Characteristics of microwave-induced borehole fracturing in hard rock with different heating rates and temperatures under true triaxial stress

Microwave-based destressing is regarded as a promising approach for proactively preventing and controlling rockbursts in deep hard rock. As the fracturing degree of microwave-induced boreholes is affected by borehole diameter, water content, mineral content, etc., it is difficult to establish relationships between them. The research aims to unify various factors with heating rate and temperature, and establish a microwave parameter design method based thereon. Tests on microwave-induced borehole fracturing in hard rock with different or similar heating rates and temperatures under true triaxial stress were conducted. The test results show that both heating rate and temperature promote radial fracture of the rock, but have little effect on the development of axial fractures. Compared with heating rate, temperature is a more critical factor influencing microwave-induced fracturing. The effects of the heating rate on rock fracturing become noticeable only at higher temperatures. When the heating rate and temperature are similar but the diameter of the boreholes is different, the crack distribution, total length, wave velocity attenuation, and fracture process are similar. It is feasible to reverse-design microwave parameters under different borehole diameters based on the heating rate and temperature. Thermal fracturing of basalt shows a distinct threshold effect between 150 °C and 195 °C (with an average of about 175 °C), and the heating rate and borehole diameter exert minor influences thereon. The results provide guidance for the design of microwave parameters in practice.