Microwave assistance effect for rock breaking of TBM disc cutter using the coupled method of continuum and grain-based model

Microwave-assisted rock breaking technique can significantly weaken the strength and the stiffness of hard rock, and can consequently enhance TBM efficiency as well as relieve wear of disc cutters. Understanding the mechanism of microwave-assisted rock breaking requires knowledge of the intersection and connectivity properties of freshly formed fractures below the cutter and the fracture network resulting from microwave-induced damage. In this study, a numerical approach, coupled with the grain-based model (GBM) and continuum based microwave electromagnetic analysis, for simulating the microwave-treated granite is proposed firstly. Subsequently, the numerical experiment of TBM cutter penetration is conducted with the consideration of five levels of microwave power irradiation as well as two waveguide arrangements. The temperature field and fracture behavior are analyzed to investigate the thermal damage of the granite after the treatment of the microwave. Furthermore, the reduction of peak normal force and variation of average coordination number are examined to evaluate the impact of waveguide position on the effectiveness of microwave-assisted rock breaking. The simulation results indicate that the suitable power of microwave irradiation (≥ 30 kW) can promote the rise of granite temperature directly below the waveguide, further resulting in an increase in the quantity of microcracks and noticeable damage to the rock block. When the microwave power exceeds 30 kW, the peak normal force and average coordination number progressively start to decline for the case of a waveguide port facing the centerline of a single cutter. In the case of waveguide facing the gap between double cutters, the decreasing trend of the peak normal force is similar to that under the condition of single cutter, but the starting threshold has been raised to 45 kW. When the microwave power is below 60 kW, the average coordination does not change significantly. After comparing the improvement effects of microwave-assisted rock breaking under the two waveguide arrangements, a preferred condition of the microwave for breaking is determined. The waveguide port facing the centerline of a single cutter, and the microwave power of 30 ∼ 60 kW can lead a success in a satisfactory effect in term of the reduction rate of peak normal force and average coordination number.