Sprayed concrete lining is a well-established method for large diameter tunnel construction throughout the UK, sometimes described as the NATM (New Austrian Tunnelling Method).
This method comprises the staged excavation of the tunnel using mechanical excavation and the application of sprayed concrete to the excavated surfaces to provide ground support. The sprayed concrete supports the excavation both temporarily and permanently.
Common methods of sprayed concrete lining (SCL) tunnels (in softer conditions) comprise a temporary primary lining of sprayed concrete, with a sheet membrane or sprayed sealant, followed by construction of a permanent cast in-situ concrete lining, usually reinforced with steel bars. The tunnel can only remain unsupported for a limited time and hence the permanent, load-bearing secondary lining (Reinforced Concrete in situ structure) is required.
This type of tunnelling has been used extensively in London, for Crossrail and London Underground. It is suitable for shorter tunnel spans and offers much more flexibility than using a Tunnel Boring Machine (TBM), in terms of size and cross section.
A key requirement is to apply the sprayed concrete lining as quickly as possible to minimize ground movements. The structural design of the sprayed concrete lining is undertaken with load factors applied in accordance with design codes to increase lining strength. This is then subjected to a check by an independent engineer.
SCL tunnelling is safe and versatile
Implementing a formalised risk management strategy is imperative for successful SCL tunnelling. This is characterised by detailed ground investigation, risk assessments and the use of sophisticated numerical models to produce a robust, fully-engineered design. A tightly controlled risk management process during construction is implemented that relies on both predictions and observations to monitor and verify the work, alongside the need for independent checks.
Terra Solutions have a range of robotic spraying equipment allowing us to offer a comprehensive range of both manual and robotic sprayed tunnel and shaft lining installations.