Pit & Quarry, May 2018
WASHING www kpijci com Putting a delay between the boreholes actually increases the fragmentation of the blast but does not throw the material as far Putting an extremely long delay hundreds to thousands of milliseconds can lead to larger fragmentation The next topic to understand is the row to row timing of a blast To help understand this imagine for a second your blast has a normal design burden of 10 ft For some reason the blast now was shot with the same amount of explosives as the 10 ft burden but a 20 ft burden was used What would be the result Well very poor breakage no throw of the muckpile vibration up to five times more than the normal burden extreme flyrock and high air overpressure Normally a good blaster or driller would never allow this to happen on the drilled burden But what about the timed burden It is critical to understand that we have numerous types of burden in a blast The drilled burden is the distance from the borehole to the nearest free face This is typically looked at in a plan view or in a two dimensional realm The toe burden is then the distance from the bottom of the borehole to the free face at the bottom of the bench This is typically the location of the largest burden throughout the powder column If this is not the largest burden area throughout the powder column would be considered the maximum drilled burden This view now looks at burden in three dimensions However blasting is actually four dimensional because the time the boreholes fire is taken into account The distance from the blasthole to the free face at the time the hole fires and pressurizes is called the actual or true burden This is the burden the borehole actually senses Therefore if a pattern is drilled firing straight on rows with a 10 ft burden but the timing row to row is too fast the true burden could actually be 20 ft Now this is obvious in cases of outof order firing due to cap scatter or poor hookup But what if the proper hookup and sequencing is used but the timing is too fast For example the previous pattern with a 25 millisecond timing between rows would have a true burden of 20 ft However by changing the timing to 42 milliseconds the true burden would now be 10 ft This could even be increased to a 67 millisecond delay between rows to allow for better breakage and throw of material This is the difference between timing a blast and sequencing a blast pitandquarry com May 2018 PIT QUARRY 29 PHOTO COURTESY OF ANTHONY KONYA Improper rhythmic timing practices can lead to worse blasts causing stemming blow out uplift poor backbreak and higher ground vibration Just modeling the timing does not necessarily give the best results Proper blast design and critical timing functions need to be incorporated
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