Study of the effect of empty holes on propagating cracks under blasting loads

In large scale rock blasting operations, such as in mining, quarrying and tunneling, there are usually a number of blast holes simultaneously, and the explosive charges are fired in different sequences. This indicates that the later-fired holes may have effect on the propagation of the cracks induced by the first fired blasting. This paper tried to investigate the effect of two empty holes on the behavior of the cracks propagating between them, and blasting experiments were conducted by using circular sandstone and PMMA specimens with a centralized blast hole, a pre-crack and two empty holes. The two-hole spacing was designed in the range from 10 mm to 45 mm, and the outer diameter of the circular specimens was 600 mm for sandstone and 400 mm for PMMA. The central blast hole was charged with a detonator and it was fired to provide dynamic loading which drives the pre-crack to propagate. Meanwhile, by using linear equation of state and the major principal stress failure criterion, numerical models were established by using AUTODYN code, and the effect mechanism of empty holes on crack propagation behavior was studied numerically. The crack propagation paths predicted by the numerical models agree well with the experimental results. The results show that: (a) Empty holes have the arrest function on outgoing cracks, and the arrest function depends on the two hole spacing; the shorter the two hole spacing is, the stronger the arrest function is; (b) As two hole spacing is less than a certain value, outgoing cracks can be completely arrested; (c) The stress perpendicular to the propagating crack plays a key role in arresting cracks.