Cutting force measurement and analyses of shell cutters on a mixshield tunnelling machine

In this study, a real-time measurement method for cutting forces of a mixshield tunnelling machine is proposed. The strain of the cutter saddle is monitored and used to back-calculate the forces acting on the cutter. The normal cutting forces are primarily analyzed in this study. Four resistance strain gauges are placed on the two free side surfaces of the saddle with effective protection, two strain gauges among which are installed in the vertical direction on one free side surface, and individual strain gauge is installed in the vertical and horizontal directions on the other free side surface. Then strain gauges are punched out from the cutter casing and sealed to connect with the data acquisition system. DataTaker DT80G intelligent data logger is employed as the core of the data acquisition system. The data logger is powered by a chargeable battery, and equipped on the back of the cutter-head and rotates with the cutter-head synchronously, which ensures continuous monitoring. The feasibility, durability, and stability of the monitoring system are calibrated and demonstrated. Subsequently, this measurement method has been implemented on the Herrenknecht mixshield machine at the Sanyanglu Tunnel in China. Monitoring was performed from chainage at No. 416 to No. 432 ring segmental lining, which lasted more than 20 days. A complete data set and corresponding tunnelling parameters of the mixshield were acquired. Statistical and trend analyses are carried out, the distribution along the installation radius, as well as the correlations between the cutting force and tunnelling parameters are analyzed. Results show that: (1) the cutting force decreases with increasing installation radius, center cutters experience maximum loading, panel cutters experience relatively equal moderate loads, and edge cutters experience the smallest loads; (2) the cutting force increases with penetration rate following a linear increasing function in general, where the cutting force mainly ranges from 20 to 70 kN; (3) after new cutter replacement, the increasing cutting force is in accordance with the increasing cutter-head thrust forces, however with continuous excavation, the increase in cutting force is slightly lower than the cutter-head thrust force, which may be due to cutter wear and mud cake wrapping; (4) the consistency between the cutting force and the cutter-head thrust force is better than the consistency between the cutting force and the cutter-head torque.