Print7.2.4: Permissible Explosives
The term Permissible Explosive can be traced back to early in the 20th century, when thousands of miners were being killed each year in the underground coal mines. Mine explosions caused many of those fatalities, and the mine explosions were set off by improper blasting practices. The U.S. Bureau of Mines conducted research to develop explosives that would not set off the mixtures of coal dust and methane that were typically found in the mines. Over the years, permissible came to mean two very important things: first, the explosive had been tested and certified by the USBM to meet the criteria; and second, the permissible explosive would be used according to a set of required practices. Without the latter, it is still possible to set off a mine explosion even though a certified-permissible explosive is used. Eventually, by the mid-20th century, mining laws mandated the use of permissible explosives and practices.
The migration from conventional to continuous mining practices in the coal mines has dramatically reduced the demand for permissible explosives in this country. Recently, around 900 metric tons of permissible explosives were sold in the U.S., whereas, in the mid-twentieth century, that number was closer to 60,000 metric tons! Nonetheless, it is commercially available and has application when it is necessary to blast in mines where methane could be present. Such applications would include: shooting the overlying strata for construction purposes in underground coal mines, e.g., for ventilation overcasts or increased headroom for belt drives/transfer points; and shooting large roof falls so that the rock can be loaded out and removed.
The permissible explosive must be used in accordance with the permissible practices. The key practices are as follows:
- qualified or certified persons, i.e., individuals meeting the requirements specified by the mining regulations, must conduct the blast.
- black powder, aluminum-cased detonators and safety fuses are prohibited;
- noncombustible stemming must be used;
- the minimum borehole spacing is 24” in coal and 18” in rock;
- no more than 20 boreholes are permitted to be fired in a round;
- no more than 3 lb of explosives can be loaded into the hole;
- the total delay period must be 1000 ms or less;
- the interval between delays must be at least 50 ms, but no more than 100 ms;
- the air must be tested for methane immediately before shots are fired, and the shot can be fired only if the concentration is less than 1.0 %;
- unconfined shooting is prohibited;
- all of the blasting circuits must be checked for continuity and resistance before the shot, using approved blasting galvanometer or blasting multi-meter.