Simulating composite behaviour in SCL tunnels with sprayed waterproofing membrane interface: A state-of-the-art review

•Sprayed waterproofing enables tension and shear transfer between lining layers.•load transfer can give efficiency gains in lining design, but not in all cases.•A guaranteed benefit of interface tensile bond is sharing of groundwater loading.•Composite action can induce tension in secondary from bending, reducing its capacity.•Concern about water saturation of membrane can be addressed by limiting water head. Introduction of sprayed waterproofing has led to innovation of composite sprayed concrete lined (SCL) tunnels, where placing the waterproofing between primary and secondary layers gives potential for composite structural action, by transmission of tension, compression and shear stresses across the interface. Numerical analysis is required to design such structures taking into account soil-structure interaction and staged construction, but there is currently very limited guidance on how to conduct such analyses. This paper reviews use of numerical analysis to simulate composite SCL tunnels, focussing on soft ground tunnelling. It introduces types of sprayed membrane, their benefits in design and current industry practice for simulating the sprayed membrane interface. Numerical strategies for simulating composite action and their verification against laboratory test data are then described. Recommendations are made of design principles to optimise design of SCL tunnels with spray-applied waterproofing. Further opportunities for research on this topic are discussed.