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Prolonged stemming material confinement offers several benefits for the blasting program that faces challenging site conditions. By maintaining energy confinement, ex‐ plosives can be loaded higher in the borehole to more uniformly break cap rock. Confinement also reduces the occurrence of decking displacement, which affects adja‐ cent charge performance in multiple explosive decking applications. "Achieving explosive energy confinement within the rock mass prevents premature venting and offers better usage of the expanding gasses to improve rock breakage," said Robert McClure, president of R.A. McClure, Inc. and a blasting industry consultant. A simple, cost‐effective solution exists that allows companies to realize pro‐ longed energy confinement. Effective Stemming Plugs Stemming plugs have been used in various applications for more than a decade to address issues such as cap rock, vibration reduction or improving a blast's perform‐ ance. Creating confinement and eliminating the venting of explosive gasses is the concept behind the use of stemming plugs. "Even slight retention time increases can help to minimize air overpressures," says McClure. There are a number of different types of stemming plug systems available on the market. Some are spherical shaped, while others look like a bullet. Certain systems require the use of accessories to load the plug in the borehole, while others are simply dropped into the hole. One such stemming plug available in the market is Rock‐ lock, distributed by Advanced Blasting Technologies, Inc. Constructed of pliable plastic, it creates a frictional gas‐ impermeable seal that prevents stemming ejection by absorbing and equalizing the pressure field across the borehole. The sphere is simply filled with stemming ma‐ terial and dropped down the column. When introducing stemming plugs, such as Rocklock, into a blast design, there are a couple of rules of thumb to follow. Selecting the right size plug is critical and de‐ pends on the borehole diameter. "The plug size should be 90 percent of the borehole diameter," explains Mc‐ Clure. Therefore, a 9‐in. stemming plug will be most ef‐ fective for a 10‐in. hole. Stemming material should be loaded to 1.5 times the borehole diameter prior to inserting the plug. So for a 10‐in. borehole, technicians should load 15 in. of stem‐ ming before the plug. The column should then be filled the rest of the way with stemming. The energy confinement provided by stemming plugs allows quarry and mine operations to realize improved blasting safety by reducing the occurrences of fly rock incidents. www.rockproducts.com ing was designed to open the blast in the center of the Field examinations help evaluate preformance. ROCKproducts • OCTOBER 2011 27 "Even a couple milliseconds of additional confinement can make a significant difference in enhancing blast performance and, therefore, curtailing fly rock," said McClure. In applications where decking is used to control vibration, prolonged energy confinement will help to prevent tran‐ sient pressures and inner deck movement. Failure of the deck to maintain confinement may lead to propagation or complete failure of the second charge, which results in poor fragmentation and increased vibration levels. Improved confinement also allows explosives to be loaded higher into the borehole while retaining the en‐ ergy, which offers significant advantages for improving cap rock fragmentation. "Stemming plugs are an eco‐ nomical alternative to the common practices of satellite hole charges and loading a separate explosive charge in the stemming zone," added McClure. "Depending on the size and design of the blast, most operations can add stemming plugs to their program for pennies per ton." Pennsylvania Quarry Evaluations Over a period of several months, in‐depth field evaluations of the Rocklock stemming plug were conducted at two limestone quarries in southwest Pennsylvania. "In all stud‐ ies, high levels of field controls were adhered to during the drilling and blasting process as they related to blast design, bench preparation, pattern layout, drilling and blast hole loading," said Douglas Bartley, president of DBA Consult‐ ing, the company conducting the tests. At Pennsylvania Site 1, the tests were conducted to answer two questions. First, do stemming plugs introduce a notice‐ able difference in a blast's performance in terms of energy containment and reduced vertical burden movement? Sec‐ ond, do plugs improve overall rock fragmentation? Technicians loaded half of a 69‐hole production blast with stemming plugs and the other half with only a crushed stone stemming material. The blast consisted of two rows of 35 holes drilled to a 58‐ft. depth and 6.75‐ in. diameter. With a 17‐ft. burden and 19‐ft. spacing, the holes were loaded with bulk emulsion explosive and det‐ onated with a non‐electric shock tube system. Blast tim‐