1. Using Konya’s Method to Identify Blast Design Problems An overview of the Havablast 4 Spreadsheet

2. The Burden Field Burden is the distance from a hole to the nearest intended free face. Konya’s method uses information on your explosive energy, rock density, hole size, and special rock conditions indicated by your input to determine the amount of burden that would be expected under ordinary conditions. Firing with too little burden can be expected to cause casting, fly rock out of the face, blowout, and noise problems. Firing with too much burden can be expected to cause ground vibration, backbreak, poor fragmentation, and cratering. Special Rock Conditions are Handled by a Burden Adjustment. The Class notes indicate how things such as massive rock a dipping fractures should be handled. Burden adjustment should be based on the condition of the rock which is a feature of the site geology - not engineering design.

3. The Stemming Field Stemming is the inert material put into the top of the hole above the explosive column. Its function is to contain the throw of material and reduce noise. Stemming is most frequently made of drill cuttings or stream gravel. Konya’s method recommends the minimum amount of stemming needed to avoid ejection from the hole (assuming the rest of the blast was well designed). Stemming is a function of the burden and thus indirectly all variables influencing burden. The ratio of the stemming length to the burden is determined by a stemming factor which was one of your inputs. The appropriate stemming factor depends on the type of material used for the stemming. Too much stemming will usually adversely effect fragmentation at the top of the bench. Too little can be expected to eject (although poor burden, cratering, and bad shot timing can also cause stemming ejection).

4. Subgrade Subgrade is the distance that a hole is drilled and charged below the bottom of the bench to be extracted. Konya’s method of determining subgrade is as a simple function of the burden. Thus, subgrade is indirectly effected by all variables that influence burden. Konya’s method assumes that benches are blasted in an essentially continuous rock mass such as that in a metal, industrial minerals, or quarry operation. In coal where there is often a drastic change in density between the overburden and coal one may not have subgrade. The extra charge in the subgrade helps to pull the toe of the bench and keep the pit floor level. (In coal the natural change in density and material causes the bench to break out along the interface without subgrade). Too much subgrade will cause large amounts of powder to detonate without a free face causing ground vibrations, back break, and in extreme cases even cratering. Too little subgrade will often cause the pit floor to rise in the direction of advance or will allow humps of rock to remain after blasting, in addition to causing difficulties pulling the toe.

5. Stiffness Ratio Stiffness Ratio is the ratio of the bench height to the burden. As the stiffness ratio approaches 2, the risk of cratering becomes greater. At two the shot can be expected to crater. Stiffness ratios above 4 tend to do little for fragmentation but they imply that a larger hole size might have been selected with savings in drilling and blasting costs likely. Stiffnesses above 4 do not immediately become a technical design problem, but should be examined if optimized economics are a factor. Stiffnesses over 6 can be associated with an increase risk of cut-offs. Because burden is a function of hole size, there is a relationship between a good selection for bench height and a good selection for hole size. The spreadsheet contains a stiffness check field. The field reports “Redesign” if the hole size will likely crater, “ok” if there are no expected technical or economic problems with the match of bench height and hole size, or “excessive” if the stiffness is high enough to create either economic optimum or cut-off risk concerns for the hole size.