Dynamic Strength and Indentation Hardness of a Hard Rock Treated by Microwave and the Influence on Excavation Rate

Mechanical excavation in hard or very hard rocks is still challenging in rock engineering. One plausible solution is to reduce rock strength prior to mechanical excavation to increase excavation and cost-efficiency. The present study utilizes a 6 kW microwave to irradiate rock specimens and evaluates the variation in rock strength and indentation hardness. Since the contact between the drill bits of roadheaders and the rock surface is a dynamic impact process, dynamic tests are conducted and compared with the quasi-static results. The experimental results demonstrate that short microwave irradiation (30 s) can produce substantial damage to the rock. The reduction of uniaxial compressive strength, Brazilian tensile strength, and indentation hardness under quasi-static loading can be about 39%, 43%, and 44%, respectively, and 33%, 26%, and 50%, respectively, under dynamic loading. In addition, it is found that the tensile strength is the most sensitive to microwave, while the crack density is the least sensitive. More importantly, this reduction is not linear and not monotonic to the irradiation time. The fast reduction of rock strength and indentation hardness after microwave irradiation implies the reduction in the drill-bit wear and increase in the excavation rate, and vigorously demonstrates the great potential of microwave-assisted mechanical excavation of hard or very hard rocks.