• Explosives and Blasting
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• v.21 no.2
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• pp.1-13
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• 2003

Overbreak or underbreak is one of the most important factors in evaluating the results of a tunnel blasting. Overbreak, which depends on the quality of rock, the type and quantity of explosives, and drilling conditions, has been a target of challenge to many blasting engineers because it directly affects construction cost. Drilling is generally known as one of the primary factors to generate overbreak. This study presents a real working model to reduce overbreak based on the analysis of drilling accuracy and overbreak generated from various working methods related to drilling. As the first step of the study, 45 experiments have been performed. The factors investigated are: marking contour line, the position of perimeter holes, the change of look-out with drilling rig position, and the proper space between perimeter holes. It is concluded that workers and engineers＇ will and efforts are the most important factors to reduce overbreak and that improving drilling method and pattern could reduce overbreak to a considerable amount.