Constant improvements in shotcrete are supporting the development of Sprayed Concrete Lining, which continues to gain prominence in the tunnelling segment.
Designed initially to be used as a temporary structure, the method has gone from being used mainly in hydroelectric projects, Scandinavia and the UK, to gain global acceptance.
Sprayed Concrete Lining sees the spraying shotcrete in layers over the tunnel´s surface to stabilize it and form a permanent tunnel lining.
Whereas the technique was previously mostly focused on lending the early structure with a lifespan of between four to five years, the improvements in quality and durability of shotcrete means it is now often being used as the final lining.
Key improvements in shotcrete
Common efforts from all the different stakeholders in the tunnel construction industry, from raw material providers, equipment manufacturers, engineers to constructors have resulted in the following improvements to SCL:
- Development of mechanised wet-mix shotcreting
- Usage of alkali-free additives
- Use of fibres for shotcrete mixing
- Mechanised shotcrete spraying machinery
- Usage of total stations for topographic surveying
- New methods of surface curing after application
- Worker training in shotcrete application
Crossrail, London
SCL: Projecting the Future
There are three main different types of Sprayed Concrete Linings:
- Double Shell Linings (DSL): consists of a primary lining that takes the temporary load plus a secondary lining to take the permanent load
- Composite Shell Linings (CSL): consists of a primary lining, sprayed waterproof membrane, and a sprayed secondary lining, where the primary lining acts compositely and takes a proportion of the long-term ground load
- Single Shell Linings (SSL): a single lining takes on the temporary and long-term loads. It can be done in several layers and is frequently used in projects with little hydrostatic charge
The ability to save time and cost-cutting on equipment and materials, as well as trimming down budget spending makes it more attractive, while it also boasts increased safety as it minimizes the risk of leaks caused by the presence of lattice girders at the end of construction[1].
Its durability and final resistance has been tested in several flagship projects such as London´s Crossrail or the A3 Hindhead.
Sources:
1 Andrew Pickett & Alun Thomas, “Where Are We Now With Sprayed Concrete Lining in Tunnels”, Tunnelling Journal, Fall 2013, http://shotcrete.org/media/Archive/2013Fal_GoinUnderground.pdf
