Effects of confinement and microwave irradiation duration on weakening strength of hard rocks

The sustained and intensive exploitation of shallow resources has driven into deeper regions. Nevertheless, traditional rock-breakage techniques are insufficient to meet the requirements of deep rock excavation. Microwave-assisted rock breakage is expected to provide a promising approach to overcome the challenging issues of low fracturing efficiency and high crushing difficulty in deep complex geological environments. However, efficient fracturing of deep hard rock generally occurs subjected to high ground temperature, high ground stress, and high pore pressure, with high ground stress. Therefore, we tried to investigate the effects of confining pressure on microwave fracturing of hard rock, and experiments were performed on samples exhibiting varying degrees of fracturing under different confinements. The results showed that with increasing microwave irradiation duration, the maximum surface temperature of the samples rose from 100–200 °C at 4 kW for 30 s to 330–380 °C at 120 s. Due to heat conduction and thermal radiation, the average heating rate decreased with longer microwave irradiation, from 7 °C/s at 30 s to 2.8–3.2 °C/s at 120 s. The inhibitory effect of confining pressure on the strength of monzonite is significant, with strength decreasing by up to 37.73% at 4 kW microwave power for 120 s compared to 30 s in uniaxial mode, and by 3.75% at 10 MPa, 8.45% at 20 MPa, and 3.86% at 30 MPa. This demonstrates that confining pressure is a critical variable when fracturing deep hard rock. These findings will contribute to advancing the application of microwave fracturing in deep engineering projects.