Tunnel construction methods
The type of tunnel construction will present specific access, intervention and operational challenges, and specialist advice will be needed on the best way to proceed. There are three main tunnel construction methods.
Cut and cover
The simplest in tunnelling terms, cut and cover tunnels are constructed by digging a trench, forming the cast concrete tunnel walls and placing a concrete slab over the top. Excavations forming the cut can be deep and require special intervention considerations and controls.
As the name implies, these tunnels are completely surrounded by the ground. The tunnel is driven through the soil under the surface by mechanical, manual or explosive means. The excavation made to commence tunnel boring can be deep and require special access and intervention considerations and controls. The characteristics of the tunnel - single or twin bore - will also affect the intervention strategy, especially in terms of the intervention distance and dead end conditions during the construction process.
Where a tunnel is to cross beneath a body of water, an immersed tube tunnel may be selected as an alternative to a bored tunnel. This type of tunnel is usually made of reinforced cast concrete rectangular sections, which are constructed in a casting basin, sealed at both ends, then floated out, sunk and joined to the previous underwater section by gaskets. The tunnel sections are normally laid in a trench on the river or sea bed and, once in place, a rock protection layer is placed over the top of the tunnel sections to protect them.
Access to the casting basin is normally via a series of tiered access roads. One end of the casting basin is fitted with a caisson to prevent the ingress of water until the basin is flooded. The flooding of the casting basin, floating out and towing, sinking and joining of the sections underwater are extremely specialist operations, which fall outside the normal scope of fire and rescue service operations. The fire and rescue service could, however, be asked to provide water safety assets to assist during a waterborne incident.