Soft rock mines are distributed through out P.R.C. And drilling and blasting is still the main method of driving a tunnel in these mines. Traditional smooth blasting has been widely and deeply studied and put into proactice in P.C.R since 1970s. But the cartridge and blast hole are both columns, the stress field in the rock around blast hole at the beginning is symmetry around hole axis, and the original cracks in the rock can be activated randomly. So serious overbreak and underbreak at the boundary of an excavation occur often, especially when the excavation is in soft rockmass, and the rates of alf-hole marks are generally about 50
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