The characteristics and mechanism of microwave-induced subsurface fracturing of hard rock at different burial depths

Microwaves can be used to fracture hard rock and have the potential to be combined with tunnel boring machines. Deep engineering rock mass is usually in a true triaxial stress state. The microwave-induced subsurface fracturing characteristics and mechanism of hard rocks at different burial depths are unclear and need to be studied for microwave application in deep engineering. In this work, using a true triaxial microwave-assisted dual-mode mechanical cutting test system, microwave irradiation tests of Chifeng basalt under different true triaxial stress states were carried out. The acoustic emission signals were monitored in real time during microwave irradiation. After microwave irradiation, the ultrasonic wave velocities were measured, and mechanical cutting tests were also conducted. The experimental results reveal that ejected rock fragments and cracks appear on the basalt surface under microwave irradiation at different true triaxial stress levels. As the true triaxial stress level increases, the ejected rock fragment mass, the area of the spalling zone, and the depth of the spalling zone center of the samples after microwave irradiation tend to increase; the total crack number and length tend to decrease; the P-wave velocity reduction rate on the sides of the sample and the cumulative acoustic emission counts exponentially decrease; and the cutting rock debris mass tends to increase. The increase in true triaxial stress promotes the formation and expansion of the spalling zone and inhibits the development of side cracks; the P-wave velocity reduction rate cannot fully reflect the cutting effect. The research results can provide guidance for the design of microwave parameters and the evaluation of microwave cracking effects under true triaxial stress conditions.