Sergo Ordzhonikidze Russian State Geological Prospecting University, Moscow, Russia:

V. M. Borychev, Senior Researcher, Candidate of Engineering Sciences
S. V. Ilyakhin, Professor, Doctor of Engineering Sciences, isv11@mail.ru
O. S. Bryukhovetsky, Professor, Doctor of Engineering Sciences

One of the main parameters of drilling and blasting is the value of overdrill. This value is usually 10–20 % of the total blast hole length, while the volume of the explosive in the overdrill can reach 50 % of the whole charge. Therefore, significant parts of the hole and charge are beyond the effective blasting zone in a bench. The choice of the optimal overdrill value will allow reduction in volume of drilling, rational placement of charge along the bench height and elimination of excessive destruction of the lower lying bench top. The analysis of the overdrill formulas known from literature shows that it impossible to compare these formulas due to different views of some influencing factors. For this reason, the formulas produce approximate results mostly, and the scatter of calculated data may reach 64 %. Thus, it is necessary to find a relation that takes into account the whole set of all basic influencing factors. As a result of theoretical processing of the existing formulas for determining the value of overdrill, the main dependence between the mine-technical conditions, drilling-and-blasting design and overdrill size has been obtained. The scope of the analysis embraces the effect of bench height hole diameter, rock hardness, charge placement conditions in hole and other factors. For this purpose, factual data were collected from 63 mines with various geotechnical conditions. There were 8 groups of rock hardness, and blast holes were group with respect to diameter: 105–115 mm; 145–165 mm and 250 mm. As a result of the performed analysis, the overdrill value formula has been finalized. The variation factor for the actual and calculated values is 21 %.

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