Pit & Quarry, February 2019
as a stemming material The rules of thumb on stemming length remain the same as in 1963 The stemming ratio should be about 07 when using crushed stone when using other materials the stemming ratio should be 10 to 12 This may still result in blowout of the boreholes SPACING RATIO Commercial blasting typically requires the use of multiple blastholes making it necessary for blasters to know whether or not there are any mutual effects between charges The effect between boreholes is determined by the spacing ratio of a blast Blasters often do not fully consider this spacing ratio Instead powder factor or basic design patterns are used For example a blaster may want to use a powder factor of 11 pounds per cu yd After designing the burden and borehole the blaster will then set the spacing to give this powder factor This is a poor method of design as it is well understood now that powder factor is not an appropriate design tool It is more of an economic tool Another common approach is to use a set pattern such as an equilateral triangle pattern These patterns typically come into play because they work well in a certain situation by giving the appropriate spacing ratio for the situation On a pattern such as the equilateral triangle pattern the spacing ratio is roughly a 115 This is about what would be used for benches around a stiffness ratio of 25 and firing each hole delayed Still when going to a much higher bench this pattern performs poorly Good blasters need to understand this spacing ratio to provide insight in designing a blast to achieve a proper borehole interaction In 1963 Ash wrote about the interaction of boreholes that was based on the timing of the blast Generally the spacing ratio would vary between 10 and 20 with the basic rules of thumb being 1 For extremely long delays KS should equal 10 2 For hole firing instantaneously along a row the KS should be equal to 2 3 For holes firing on a delay the KS should be between 10 and 20 and typically between 12 and 18 This is because the blasting is four dimensional as timing is a crucial part of the design Changes in timing lead to different effects and stress fields in the rock The spacing and timing then determine the fragmentation Should the spacing be Real men weave wire Accuracy Quality Delivery Real people make sure your screen media can withstand the toughest screening application We know down time means deadtime Count on Unifieds team to help you meet the demand Visit us at Booth 38012 pitandquarry com February 2019 PIT QUARRY 67
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