User's Manual

83
RIOTRONIC X+
User Manual
9. Annex
9.1. Rules of thumb in blasting design Recommendations for timing
Some of the basic ‘rules of thumb’ for designing timing in quarrying are noted below. In each
situation, local site conditions will determine what the optimal timing should be. MAXAM
recommends that a competent blasting engineer, who is familiar with the ground conditions,
performs all timing design. Explosive rock interactions happen very fast. When an explosive
goes off in a drill hole in hard rock, the first thing that happens is that the blast hole expands.
This very rapid expansion causes compression and shear waves to travel out through the
rock at high speed. Depending on the rock type the compression wave can travel at speeds
between 2000 and 4000 meters per second. When this compression wave hits joints, the face,
or other faults, it is reflected as a tensile wave causing cracking to occur. The high-pressure
gases then travel into the rock mass, opening up existing cracks and extending them. Crack
propagation in hard rock is thought to travel at between 300 and 600 meters per second.
How does this affect timing in hard rock quarries? If the cracks from one blast hole are
travelling at a speed of 450 meters per second, they will reach the next hole in the row in
around 13 milliseconds (assuming a six-meter spacing). It is thought that, in order to promote
the best fragmentation, the next hole should fire before it is unburdened by the hole firing
before it.
Figure 111. Crack propagation in a blast
In fact, a relationship between timing along the row and fragmentation is thought to look
like the following (Figure 112).
Figure 112. Relation between timing and fragmentation