Vertical attitude and its control strategy for shield tunneling in soft soil

This study proposes a method for calculating the loosened soil pressure applicable to shield tunneling in cohesive soil layers based on the modified ellipsoid theory as a solution to existing difficulties in attitude control of shield machines when traversing soft and weak strata such as loess and coastal soft soil. Specifically, we established a theoretical model for shield-machine-soil interaction using equivalent soil springs to analyze the vertical displacement, pitch angle changes and attitude adjustment of the shield machine. By comparing with the construction monitoring data of the shield machine traversing the loess plateau in the South Line of Hanjiang-to-Weihe River Water Diversion Phase Ⅱ Project, we verified the accuracy of this theoretical model in predicting the attitude changes and correcting thrust calculation of shield tunneling in soft soil strata. Sensitivity analysis of parameters shows that 1) the vertical displacement and vertical force of the shield machine exhibit a parabolic variation; 2) the pitch angle of the shield machine and the vertical plane torque show a cubic parabolic variation. With the increase of torque, the attitude changes from downward pitch to upward pitch; 3) as the proportion of soft soil in the strata increases, the settlement and downward pitch of the shield machine increase; 4) adjusting the propulsion system generates low significant thrust difference in the upper and lower parts of the shield tail, which could realize settlement control of the shield machine and correction of pitch angle.