Spray-applied membranes for waterproofing of sprayed concrete tunnels have led to the possibility of shear transfer between primary and secondary linings through the membrane interface, with potential for reducing overall lining thickness. Laboratory tests have shown a reasonable degree of composite action in beam specimens. In this paper, a numerical model previously calibrated against such tests is applied to a whole tunnel, considering soil-structure interaction and staged lining construction. The model shows composite action and load sharing between the lining layers is expected in the tunnel as in the beams. Parametric studies over the practical range of interface stiffnesses show that composite action is maintained, although at high interface stiffness too much bending may be attracted to the secondary lining, requiring additional reinforcement. An efficient composite shell design with minimal additional reinforcement is achievable if the secondary lining thickness is reduced from current practice. Robustness of the system measured in terms of the interface’s ability to transfer stress under unequal loading, causing distortion on the tunnel, is found to be generally adequate. However, adjacent construction in close proximity may give insufficient margin on membrane tensile debonding, especially if membrane conditions are partially or fully saturated.\ud
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