Dynamic Simulation and Optimization of Synchronous Grouting for Shield Tunnels in Cavity‐Containing Strata

In water‐rich strata containing cavities, flowing water washout presents a significant challenge for synchronous grouting in shield tunnels. This washout compromises the filling efficacy of the shield tail void and the cavities behind the lining. Consequently, it can trigger severe engineering risks such as ground settlement. To address this issue, a dynamic CFD simulation was employed. The study evaluated the effects of grout material parameters, construction parameters, and flow velocity on both grout diffusion and antiwashout performance. The results indicate that cavities act as high‐permeability channels. These channels reduce diffusion resistance and expand the grout–soil contact area. As a result, the volume and distance of grout diffusion into the stratum significantly exceed those observed in cavity‐free conditions. Furthermore, grout diffusion morphology and retention capacity under flowing water are governed nonlinearly by the W/B and Ben/W ratios. The influence of B/S and C/F, however, remains negligible. Increasing the W/B ratio enhances the grouting volume and diffusion range, but it compromises antiwashout performance due to reduced viscosity. In contrast, the Ben/W ratio effectively mitigates ineffective grout dissipation through its swelling and thickening effects. This ratio serves as the most sensitive parameter for controlling washout in flowing water. Regarding construction parameters, elevated grouting pressure promotes permeation and compensates for energy dissipation caused by flowing water. Extending the shield tail void formation time improves diffusion by prolonging infiltration. However, this optimization effect is secondary to that of grouting pressure. Finally, continuous shear scouring by flowing water induces significant boundary shrinkage and mass loss on the upstream side of the grout. When the flow velocity exceeds 0.40 cm/s, both diffusion and filling loss rates surge, indicating a structural failure of the grout skeleton.