Pre-split blasting is a primary technique for creating slopes with a minimum amount of unwanted damage to the remaining rock mass. Rock slopes created in such a way have both safety-related and economic advantages. The applications extend from road cuts to rock quarries to large open pit mines. For example, as deep open pit mines approach the end of their operating lives, slope steepening is one option to be considered. To be able to safely achieve and maintain steepened final slopes, good blast design techniques must be available and applied together with close operational control. Pre-split blasting is an important element in this slope creation process.The effective use of de-coupled charges in the perimeter blast holes is an important ingredient for achieving the desired result. The present procedure for selecting the hole size, hole spacing and explosive combination used for pre-split blasting is largely based on work done more than 30 years ago by Sanden (1974). It assumes that the explosive gases expand adiabatically within the annulus separating the charge and the hole wall. Recent work by Meng (2005) has shown that the expansion in the annulus occurs isothermally rather than adiabatically. This affirms the writings of Hino (1959) and Johansson and Persson (1970). The result is that the basis for the currently used pre-split design procedure must be re-assessed and appropriately modified.The paper examines in some detail hole spacing, explosion pressure, borehole wall pressure and rock properties with regard to pre-split design. A practical isothermal expansion-based approach for calculating the pressure on the borehole wall is presented.A simple approach to pre-split design is then presented based upon using commonly available explosive and rock properties. The practical application of the new technique is demonstrated through an example.
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