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
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GROUND SUPPORT MAY 2017 • E&MJ; 39 www.e-mj.com ent types of reinforced shotcrete have different compressive strengths. Shot- crete with steel fibers will have a higher compressive strength. "If you don't put any reinforcement in it, just shotcrete by itself, it gets up to a point then it breaks. But if you've got poly or metal fibers, it goes up to a strength, but then it de- forms," Sandbak said. "The strength load capacity it could take was originally up there at 15 or 17 kiloNewtons (kN), but the residual load it can take goes down to two or three kN after it has been broken or moved." (See Figure 5.) Adding mesh provides additional sup- port in case of load displacement due to movement. "Even after it has moved several inches, mesh is still providing support," he said. "After it cracks, yes it does lose some support. In our case, it is combined with mesh so we get some retraining elements." Mesh proves to be critical in support- ing shotcrete in rock prone to movement, the report said. "Shotcrete reinforced with mesh will resist cracking and have a higher residual load in displacement environments than fiber-reinforced shot- crete," the report stated. "The energy ab- sorbed is also much higher for the mesh at 700 joules versus 300 joules for the poly or metal fibercrete." This is because the mesh provides the tensile strength shotcrete lacks. "Mesh combined with shotcrete give you the best of both worlds," Sandbak said. Turquoise Ridge uses No. 6 mesh screen. It "provides about 3.3 tons (of bag strength), or a local safety factor of 3.3," the report stated. (See Figure 6.) "The current thickness of shotcrete (3 in.) at the current 5,000 psi compressive strength is expected to provide another 1.5 tons of support or a safety factor of 1.5." (See Figure 7.) Going Forward Combining mesh and shotcrete "increas- es the overall safety factor by at least 0.4 for the 14- by 14-ft top cuts, and 0.2 for the larger sized drifts," the report stated. "This suggests that a 15- by 15-ft topcut with the allowable 2-ft-wide overbreak is still above the safety factor of 1.5, or the addition of shotcrete gives us more leeway on allowable overbreak to near 18 ft." (See Figure 8.) Fundamentally, the local safety factor is gauged to keep the area between the bolts intact, Sandbak said. "Once it un- ravels, the larger wedge is going to come out," he said. "With shotcrete and mesh, we retain that really high safety factor be- tween the bolts." Sandbak said that the report reveals the viability of an unconventional strat- egy. "Shotcrete has always been viewed as secondary support: something to coat the wire mesh to prevent small rocks from falling out," he said. "The objective going forward is to try to spray shotcrete first, and then place bolts and mesh on top of the shotcrete." Ideal, he said, would be to "get an early strength shotcrete of at least 150 psi (1 MPa) in one hour so as to limit initial movement, and facilitate the drilling for bolts without unravelling the shotcrete or rockmass." Figure 6—Mesh Equivalent for Shotcrete (from Pakalnis, 2014). Figure 7—Graphs showing relationship between support capacity, shotcrete thick- ness, and shotcrete 28-day compressive strength for 3- by 3-ft wedge between bolts (1-ton weight). Based on using 17% support capacity for RMR = 20 (Very Poor Rock). Note 5,000 psi compressive strength design standard for shotcrete. Figure 8—Combined Safety Factor versus drift width analysis for bolts and combined bolts, mesh and shotcrete. Minimum Safety Factor 1.5. based on RMR <25 (Very Poor-Poor Rock). Figure 9—Optimal rock reinforcement scenario -- shotcrete first, then bolts. (Source: Barret, S.V., and McCreath, D.R. 1995. Shotcrete support design in blocky ground. Tunneling Underground Space Technology. 10(1):79–89.)