Consideration of in-plane tensile stresses in precast tunnel segments induced by thrust forces

Consideration of in-plane tensile stresses in precast tunnel segments induced by thrust forces. In-plane tensile stresses due to concentrated jacking loads induced by the tunnel boring machine (TBM) in precast concrete (PCC) tunnel segments is a subject that is rarely found in literature. This is mainly due to TBM advance being a temporary construction state leading to local tensile stresses with the segment reinforcement required typically does not exceed the predefined minimum reinforcement specified e.g. in design standards and guidelines. The relevance of the in-plane stress state is often misjudged although the related tensile stresses govern with respect to the majority of damages, i.e. cracks in segmental linings, according to our experience in tunnel projects over many years. A careful consideration of this load case is necessary, especially in cases where the minimum reinforcement may be decreased or an alternative type of reinforcement such as steel fibres is utilised. The present article provides some useful tables for analysing in-plane tensile stresses due to TBM-jacks (without further numerical calculations) considering the governing input parameters. The tables provide a quick method to optimise the various jack configurations and jack shoe sizes and may also be used as the basis for detailed design. In addition, the consideration of unavoidable tolerances in the structural system is discussed. For these cases guidance is provided regarding the structural models of the relevant load situations and the different design philosophies, in particular for calculation of the crack width. A brief discussion in the context of the divergent requirements (cost-efficient construction, robustness) and recognition of relevant influences follows. Finally, the authors propose an engineering design strategy for this particular load case.