Auxiliary air pressure balance mode for EPB shield tunneling in water-rich gravelly sand strata: Feasibility and soil conditioning

Machine loads are always high, or soil conditioning is required to be rigorous when the pressure chamber is full of excavated soil during earth pressure balance (EPB) shield tunneling. This study investigates the feasibility and soil conditioning of the auxiliary air pressure balance mode for EPB shield tunneling in water-rich gravelly sand strata to decrease the machine load based on a project in China. It is indicated that the suitable slump value for the mode to avoid muck spewing should be lower than that for the tunneling mode with the pressure chamber fully of excavated soil. The water pressure monitored in the ground around the tunnel experienced rising, falling and stable stages during the EPB shield tunneling of each lining ring. The peak value of water pressure occurred immediately after the EPB machine stopped and decreased quickly during the first 30 min thereafter, followed by a gradual decrease. The time required for the water pressure to again reach its initial value was 70-90 min and did not influence the tunneling behavior of the following lining ring. The monitored ground displacement was much lower than its allowable value. The monitoring results indicated the success of the application of the auxiliary air pressure balance mode with the discharged muck with a slump value of 2-10 cm in the strata, satisfying the controls of ground displacement and advancing speed. However, the higher soil workability induced a lower thrust force and cutterhead torque, more stable cutterhead torque and chamber pressure, and a higher shield advancing speed. Hence, the ideal slump value was determined to be 8-10 cm for the studied project constructed by the EPB shield with the auxiliary air pressure balance mode in the water-rich gravelly sand strata. Finally, future research is recommended for the application of the tunneling mode.