Web Exclusive

Top-priming of                                                                  optimization of burn cuts. Burn templates were
                                                                                used to try and assure the miners were getting
underground burn cuts                                                           the holes properly located. There are a myriad of
                                                                                patterns, many using relief holes reamed larger
by Larry C. Hoffman                                                             to allow those first few holes extra expansion
                                                                                room.
          The most difficult part of underground
                               blasting operations is usually the design and        Virtually all small underground blast
                          execution of the burn, or shatter cut ... the first   holes are primed at the bottom of the bore, to
                          removal of enough rock from a mining face to          minimize any chance of the initiation sequence
                          give the rest of the round room to break. If the      being cut off by a previous blast. This was an
                          cut fails, the rest of the round will have no relief  ever-present serious danger in the days of fuse
                          as the successive holes go off, leaving almost no     blasting, especially in big headings, where the
                          rock broken and a difficult, often dangerous,         last burning fuse would be well short of the cap
                          recovery situation.                                   when the first one went off.

                              In the early days of drilling and blasting            Burn cuts were particularly susceptible to
                                                                                cutoffs and early detonation, with the holes
Doug Peters,          underground, it was almost impossible to take             closely spaced. It was common for miners to
Montana Tech          a round deeper than the shortest dimension of             reject the idea after one failure and go back to
mining engineer-      the heading. A variety of cuts were used to get           their favorite traditional cut.
ing senior, drilling  the first rock blasted out to give the rest of the
a burn cut.           round a place to blast into. V, pyramid, wedge                With the development of closely timed
                      and hammer cuts were common, each with their              electric blasting caps and later nonelectric
                                                                                detonators, the miner could be assured that all
                      advantages and detriments in different rock               the blast initiation process was safely in the end
                                                                                of the drill holes, relatively safe from damage or
                      formations. Every miner had his favorite cut.             premature detonation, as the charges went off.

                      In the 1930s, it was found that drilling closely              The problem with bottom-priming the first
                                                                                cut hole is easy to see, but largely ignored.
                      spaced parallel holes, with some of them left
                                                                                    When that first blasting cap goes off, in
                      empty for the loaded holes to blast into, gave the        the end of the hole, the rock is broken into
                                                                                the adjacent hole or holes. However, as the
                      necessary relief to allow the rock to be broken           detonation progresses outward toward the face,
                                                                                more and more rock is broken with less and less
                      and ejected clear of the face. In the days of             expansion room. Until the explosion reaches the
                                                                                face and can blow out, most of the rock has no
                      mounted drifter drills, it was difficult and time         place to go and is just “stored up” until it has an
                                                                                exit.
                      consuming to set up and drill many parallel
                                                                                    The use of larger reamed holes has the
                      holes, so the burn cut did not become popular             advantage of both giving the initial breakage
                                                                                more expansion room and a larger channel for
                                      until the advent of the                   pulverized rock to escape. Expansion holes, and
                                                                                lots of them, also give more latitude to less-
Larry C. Hoffman, member SME, is      integrated airleg drill in the            expert miners, even engineers.
chief engineer, Blue Range Energy     early1950s.
Co, email hardrock4800@gmail.                                                       The old school of blasting for decades was
com.                                      Many studies have                     that no primers could be left exposed because
                                      been done and volumes                     timing irregularities could cause them to be
                                      written on the design and                 blasted loose before they could do their job.

2 mARCH 2017     Mınıng engıneerıng	                                                That’s no longer true.
                                                                                    The simple change of top-priming the first
                                                                                cut hole completely alters the dynamics of
                                                                                shooting a round.
                                                                                    Modern shock-tube detonators offer
                                                                                extremely reliable timing that opens up a new
                                                                                method of designing and blasting confined
                                                                                underground rounds, those broken every day to

                                                                                                                   www.miningengineeringmagazine.com