Strain-based design for segmental tunnel lining under thrust jack load

A strain-based design approach is proposed to assess precast concrete tunnel lining (PCTL) under thrust jack load exerted by a tunnel boring machine (TBM). This approach is intended to provide service-level PCTL thrust jack load capacities to mitigate TBM thrust-induced cracking. The stress-based methods currently adopted by design guides neglect compression-induced tensile strain and cracking by Poisson's effect. Hence, when concrete tensile strength is used as design criteria, stress-based approaches significantly overestimate service-level strength. A strain-based design approach is therefore developed via parametric examination of stress distributions in PCTL under thrust jack load, which can be used as an alternative or supplement to finite element analyses (FEA) in practice and consists of design charts accounting for variations in: (1) Poisson's ratios, (2) contact area between pads and segments, (3) number of pads and (4) segment geometries. The design approach was validated against FEA and full-scale experiments. The sensitivity to eccentric loading and segment curvature were also examined. Loading eccentricity induces higher radial bursting strain due to reduced contact between pads and segments, resulting in higher transverse bursting strain on the eccentricity side of the segment. Tunnel curvature has no significant effect on stress or strain induced by thrust jack loading.