Torque Calculation of Shield Cutters in Gas-Bearing Strata and Its Effect on Surface Deformation

When a shield tunnel advances through gas-bearing strata, shallow gas can adversely affect normal construction operations. Considering the characteristics of the matric suction in gas-bearing soils, apparent cohesion is categorized into three stages. For the first time, the shear strength theory of unsaturated soils is applied to calculate the cutterhead torque in gas-bearing formations. The study results demonstrate strong agreement between theoretical calculations and field measurements, enabling accurate prediction of the maximum cutterhead torque during shield tunneling in such conditions. A parameter analysis reveals that the cutterhead torque increases by 39.4%, 26%, and 15.6% in gas-bearing soils with pressures of 300, 200, and 100 kPa, respectively, compared with saturated soils. After gas release, the torque in the soil is only 6.2% higher than in saturated soils and 23.8% lower than before degassing, indicating that controlled gas venting significantly enhances tunneling efficiency. To further evaluate the effects, the torque penetration index (TPI) is introduced. A positive correlation is observed between soil deformation and TPI fluctuations. During twin-tunnel construction, secondary disturbances occur in gas-bearing soils, with the leading tunnel exhibiting greater deformation and a higher TPI than the trailing tunnel, contrasting sharply with conditions in non-gas-bearing soils.