Static response of double shell concrete lining with a spray-applied waterproofing membrane

The New Austrian Tunnelling Method (NATM) is widely used for the design and construction of tunnels and one of its defining characteristics is double shell lining. When evaluating the long-term stability of a tunnel, the primary lining of sprayed concrete is not considered due to concerns about long-term alternations in properties; only the secondary lining is used when assessing long-term stability for permanent loads. However, several studies have now shown that primary sprayed concrete linings (SCLs) have reasonable mechanical properties even after years of operation and can therefore be used in evaluating long-term stability in low aggressive environments. Because of this, considerable savings might be achieved in future stability evaluations by taking into account the shear capacity of spray-applied waterproofing membranes applied between both linings. Such double shell linings can be considered to be compound structures able to withstand permanent loads.