Even with the most advance blasting techniques, the presence of oversized boulders appears to be inevitable. Secondary rock-breakage operations impose undesirable extra costs. Secondary blasting is time-consuming and can damage the infrastructure, whilst impact hammers are cumbersome and cannot be used where boulders are not readily accessible. Amongst various types of high-pressure, low-volume water-jets introduced for cutting rocks, pulsed water-jet has demonstrated ability to break large boulders of competent hard rocks. Pulsed water-jet, with high power intensity and a low reaction force, can potentially provide an effective alternative tool for secondary rock-breakage. However, widespread use of the technology in practice relies on comprehensive understanding of the rock-breakage mechanism of such device.

Effective rock-breakage by a pulsed water-jet depends on the shape, velocity and impact stresses imposed by the individual water pulses. Moreover, the water-slug length and the separation distance between sequential slugs influences the breakage. This paper investigates the rock fracture process by a pulsed water-jet device to recognize the relative contributions of the slug-length and pulsation frequency on the generated local and internal damage. The experimental studies have been carried out on marble and granite samples. The cracks that were formed as a result of high-intensity impacts propagated by the water flows from the individual water-pulses, leading to major rock-failure. The length of the water-slug mainly controlled this process. The results also indicated and confirmed the occurrence of internal damage in tested samples. Both the slug-lengths and the pulsation frequencies of the water-jets were found influential; however, depending on the rock type, one played a more significant role than the other. An understanding of the individual and interactive effect of the slug-length and pulsation frequency on the failure process of a rock target is a prerequisite to the design and operation of an efficient pulsed water-jet apparatus.

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