Tunnel Segments FAQ
Below are some questions that we are asked on a regular basis.
Concrete moulds can be made to build almost anything out of concrete. The mould only needs to withstand the pressure of the concrete and it has to be shaped to contain the form you want.
There are currently 364 tunnels in the UK. The definition of a tunnel is an underground passageway with no defined minimum length, though it may be considered to be at least twice as long as wide. Tunnels in the UK are normally used for road networks, rail and waterways or aquaducts for people to travel through by foot or car but tunnels can also be used to route power or telecommunication cables or for wildlife to cross motorways.
Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and shape of the tunnel and appropriate risk management.
There are three basic types of tunnel construction in common use:
• Cut and cover tunnels, constructed in a shallow trench and then covered over.
• Bored tunnels, constructed in situ, without removing the ground above. They are usually of circular or horseshoe cross-section.
• Immersed tube tunnels, sunk into a body of water and sit on, or are buried just under, its bed.
Herrenknecht AG is a German manufacturer of tunnel boring machines of all sizes. Herrenknecht is headquartered in Allmannsweier near Schwanau and is the world market leader for heavy tunnel boring machines. Roughly two-thirds of the 3,300 employees work at the company headquarters and roughly 300 work at three different locations in China.
Concrete formwork is essentially the frame or mould that holds wet concrete in place while it sets (or cures) fully. There are two main types of formwork – temporary and permanent.
Temporary formwork is a reusable formwork that is removed from around the concrete once it is set. It is generally made from timber, steel, plywood or plastic.
Permanent formwork is formwork that stays in place and is never removed and an excellent example of this is Insulated Concrete Forms (ICFs). ICFs are made from a foam material (EPS – expanded polystyrene) that has a “hollow” in the middle into which concrete is poured. The combination of the foam and the concrete provides excellent thermal insulation to the building.
In early days manual shield tunnelling, was used as a way to protect labourers who performed the digging. The shield would protect workers and be inched forward, and workers would progressively replace the shield with pre-built sections of tunnel wall. The shield divided the workface into overlapping portions so that each worker could excavate. This was the method that was used to build the London Underground.
The tunnel lining is the wall of the tunnel. It usually consists of precast concrete segments which form rings but the advantage is they can be cast to suit any shape. Cast iron linings were traditionally used in building the London Underground tunnels, while steel liners were sometimes used for lining soft ground tunnels. Concrete linings provide a smooth surface and are fire resistant.
A concrete shaft is a shaft filled with concrete that helps to support a bridge or can be used in conjunction with tunnel panels to provide additional strength and durability.
The longest road tunnel in the UK is the A3 Hindhead Tunnel at 1.2 miles, which is between London and Portsmouth.
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