 1930, the first crossing between two countries under international water, where we have been serving as the owner’s engineer ever since.

Putting Our Experience to Work 

Immersed tunnels consist of very large precast concrete or concrete-filled steel tunnel elements fabricated in dry docks and then installed underwater. They are first fabricated in dry docks, shipyards or in improvised flooded basins, sealed with bulkheads at each end, and then floated out. Once they reach their final location, the elements are immersed, lowered into a prepared trench, and joined to previously placed tunnel elements. Immersed tunnels elements are then back filled and the bottom bed reinstated.

There are two main types of immersed tunnel: steel and concrete tunnel elements. Steel elements use structural steel, usually in the form of stiffened plates, working compositely with the interior concrete as the structural system. Concrete elements, on the other hand, rely on steel reinforcing bars or pre-stressing cables. WSP is one the few firms in the world that have experience with both types of immersed tube tunnels.

We designed a steel immersed tunnel across San Francisco Bay as part of the BART (Bay Area Rapid Transit) system. The tunnel consisted of 57 elements totaling 5.8km, the longest of its kind in the world. Thanks to innovative seismic retrofitting, the tunnel was able to survive the 6.9 Richter scale Loma Prieta earthquake in 1989 without any damage.

In Baltimore, we were part of a joint venture that designed the Fort McHenry Tunnel, which was the widest immersed tunnel built at that time. It was the first immersed tunnel to have double tubes, carrying a total of eight lanes of traffic, laid side-by- side in a single trench under Baltimore Harbor.

WSP also designed the bridge-tunnel crossings of Hampton Roads, Virginia, where immersed tunnels were built between two artificial islands that connected to the mainland via viaduct bridges. This approach was later adopted by other projects worldwide, including the Oresund Bridge-Tunnel between Denmark and Sweden.

Concrete Immersed Tunnels

In Istanbul, we worked on the concrete/steel sandwich construction Marmaray Tunnel, the first tunnel to connect two continents. The rail and transit tunnel is 1.4km long, consisting of 11 elements, each weighing up to 18,000 tons. At 200 feet (60m) under the water’s surface, it is the deepest immersed tunnel in the world.

In Virginia, the Elizabeth River Tunnel (Second Midtown Tunnel) was delivered as part of a public-private partnership procurement process and was opened to traffic in 2016. The tunnel consists of 11 concrete elements fabricated in Sparrows Point, Maryland, and towed down the Chesapeake Bay to the project site in Portsmouth, Virginia, for immersion and placement under the Elizabeth River. 

WSP is also performing the independent design review of the Fehmarn Belt fixed link between Denmark and Germany. Upon its completion, it will reduce the journey from Hamburg to Copenhagen by an hour and 30 minutes. At 17.6km long, it will be the longest immersed tunnel in the world. The precast concrete tunnel sections will have a rectangular cross-section of 42m wide by 10m high, consisting of four separate passageways: two for vehicles and two for rail.