Calculation of foam injection ratio and regulation method of muck compressibility under shield soil chamber pressure conditions

The proper compressibility of shield muck is crucial for ensuring the safety and efficiency of shield tunneling. Foam conditioning is an effective method for improving the compressibility of muck. However, the amount of foam injected in practical construction is mostly based on experience or subjective judgment. A novel foam injection ratio calculation formula was derived based on Boyle’s law and the pore pressure calculation model. This formula comprehensively considers the impact of chamber pressure, pore water, soil gradation, and existing muck conditioning parameters on foam consumption. Starting with the premise that pore foam lifts the soil skeleton, reducing the effective stress in the soil to zero, a method for regulating the compressibility of muck in shield tunnel construction sites was proposed. This method presents the advantage of accurately quantifying the amount of foam injection. Verification results from field applications confirm that optimizing the muck conditioning parameters through this new method leads to improvements in workability, permeability, and muck compressibility, meeting the requirements for efficient shield tunneling. The fluctuation of chamber pressure is reduced, lowering the risk of tool wear. Additionally, there is a reduction of 29.96 % in average total thrust and 29.82 % in cutterhead torque, with a corresponding increase in tunneling speed by 24.42 %. Furthermore, the advantages and limitations of the proposed method were discussed.