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.
Highlights
High explosives have been widely used in hard rock mining and various kinds of rock engineering for over one century
Since the 1970s, one more viewpoint has been found to be more acceptable on the mechanism of rock fragmentation. This viewpoint states that it is the combined effect of both stress wave and gas pressure that dominate rock fragmentation (e.g., Kutter and Fairhurst 1971; Field and Ladegaard-Pedersen 1971; Bhandari 1979; Dally et al 1975; Fourney et al 1993; Fourney 2015). Another reason for the unsatisfactory blast results is that many basic parameters in rock blasting have not been well determined such as crack propagation velocity, gas penetration speed, and characteristics of stress waves induced in the rock
The light areas in frame 1 (F1) and frame 2 (F2) in Fig. 2 are probably the regions occupied by the shock waves, i.e., these light areas are not gases
Summary
High explosives have been widely used in hard rock mining and various kinds of rock engineering for over one century. 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). Another reason for the unsatisfactory blast results is that many basic parameters in rock blasting have not been well determined such as crack propagation velocity, gas penetration speed, and characteristics of stress waves induced in the rock. To understand the mechanism better and improve blast results, various model blasts have been carried out, dealing with stress waves, gas pressure, and fragmentation
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