The three most predominant methods for hard rock excavation and fragmentation are the use of explosives, mechanical impact/cutting and hydraulic fracturing. However, those methods have inherent drawbacks, such as non-applicability or poor performance in extremely hard and abrasive rocks. Novel rock fracturing and fragmentation methods are in need to either work individually or in combined forms to break rocks. Research shows that some rock forming minerals and water can be heated up rapidly by microwave, to induce microcracks and fractures in rocks. Microwave therefore can be regarded as a promising technology of hard rock fracturing and fragmentation, with the potential of energy and cost efficiency. This keynote first provides a brief review of the research on microwave effects on rock fracturing, followed by descriptions of experimental studies of crack formation in different rocks treated by a low power industrial microwave. Possible fracturing mechanisms by microwave treatment are discussed, and the applications of microwave treatment assisting rock excavation coupled with mechanical means are outlined at the end of this keynote.
Fracturing and fragmentation of hard rocks is one of the most important tasks in rock engineering in the fields of mining, petroleum, and tunneling industries. While drilling and blasting remains to be the most powerful and effective method to break hard rocks, the uses of explosives are often limited by various constraints. Hard rocks are excavated and fractured by other means, including mechanical cutting and drilling, hydraulic fracturing and heating. Mechanical cutting becomes a dominant method for large scale rock excavation, particularly in tunneling, with the extensive uses of tunnel boring machines (TBMs) and roadheaders. However, hard rocks can pose great challenges to mechanical cutting and fracturing due to the extreme hardness and abrasiveness, which often lead to reduced advance rates and increased cutting tool wear. Other methods of rock fracturing and cutting have been investigated, including waterjet, laser, millimeter wave and microwave, as a sole mean for rock cutting or in combination with mechanical excavation.
New technologies explored for possible use of rock fracturing and cutting are the electrical methods and the electromagnetic (EM) methods. The electrical methods include plasma blasting, electron beam and electric current. The EM methods use electromagnetic waves, including laser cutting, infrared irradiation, torch heating, and microwave heating. Mostly those technologies are investigated as a sole mean for rock fracturing or vaporization. The study presented in this keynote uses microwave technology to induce microcracks in hard rocks hence to ease the mechanical excavation. The objective is on adapting and developing low powered microwave tools to assist mechanical excavation of hard rocks, as an economical alternative.