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  A new drilling pattern of contour blastholes for overbreak control

  The problems of overbreak and underbreak exist widely in drilling and blasting tunnels. The traditional contour blastholes (TCB) pattern has disadvantages in controlling overbreak and underbreak due to the impact of the initial support. For example, it generally increases the look-out angle and causes the overbreak. However, its effect is rarely mentioned in previously published studies. Based on it, this paper proposed the new long and short associate contour blastholes (LSACB) pattern to solve the problem. After that, ALE numerical models were developed in LS-DYNA to discuss the rock damage evolution of the LSACB pattern. Finally, two patterns were applied in the project of a certain tunnel in Qingdao, China. The measurement results of contour profiles showed that the LSACB pattern was better than TCB pattern in overbreak control. This paper not only showed that the initial support was indeed the primary cause for overbreak but also effectively solved the problem, which provided an excellent reference to similar projects.

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