Fracture Initiation, Gas Ejection, and Strain Waves Measured on Specimen Surfaces in Model Rock Blasting
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ORIGINAL PAPER
Fracture Initiation, Gas Ejection, and Strain Waves Measured on Specimen Surfaces in Model Rock Blasting Zong‑Xian Zhang1 · Li Yuan Chi2 · Yang Qiao1 · De‑Feng Hou1 Received: 3 June 2020 / Accepted: 30 October 2020 © The Author(s) 2020
Abstract Crack velocity, gas ejection, and stress waves play an important role in determining delay time, designing a blast and understanding the mechanism of rock fragmentation by blasting. In this paper, the emerging times of the earliest cracks and gas ejection on the lateral surfaces of cylindrical granite specimens with a diameter of 240 mm and a length of 300 mm were determined by high-speed photography, and the strain waves measured by an instrument of dynamic strain measurement during model blasting. The results showed that: (1) the measured velocity of gas penetration into the radial cracks was in a range of 196–279 m/s; (2) the measured velocity of a radial crack extending from the blasthole to the specimen surface varied from 489 to 652 m/s; (3) the length of strain waves measured was about 2800 µs, which is approximately 1000 times greater than the detonation time. At about 2850 µs after detonation was initiated, gases were still ejected from the surface cracks, and the specimens still stood at their initial places, although surface cracks had opened widely. Keywords Model blasting · Rock fracture · Gas ejection · Stress wave · High-speed camera
1 Introduction High explosives have been widely used in hard rock mining and various kinds of rock engineering for over one century. However, up to now, blast results in hard rock mining and rock excavation have not been satisfactory yet. For example, energy efficiency in rock blasting has been very low (Langefors and Kihlström 1963; Ouchterlony et al. 2004; Sanchidrián et al. 2007), and blast operation has been dominated by empirical design which results in considerable mineral loss, poor safety, high vibrations, explosive wastage, and induced seismic events (Zhang 2016). One reason for the unsatisfactory blast results is that the fragmentation mechanism in rock blasting has not been very clear so far, although detonation theory and blasting science have been developed for several decades (e.g., Johansson and Persson 1970; Langefors and Kihlström 1963; Persson et al. 1994; Cooper 1996; Fickett and Davis 2000; Zhang 2016). In the earliest studies on rock * Zong‑Xian Zhang [email protected] 1
Oulu Mining School, University of Oulu, 90014 Oulu, Finland
State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100080, China
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blasting, there were two viewpoints on the mechanism of rock fragmentation. One viewpoint considered that stress wave played a predominant role in rock fragmentation (e.g., Hino 1954; Duvall and Atchison 1957), and the other indicated that high-pressure gas took the dominant part in rock fragmentation (e.g. Langefors and Kihlström 1963; Clark and Saluja 1964). Since the 1970s, one more viewpoint has been found to be more acceptable on t
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