Sprayed Concrete Design Optimization: The Composite Shell Lining
State-of-the-art sprayed concrete technology and innovation in sprayable waterproofing membranes have enabled tunnel engineers to design a considerably more economical tunnel lining system, the composite shell lining (CSL) system. It is a further development of the single shell lining system, and consists of two concrete linings, primary (outer) lining and secondary (inner) lining, and a double-bonded spray-applied waterproofing membrane embedded between them.
Sprayed Concrete: CSL systems (Composite Shell Lining systems)
CSL systems can be used for tunnels located in ground with limited, manageable water ingress, or for tunnels where water ingress has been treated with pre-injection. Although the composite shell lining is predominantly used in tunnel construction, it is suitable for all underground structures.
The primary sprayed concrete lining of a CSL system is designed as a permanent solution, and the two bonded concrete linings allow the thickness of the secondary lining to be significantly reduced, leading to considerable savings. The most important savings result from less excavation being necessary, a reduced volume of construction materials and no need for formwork. Additional savings in comparison to conventional waterproofing sheet membranes result from higher flexibility of the construction and reduced long-term maintenance costs.
Example of using the composite shell lining system
Project: Metro Lausanne, Switzerland
The final structural lining design of the Tunnel de Viret, part of the extended M2 line of the Metro Lausanne, was built using the composite shell lining system. The main challenge was to excavate underneath the city, including the cathedral and the ground water table beneath it. For this reason, a waterproofing solution which minimizes the risk of water drainage was needed to avoid ground settlement and damage to the cathedral.
Permanent sprayed concrete was used with the MasterSeal 345 spray-applied waterproofing membrane, creating a composite shell lining. The thickness of the secondary lining was considerably reduced in comparison to the original lining design, leading to a 23% reduction in the total lining thickness. No defects or water ingress have been reported to date since construction in 2008.
Additional project benefits:
- Considerable cost savings per linear tunnel meter
- Reduction of construction time by two months
- No formwork needed for the secondary lining through use of sprayed concrete
Permanent sprayed concrete was used with the MasterSeal 345 spray-applied waterproofing membrane, creating a composite shell lining. The thickness of the secondary lining was considerably reduced in comparison to the original lining design, leading to a 23% reduction in the total lining thickness. No defects or water ingress have been reported to date since construction in 2008.