Coal Age

SEP 2018

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24 September 2018 blasting continued and has lost every time. Advanced methods exist to prove this, but require a special study and unique conditions. • These caps are safer. They provide no real safety benefit except the ability to check them to reduce risk of misfires. In many cases, prema- ture initiations have occurred with these that would not have occurred with other caps and for this reason they are not allowed on many U.S. government projects. • Electronic caps are required to perform signature hole analysis. The authors began using signature hole analysis in the 1980s predict- ing ground vibration waveforms to within 95% accuracy using electric and nonelectric caps. What electronic caps give the mine is the ability to precisely time blasts by selecting the exact time for a borehole to detonate, which will minimize the cap scatter. Unfortunately, most com- panies do not have the systems to de- termine the actual best time for either ground vibration or fragmentation and blindly switch. One method that has been devel- oped that perhaps most benefits and is most widely available to immediately see an impact from electronic caps is that of Signature Hole Analysis. It is a process in which one waveform from a single borehole is collected, then a superposition of the waves and a sim- ulation is done to determine the best rhythmic timing sequence for a blast. Unfortunately, from leading experts ex- perience, the way this is conducted by most companies is completely wrong. Understanding Confinement To begin to understand this, one first must understand the most basic princi- ple of blasting — confinement. Confine- ment of a blast is the first step to under- standing blast fragmentation, muck pile heave, poor toes, air overpressure, and most importantly for this case, ground vibration. As previously discussed, shock waves have no impact on these explosive results and the gas pressure generated by explosives is what is now known to be the major factor in rock blasting. As such a certain amount of explosive gas pres- sure (pressure) can be compared to the total confinement of the shot (burden). The ratio of this gives the confinement of the blast. Why does this matter? Be- cause, an over-confined blast can lead up to five times the ground vibration of a normally confined blast. A blast that is under-confined has much less ground vibration than a normally confined blast. This means that the seismic wave- form changes as the confinement chang- es. Let's look at a very minor aspect (compared to burden) of a blast then — the stemming. If a blast has the proper amount of stemming, which does not blow out, it has its maximum pressure for breaking and moving burden. What if the stemming blows out and releases half the explosive pressure of the blast? This leads to a state of over-confinement and the blast can produce up to five times greater ground vibration. Now let's begin to look at a signa- ture hole analysis. The goal of a signature hole analysis is to obtain the vibration- al waveform from one borehole to be simulated into the full vibration wave- form. Using nonelectric detonators, an accuracy of 90-95% of the entire vibra- tion waveform can be predicted. This means not only can one predict the peak particle velocity almost exactly, but could also predict at what millisec- ond after the blast it will occur, as well as the rest of the vibration waveform. This is then achieved by using a seis- mograph near the structure that is to be protected and putting delays of 1,000 to 2,000 ms between the first borehole in the shot and the second borehole in the shot. These same delay schemes can be done inter-row (or a hole in the mid- dle of the shot) and on the last hole in a shot. This allows for all the variables with the blast to be identical to what will be done in the future and a realistic simula- tion to be completed. Again, this can be completed using any type of cap, however, an electron- ic cap can minimize ground vibration. For example, let's say that by putting a 47-ms delay between boreholes would cut ground vibration from 2 inches (in.) per second (with a 17/42-timed zig- zag pattern) to 1 in. per second (with a 47-ms rhythmic timing pattern). This would be the best and ideal case, how- ever, using a 42-ms nonelectric delay may give you a ground vibration of 1.05 in. per second. It is the mine op- erator's call as to whether the 0.05-in.- per-second reduction in ground vibra- tion is worth spending five to 10 times as much on initiators. Signature Hole Analysis In addition to most companies not re- leasing this information, some com- panies worldwide are performing the signature hole analysis complete- ly wrong. This is in an effort to make additional money on the testing pro- cedures and to persuade miners into using electronic caps. These studies typically cost mines tens of thousands of dollars, disrupt production, and pro- duce no valuable data or information. How are these incorrect studies done? First a company will come out and set up 10-plus seismographs all around an area or protected structure. These will often be different distances away from a planned location. The compa- ny will then have the mine drill two to four holes in this planned location well behind the face of the blast (completely confined) and vary the load (typically .5, 1, 1.5 and 2 times the normal borehole load). They will then fire these individ- ually and measure the ground vibration on a large array of seismographs. This is in an attempt to "vary the scaled dis- tance," which has zero benefit technical- ly because scaled distance is not used in signature hole analysis. In addition to this, not only is the vibration's peak particle velocity (PPV ) completely wrong from what it would be in a normally confined borehole, but the frequency and wave trace from these boreholes is also completely wrong. They will give some attempt at modeling and say that the maximum PPV from this blast should be under, for example, 8.91 in. per second. The blast is fired, and the actual result is 3.2 in. per second. This is not a signature hole analysis and does not provide any data to the mine, this will not stand up in court, and this provides no guarantees on any ground vibration, let alone gives any real timing or loading advice to the mine. Additionally, we have seen this pro- cess perverted even further with compa-

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