Numerical investigation of segment uplift induced by fresh grout in shield tunneling: an immersed boundary-based fluid–structure interaction model

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During shield tunnel construction, the initial buoyancy generated by synchronous grouting frequently induces segment uplift, which may lead to defects such as misalignment and cracking. These defects compromise construction quality and threaten the long-term structural safety of the tunnel. Conventional numerical models typically simulate grout consolidation using an equivalent layer method, but they cannot accurately capture the dynamic spatial distribution of the grout or the fluid–structure interaction (FSI) between the grout and the segments. To overcome this limitation, this study develops an FSI model for segment uplift based on the direct-forcing immersed boundary method (IBM). The model represents the segment–grout interface via IBM, discretizes the governing equations with a collocated-grid finite volume method, and solves the flow field using the PISO algorithm. A time-dependent viscosity function is introduced to simulate the progressive consolidation and hardening of the grout. The model is validated against field measurements from two shield tunnels of different diameters: Hangzhou Metro Line 7 (6.2 m) and Jinan Rail Transit Line 9 (6.4 m). For the Hangzhou case, the average relative error across five monitored rings is 8.58 %. For the Jinan case, the proposed IBM-FVM model is quantitatively compared with the widely used equivalent layer method, yielding relative errors of 2.71 % and 8.06 % against the measured values, which are significantly lower than the 8.53 % and 13.55 % produced by the equivalent layer method. The simulation results reveal distinct spatiotemporal characteristics of grout flow during uplift: it evolves from an initial layered pattern to a uniform directional flow during rapid uplift, and eventually reaches a steady state. The influence of shield attitude is further examined by varying the pitch angle, revealing a clear linear relationship between the final uplift displacement and the inclination angle; a downward inclination can effectively offset part of the uplift. An orthogonal experimental design combined with parametric simulations is employed to investigate key grout properties. Through targeted case comparisons, the interrelationships among apparent density, initial setting time, and initial viscosity are decoupled. The results demonstrate that the dominant rheological factor governing early uplift resistance is the initial viscosity, rather than the reduced buoyancy caused by grout density. Among the parameters, the water–binder ratio exerts the strongest influence. These findings provide a theoretical basis for optimizing synchronous grouting practices and controlling segment uplift in engineering applications.