Engineering & Mining Journal

AUG 2018

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Page 37 of 107

BLASTING 36 E&MJ • AUGUST 2018 • By reducing cap scatter, ground vibra- tion will improve. This again has no technical backing as none of these phe- nomena are correlated to cap scatter. In fact, in many cases, just substituting electronic caps for nonelectric caps causes an increase in ground vibration. • By reducing cap scatter, the 8-ms rule no longer applies and blasters can now use a 4-ms or 1-ms rule for delaying of charge weights. This has been attempted and has been challenged in numerous court cases 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, premature 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 predicting ground vibration waveforms to within 95% accuracy us- ing 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 companies do not have the systems to determine the actual best time for either ground vibra- tion or fragmentation and blindly switch. One method that has been developed that perhaps most benefits and is most widely available to immediately see an im- pact from electronic caps is that of Signa- ture Hole Analysis. It is a process in which one waveform from a single borehole is col- lected, then a superposition of the waves and a simulation is done to determine the best rhythmic timing sequence for a blast. Unfortunately, from leading experts expe- rience, 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 pressure (pressure) can be compared to the total confinement of the shot (burden). The ra- tio of this gives the confinement of the blast. Why does this matter? Because, an over-confined blast can lead up to five times the ground vibration of a normally confined blast. A blast that is under-con- fined has much less ground vibration than a normally confined blast. This means that the seismic waveform changes as the confinement changes. Let's look at a very minor aspect (com- pared 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 signature hole analysis. The goal of a signature hole analysis is to obtain the vibrational wave- form from one borehole to be simulated into the full vibration waveform. Using nonelectric detonators, an accuracy of 90-95% of the entire vibration waveform can be predicted. This means not only can one predict the peak particle veloc- ity almost exactly, but could also predict at what millisecond after the blast it will occur, as well as the rest of the vibration waveform. This is then achieved by using a seismograph 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 middle 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 simulation to be completed. Again, this can be completed using any type of cap, however, an electronic 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 pat- tern) to 1 in. per second (with a 47-ms rhythmic timing pattern). This would be the best and ideal case, however, using a 42-ms nonelectric delay may give you a ground vibration of 1.05 in. per second. It is the mine operator's call as to wheth- er the 0.05-in.-per-second reduction in ground vibration 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 companies worldwide are performing the signature hole analysis completely wrong. This is in an effort to make additional money on the testing procedures and to persuade miners into using electronic caps. These studies typically cost mines tens of thou- sands of dollars, disrupt production, and produce no valuable data or information. How are these incorrect studies done? First a company will come out and set up 10+ seismographs all around an area or protected structure. These will often be different distances away from a planned location. The company 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 individually and measure the ground vibration on a large array of seis- mographs. This is in an attempt to "vary the scaled distance," which has zero ben- 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. [Signature hole analysis] studies typically cost mines tens of thousands of dollars, disrupt production, and produce no valuable data or information.

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