Engineering and Mining Journal - Whether the market is copper, gold, nickel, iron ore, lead/zinc, PGM, diamonds or other commodities, E&MJ takes the lead in projecting trends, following development and reporting on the most efficient operating pr
Issue link: https://emj.epubxp.com/i/822333
GROUND SUPPORT MAY 2017 • E&MJ; 37 www.e-mj.com quality of the rock, the report stated. "The support capacity of the system is deter- mined by the bond strength of the bolts holding up the wedge, which is in turn based on the Rock Mass Rating (RMR)." Bond strength of a bolt will vary from mine to mine. In strong rock, bolt bond strength is greater. For high RMR, each foot of bolt can support a greater volume of rock, Sandbak said. For RMR less than 25, classified as very poor to poor rock, "we're only going to give it one ton per ft," he said. Therefore, total support capacity, the report stated, "is the sum of the individu- al bolt values defined by either the break- ing strength or the bond strength of the bolts (whichever is lower)." Easiest to envision is bolt support in strong rock, Sandbak said. "In strong ground they can get double or triple the strength, so you don't need as many," he said. "Bolts can only hold so much in weak ground." The equation seems simple when sup- porting high RMR rock. Superficially, it would require only adding together the individual bolt values. In low RMR rock, however, the bond strength is low. As the drift widens, each additional bolt can support less rock. "Pretty soon, it's got a diminishing return because it is a volume thing," Sandbak said. A 10- by 10-ft drift in low RMR might require only three bolts in the back. "For a 12 by 12 we needed four bolts," he said. "As we go out to 14, or anything larger than 14, we went to six bolts." (See Figure 1.) The volume of rock to be supported would be greatest in the center of the drift. Turns out, according to the report, theoretically the values for the center bolts would be less than that of a 3-in. wedge of shotcrete. Bolts, Shotcrete and Paper On paper, in certain types of rock, shot- crete can play more than its traditional role. Calculations reveal that it can serve as primary support. "The support ca- pacity of shotcrete is equal to the shear strength times the area affected," the re- port stated. Presuming ideal adhesion, "a 3-in.-thick, 3-ft-wide by 14-ft-long wedge of shotcrete in the back has a support ca- pacity of 43 tons if this slab could remain intact," the report stated. "Therefore, the calculated safety factor of shotcrete is 3.8, or nearly doubles that of the safety factor of using bolts." (See Figure 2.) The report states shotcrete does not adhere well to certain types of low RMR rock. In those types, it would primari- ly be used for retention. In higher RMR rock, however, "if you just go by the sheer strength, that little box of shotcrete, is better than that entire bolting sequence of six bolts in that same 3-ft wedge," Sandbak said. "That is twice as strong as our support for bolts in that same (14-ft drift)," he said. "If you had all things being equal, shotcrete is better." All things are never equal for long, es- pecially underground. Figure 1—Safety Factor Design versus width and number of bolts. Based on bond strength of 1 ton/foot; average for Swellex bolts in Type IV Very Poor Ground (RMR<25). Figure 2—Comparison of unbroken shotcrete support capacity versus bolt support capacity for a 14- by 14-ft flat back or topcut heading. Figure 3—Percent of shotcrete support capacity based on RMR. Shows how capacity increases with increasing RMR. Used 17% for RMR=20 for ore body calculations. Based on Bieniawski and Lowson, 2013.