Engineering & Mining Journal

MAY 2017

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GROUND SUPPORT 36 E&MJ; • MAY 2017 www.e-mj.com Pursuing gold that cannot be seen with the naked eye in progressively weaker rock at its Turquoise Ridge mine in Nevada compelled Barrick to reconsider its drift support plans. Dissatisfied with the re- search available on the topic of shotcrete support, Senior Geotech Engineer Louis Sandbak and his team gathered the data from various tests conducted there and released an erudite white paper packed with numbers. It provided the proof need- ed to justify some costs and formalized some common understandings. Sandbak said historically miners normally erred on the side of caution and acted on indus- try standards that typically weren't easily sourced. What everyone knew, he said, was backed statistically when testing in the gold mine revealed shotcrete used in conjunction with bolts and mesh in some cases doubled the local safety factor. 1 Now available online, the paper can be referenced by miners seeking to ensure safety when working in medium and weak rock. Some of the conclusions, he said, could change the way shotcrete is used. The example drift is 14 ft (4.3 meters [m]) wide. The report described it as "sit- uated in ore of the Type IV and Type III weak rock class categories that necessi- tate the use of bolts, mesh and 2 in. to 4 in. of shotcrete." Sandbak described it as very weak. "We could have ground that we have standup times of almost immediate collapse to hours or days," he said. "If it didn't have any support it would just collapse or cave." Five years ago, the original drift size was 10 ft by 10 ft. Initially, drift support plans discounted shotcrete and mesh. "All of our safety factors that we based our drift size on were based on bolts only," Sandbak said. Nonetheless, the mine was logically using shotcrete and mesh as the drift widened and the company sought to deploy more equipment and automation. "In our weakest rock, shotcrete seems to do better in some cases if we can confine our ground with it," he said. The company then deployed "all au- tomated bolters," and the drift size hit "15 by 15, using 3 to 4 in. of shotcrete," he said. "We wanted to go to fast-set- ting shotcrete to support mechanization such as roadheaders." Meanwhile the pa- perwork behind the action still factored strictly bolts. The opportunity seemed obvious. "This was an attempt to say, hey, the mesh and the shotcrete add a great deal to the stability, especially in our weakest rock," he said. "We did this research to understand how each piece of the support (bolts, mesh, and shotcrete) combine to maximize our drift sizes in different types of ground encountered at Turquoise Ridge." The goal was to arrive at an equation, a formula, that determines how many bolts, and how much shotcrete and mesh, are needed for a specified area to have a mini- mal safety factor of 1.5. Sandbak present- ed the findings in Safety Factor Design Analysis: Integration of Bolts, Mesh, And Shotcrete Support in Weak Rock Mass- es, Turquoise Ridge Mine, Nevada at the 2017 Society for Mining, Metallurgy and Exploration's annual conference in Den- ver (see SME preprint No. 2017-110). It proved not only the importance of shot- crete for retention purposes. According to the report, shotcrete and mesh can be critical for support in certain types of rock. The report takes the reader to this conclusion after looking at the current re- search on the topic. First, it reviews how the safety factor is gauged for drift sup- port using only bolts. Bolts Only To attain the desired safety factor using only 8-ft bolts, the key variable is the Design Considerations for Ground Support in Nevada's Weak Rock A new analysis demonstrates how the use of bolts, mesh and shotcrete influence the safety factor for ground support programs By Jesse Morton, Technical Writer 1 The target safety factor translates to approximately one ton of rock supported in a 4- by 4-ft cornered by 8-ft bolts. Miners spray on an initial early-strength shotcrete layer with fibercrete (poly or metal). The goal is to stabilize the immediate rock zone followed by placement of primary support of mesh and bolts.

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