Engineering & Mining Journal

MAY 2017

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GROUND SUPPORT 38 E&MJ; • MAY 2017 www.e-mj.com Cracking Up Shotcrete by itself is vulnerable to load displacement due to rock movement. If the rock it supports shifts a few millimeters, it could crack or worse. When that hap- pens, it would lose its supportive qualities. "Shotcrete is relatively weak in tension, and after significant cracking loses 80%- 90% of its strength," the report stated. There are a number of ways shotcrete can fail. "Failure mechanisms of sprayed shotcrete includes adhesion, a punching through shear failure, flexural failure, and flexural shear failure," the report stated. As mentioned, sometimes shotcrete won't adhere to weak rock with lots of clay. "Sometimes you might have shear and it punches through in between the bolts, or as a flexural failure it squeezes and it shears," Sandbak said. "If you get stress between the bolts, it could crumble and form its own thing and fall out as a flexur- al failure. If you get a lot of movement, it could shear to the bolts and then fall out." On paper, once shotcrete cracks it loses its support capacity, Sandbak said. "As soon as it breaks and cracks, they give shotcrete zero support. It is no lon- ger viable," he said. In practice, howev- er, if used in conjunction with or behind mesh, it still provides some support. "It is weak," he said, "but if you combine it with mesh or combine it with fiber or anything else, it gives you more. It is not zero anymore after it breaks." Therefore, shotcrete should be rein- forced with mesh. "Bolts provide confine- ment and compression," the report stated. "And mesh restricts the bulking of the rock mass, and ensures the interlock friction to keep the rock mass in compression." Applications at Turquoise Ridge Practical application of these findings means that in some drifts in some mines, shotcrete can be used with bolts to possi- bly double the safety factor. "For RMR's from about 35-60 (Fair Rock) most char- acteristic of rock surrounding the ore and in development drifts, shotcrete can be thought of as acting as an arch," the report stated. "This means increased flexural and shear forces acting on the shotcrete due to better adhesion with the rock, and taking more of the load and ax- ial compression." For some drifts, shotcrete alone will provide sufficient support. "In really strong ground you could do shotcrete by itself and it would probably do it because it wouldn't crack," Sandbak said. However, for use at Turquoise Ridge, where the tests were conducted, its theoretical value gets reduced by al- most an order of magnitude. "Shotcrete is only utilized in that weak ground at only 17% of what they claim originally that strength would be," he said. "Shotcrete doesn't provide the best support here by itself because it doesn't stick very well." Test results captured this reality. In a lab, shotcrete's calculated support ca- pacity is 170 pounds per square inch (psi), the report stated. In "actual tests from Turquoise Ridge, the adhesion strength of shotcrete tested is approxi- mately 145 psi or 1 megaPascal (MPa) after 30 days," it stated. At Turquoise Ridge, shotcrete is used for retention. Thanks to the testing conducted there, an engineer elsewhere can calculate the support capacity of shotcrete based on the RMR of the rock it will support. The Equation As mentioned, the report puts the shear strength of shotcrete "partially confined by mesh" and supporting low RMR rock at 17% of the calculated support capaci- ty. "As the RMR increases, the shotcrete capacity increases from 13% for the low- est RMR to 50% when the RMR is 80," the report stated. 2 The formula takes into account the shotcrete's compressive strength. Differ- Figure 4—Design shotcrete strength based on RMR for shotcrete compressive strength of 32 MPa (5,000 psi) and safety factor of 1.5. Figure 5—Comparison of load versus displacement and energy versus displacement for RDP test samples reinforced with polyfiber, steel fiber, and wire mesh (from NIOSH, 2015). Welded mesh is superior to either poly or metal fiber shotcrete for peak, residual loads, and toughness (energy absorbed) even beyond 40-millimeter (1.6-in.) test standards. 2 Source: Bieniawski, Z.T., and Lowson, A, R., (2013), "Critical Assessment of RMR based Tunnel Design Practices: A Practical Engineers Approach" Rapid Excavation and Tunneling Conference, Washington DC, Design and Planning Session.

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