Numerical analysis for effects of single blast hole in mudstone rock-mass at Gotvand Olya dam

Document Type : Research Article

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Abstract

Drilling and blasting have numerous applications in civil and mining engineering. However, there are many unfavorable associated side effects and hazards, such as ground vibrations, air blasts, fly rocks, back-breaks, unwanted displacements, crack formation and propagation, and extended crushed zones, all of which need to be predicted and controlled effectively. Ground vibrations caused by blasting can damage the zones in the vicinity of the explosion block and its associated civilian structures and equipment. In addition to environmental and structural damages, air blasts can irreversibly damage the health by affecting the hearing sense and mental stability of the personnel.  Damages to the front face, caused by open-pit and underground explosions, not only increase the maintenance costs, but also make the appearance unacceptable. An important factor in reducing hazards in controlled blasting is the prediction of crack formation around the blast-hole and its propagation, which has been the subject of research since early 1950’s using field experiments, analytical methods and numerical simulations, paving way for many semi empirical correlations presented in the literature on this matter. As a rule of thumb, the radius of the crushed zone and the length of the radial cracks, are assumed to be in the order of 3 to 5 times and 40 to 50 times that of explosive radius, respectively. Hence, the radius of the crushed zone and the radial crack lengths were evaluated to be 10.16 cm and 114 cm, respectively. In this study, the results of the field studies from single blast-holes in the mudstones of Gotvand Olya dam were compared with several empirical correlations, using a blast-hole of 76 mm diameter, 2 m depth, 1 kg emulate 27 charge and a single instantaneous electrical cap. Two seismometers of VIBROLOC placed 8 m and 13 m away from the blast-hole recorded the ground vibration at 17.22 and 9.02 mms-1, respectively. The crushed zone radius and the radial crack length were measured to be 25 cm and 90 cm, respectively. The crack propagation and the ground vibration were compared with the field study results using a UDEC discrete element method. In the simulation exercise, the dynamic loading on the surrounding walls of the blast hole were assumed to be uniform and in radial direction. Also, the blast was assumed to take place instantaneously along the cylindrical charge and the semi-empirical relationship of Liu and Tidman was used to evaluate the maximum detonation pressure produced. The simulation results included a variation in the peak particle velocity with respect to the distance from the blast hole centre, a variation in the particle displacement, a variation in the applied stresses caused by the shock wave travelling, reflecting the stress wave from a free face. The numerical analysis indicated the crushed zone radius and the radial crack length to be 20 and 90 cm, respectively. Also, the ground vibrations at 8 m and 13 m distances away from the blast-hole were simulated to be 17.2 mms-1 and 9.27 mms-1, respectively. Amongst the empirical correlations used, Ash correlation (1963) revealed a radial crack length of 110 cm, and Essen et al. (2003) evaluated a crushed zone radius of 19 cm, indicating more accurate estimations. This study indicates that the numerical analysis used is capable of presenting acceptable accuracy.
 

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References

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Iranian Journal of Geophysics
Volume 6, Issue 2
September 2012
Pages 56-72

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