RULE 1: The distance between holes (spacing) should not be greater than one-half the depth of the borehole.
When the effect of a blast is simulated on graph paper using an assumed or idealized angle of breakage of 90 degrees, the diagrams indicate that in each instance where the distance between holes in a row is greater than one-half the depth of the hole, the angles of breakage intersect so far above the bottom of the holes that the primary relief for each hole is to the surface. This causes both a great deal of vertical throw and a very uneven bottom. The greater the disparity between depth and spacing, the more pronounced the effect will be, to the point where the angles of breakage intersect above the surface of the shot.
RULE 2: In any blast where there is hole-for-hole delay, the spacing to burden relationship should be seven to five.
Several investigations about the science of rock mechanics have suggested that the optimum spacing to burden relationship should be 2:1, with the burden equal to one-half the spacing. Field experience shows that this relationship has two drawbacks. First, the blaster may assume that this relationship will apply when there is no delay system at work, when in fact, the optimum spacing to burden relationship in all instantaneous blasts should be 1:1 to ensure equal distribution of explosives in the blast. Second, if an instantaneous blast is fired with a spacing to burden relationship of 2:1, the back wall of the blasted area will, in most cases, be “sawtoothed.”
When delays are used, particularly when there is hole-for-hole and row-for-row delay, with no two holes firing on the same period, the angle of breakage approaches the idealized ratio of 2:1. The slight addition of burden avoids the possibility of “blowout,” or violent throw from relieved burdens during the shifting of burden from one hole to another.
RULE 3: Stemming should be equal to the burden.
The purpose of stemming, it has long been assumed, is to return the borehole to its original condition as much as possible in order to reduce noise, and possibly rifling at the top portion of the hole. Stemming also serves to confine and maximize efficient use of the explosive’s energy. If the explosive detonation process takes place up the borehole, the surface of the rock above the stemming is as much a free face (assuming there is a free face) as the free face that is parallel to the boreholes. If the stemming is greater than the burden, the rock at the top of the borehole will have less cracking from reflection and refraction of compressive and tensile waves. Then stemming should equal burden, and be of such material as to return the rock close to its original condition. Drill fines, tamped into the hole are ideal.
RULE 4: Subdrill (if necessary) should be between .3 and .5 of spacing.
Some investigators state that subdrill should be equal to .3 of burden. This is true in instances where spacing and burden are equal, such as with instantaneous blasts. It will also work when there is row-for-row delay. In blasts where the delay system is both row-for-row and hole-for-hole, however, the subdrill should be determined by the largest dimension, which is the spacing. An average subdrill of .4 of spacing is best to use for planning purposes.
