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Immersed tunnels consist of very large pre-cast concrete or concrete-filled steel tunnel elements fabricated in dry docks and then installed underwater.


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WSP has pushed the limits of tunnelling 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 tunnelling techniques, and fibre-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. At WSP, 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 US 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.

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Putting Our Experience to Work for You

Immersed tunnels consist of very large pre-cast concrete or concrete-filled steel tunnel elements fabricated in dry docks and then installed underwater. They are first fabricated in convenient elements in dry docks, shipyards, or in improvised floodable 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 backfilled 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 prestressing cables. WSP is one the few firms in the world who have experience with both types of immersed tube tunnels and the tunnel was opened to traffic a few hours after the earthquake.

Steel Immersed Tunnels

In San Francisco, US, we designed the steel immersed tunnel for the Cross Bay Tunnel, a part of the BART public transit system. The tunnel consisted of 57 elements totalling 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 without any damage.

In Baltimore, US, we designed the notable steel immersed 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 immediately 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 in the world, including the Oresund Bridge-Tunnel between Denmark and Sweden.

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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 60m (200ft) under the water’s surface, it is the deepest immersed tunnel in the world.

In Virginia, US, 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 thirty 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.

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WSP in Action

In Singapore, we designed the Tuas Undersea Cable Tunnel, the only use of on-end casting of immersed tunnel elements in the world. The tunnel is 2.1km long, 11.7m wide by 4.3m high, and the elements were cast vertically on-end in segments of 4m long each, to minimize shrinkage cracking and ensure a watertight structure. The segments were then rotated, assembled, grouted, and pre-stressed to form 100m long elements.

Our Research and Development programs have produced tools of the trade, including our tunnel engineering handbook and practical guide to tunnelling beneath open water which are considered to be authoritative resources for tunnel design, risk management, and site investigation guidance worldwide.