The compressed air method in shield tunneling exhibits significant advantages in underwater tunnel construction

yet it lacks a comprehensive theoretical analysis model. A theoretical model for the relationship between air pressure in the shield chamber and the inflow and outflow of compressed air is firstly established

based on the principles of mass conservation and the ideal gas law. A dynamic air pressure boundary at the tunnel face is used to integrate the theoretical model with a numerical model of the surrounding rock that considers gas-liquid two-phase flow. The model is validated through field data. Analysis of key factors affecting chamber pressure during shield shutdown reveals that chamber pressure initially increases

then decreases

and eventually stabilizes upon compressed air injection. Higher air injection rates lead to increased peak and stable chamber pressures. Compressed air within the chamber reduces the rate and amplitude of pressure fluctuations

with a larger volume of air amplifying this effect. The compressed air method is best suited for strata with low permeability. As the permeability coefficient of the stratum increases

the ability of stratum to contain compressed air decreases

leading to the formation of air discharge channels extending to the ground surface. In strata with higher permeability coefficients

abruptly halting air injection can cause a rapid drop in chamber pressure and groundwater influx

threatening construction safety.