Estimation of the cutterhead torque for earth pressure balance TBM under mixed-face conditions

•The cutterhead torque of modified EPB TBM in mixed-face ground is analyzed.•The force formulas for drag bits under mixed-face ground on cutterhead are derived.•In mixed-face ground, cutting torque is significant when hard rock has a greater proportion. In this paper, the cutterhead torque of the modified earth pressure balance (EPB) tunnel-boring machine (TBM) in mixed-face ground is analyzed. The theoretical model of the cutterhead torque of EPB TBM in single soft ground is generalized to the mixed-face ground. To determine cutting torque, a theoretical model for drag bits and disc cutters, which are suitable for mixed-face ground and nonhomogeneous distribution of drag bits on cutterhead, is established and the force formulas are derived. The components of cutterhead torque are investigated in detail. It is found from the results that the friction torques acting on the frontal surface, circumference and back surface of cutterhead, cutting torque, shearing torque acting on opening and agitating torque are important torque components and occupy a fairly large proportion. Moreover, in mixed-face ground, cutting torque induced by disc cutters is an important part, especially when hard rock has a greater proportion.