Pilot Tests on Thermal-Mechanical Composite Rock-Breaking Methods

In this paper, theoretical analysis and laboratory tests were performed on the thermal-mechanical composite rock breaking method to verify its feasibility. Thermal analysis was conducted on the rock finite element model by means of transient heat transfer to evaluate the effect of heating power on rock breaking. And then, simulated drilling tests and comparison tests were carried out by using the self-made drilling bits and test benches to verify the feasibility and rock breaking velocity increasing capacity of the thermal-mechanical composite rock breaking method. Based on the theoretical analysis, the penetration rate contributed by the heat of combustion was 18.0 m/h if thermal-stress rock cracking due to the heat was not taken into consideration. Based on simulation tests, the average penetration rate in granites was 24.5 m/h. And in the process of drilling, the aggregation state of rock compositions around the borehole was changed under high temperature, and consequently a ceramic layer of 1.8 mm thick was formed. And in the same test conditions, the penetration rate obtained by the thermal-mechanical composite rock breaking method was 8.3 times the conventional one. It was shown that brittle failure occurred on the rocks with lower coefficient of heat conductivity, tensile strength and shear strength under the effect of heat impact caused by heat of combustion, and the compressive strength dropped. And by virtue of the thermal-mechanical composite rock breaking method, drilling efficiency was increased significantly.