Integrated waterproofing evaluation method for longitudinal joints of shield tunnel subjected to extreme surcharge: Numerical analysis and experimental validation

Sufficient waterproofing performance is critical to ensure a normal operation and safety of tunnels, and certain safety margin is essential to cope with uncertain external loads. This study proposes a novel methodology for evaluating the waterproofing performance of shield tunnels subjected to extreme surcharges, by integrating an analytic derivation, numerical analysis, and experimental validation. First, the analytical derivations of stresses at longitudinal joints and a finite element (FE) model for calculating the internal forces of tunnel linings under external loads are established; these are subsequently combined to compute the gasket deformation. Then, the stress-deformation curve of the gasket is fitted by a material-scale FE model to evaluate the contact stress against the underground water pressure. Finally, the methodology is validated via measurements from a full-scale experiment. The results help establish that the presumptions derived from the proposed method are consistent with the experimental results. It is also anticipated that the waterproofing performance would reach the actual failure point at the approximate 4-7 m depth of the equivalent dumped soil, which is remarkably consistent with the on-site observations in the Shanghai Metro tunnel. This study provides an efficient and economical method in terms of engineering applications, with emphasis on the importance of structural analysis in the waterproofing evaluation.