During the last two decades, significant progress was made in sprayed concrete technology, with advanced admixtures, as well as in the application of sprayed concrete, with sophisticated spraying robots and in waterproofing, with spray applied membranes. Also numerical design techniques have improved. All these factors have enabled designers to use sprayed concrete linings increasingly for long-term service life. In many cases the traditional double-shell lining system was replaced by the composite shell lining system, which consists of two concrete linings with a sprayed waterproofing membrane between linings. This paper introduces thisinnovative tunnel lining system and related design aspects. It also describes the main advantages of this lining system in comparison to the traditional double shell lining system.
