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Over break and under break are important measures of conformance to the designed perimeter in a civil or
mine tunnel excavated by blasting. Three randomised paired trials of a new low-density bulk explosive
compared with 70 g/m detonating cord loaded in the perimeter were made in different development headings
of an underground zinc mine. The low-density bulk perimeter explosive produced more over break but less
under break in two of the three comparisons with the detonating cord. However, a statistical analysis on the
small number of paired comparisons indicated insufficient power in a paired t-test and further paired
comparisons are recommended. Rather than the few well defined half-barrels of the detonating cord,
characteristic scorch/crush zones were evident in the low-density bulk explosive rounds and miners observed
somewhat increased barring down time in these blasts. Such observations are understandable due to the
higher effective perimeter charge of the low-density bulk perimeter explosive. The maximum measured over
break was less than 12% and the maximum measured under break was less than 8% for either blasting
system. The miners stated that the speed of loading full faces was faster with the low-density bulk explosive
system, compensating for longer barring-down times. Work is ongoing to improve the low-density bulk
explosive as an alternative for mines seeking rapid development mining and tunnelling.
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