Engineering & Mining Journal

JUL 2014

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In the meantime, FCX converted its DOZ orebody, which it began mining in 1989 using open stoping, to block caving in 2000, with production expected to continue until 2019. With an output of 49,400 mt/d during 2013, ramping up to 80,000 mt/d this year, the DOZ is now one of the world's highest-capacity underground metal mines. Even that will look insignificant once the Grasberg block cave comes on stream, with the company reporting in its 2013 Form 10-K that it is planning an output of 160,000 mt/d—adding to 80,000 mt/d from the DMLZ. Total investment for devel- oping these two mines, plus the common infrastructure project, will be around $7.8 billion between 2008 and 2021. In Chile, meanwhile, Codelco is continu- ing with its evaluation of an underground mine to replace the depleting Chuquica- mata open pit. The current concept in- volves a 7.5-km-long access decline leading to four production levels, with block caving being used to produce 140,000 mt/d of ore. And in the U.S., the possibility of min- ing below the existing Bingham Canyon pit raises the prospect of extending the mine's life beyond 2050. In a presentation given in September 2012, Kennecott Utah Copper's then president and CEO, Kelly Sanders, gave a conceptual outline of possible underground development that includes three block caves out of six sep- arate ore zones. Mine Design Moves Deeper In the mid-1990s, the South African rock mechanics expert, Dr. Dennis Laubscher, published what turned out to be a landmark paper on the design of caving operations, with the subsequent compilation of the definitive block caving manual. Based on practical experience in the asbestos indus- try in Southern Rhodesia (pre-Zimbabwe) and elsewhere, Dr. Laubscher also intro- duced his MRMR (mine rock mass rating) system—the citation at the presentation to him of the 2007 Brigadier Stokes Memorial Award of the South African IMM noted, "There is not a block caving mine anywhere in the world that has not used his empirical correlation, or stability graph, to evaluate the cavability of its deposit." The 1990s was a decade of consider- able interest in block caving, with the establishment in 1997 of the International Caving Block Study, and the resultant pub- lication of Block Caving Geomechanics . Fast forward to the Caving 2010 con- ference, and Professor Gideon Chitombo of the University of Queensland—who was the project manager for the block caving study—provided an update to Laubscher's contribution in one of the keynote presen- tations at the meeting. Amongst the more recent developments, he said, has been "an emerging approach to characterize and predict the behavior of a rock mass. The synthetic rock mass (SRM) is a numerical methodology designed to help predict the large scale response of the rock mass to caving based on its strength, discontinuity characteristics and discrete fracture network (DFN) techniques. "The general knowledge and under- standing of caving methods has significant- ly improved and has gone beyond a few indi- viduals or companies being the sole custo- dians of caving knowledge and expertise," Chitombo said. "However, as an industry, there are still some major challenges that may collectively threaten the viability of cave mining." Examples, he said, include: • Not being able to achieve continuous caving; • Differential cave propagations due to the presence of different geological lithology; • Seismicity caused by unfavorable under- cutting practices; • Early dilution or waste ingress and accel- erated fines; • Migration containing waste; • Structural collapses and instabilities due to mining of large panel widths; • Extraction level instabilities due to poor undercutting practices; and • The presence of remnant pillars or com- paction of caved materials, a conse- quence of poor draw practices. In the conclusion to his address, Chitombo said, "The caving industry is now moving toward the next generation of caving geometries and scenarios ('super caves') where current practice and know- ledge may be reaching its limits. Laubscher's rules, which have a strong empirical and experiential basis, now need to be supplemented by methods that con- sider more of the governing physics of the caving processes." Chitombo's comments were particularly apposite given the general trend toward using block caving at greater and greater depths. Opportunities for near-surface applications are becoming fewer and fur- ther between, while there is increasing interest in using the technique on both new production levels at existing mines and in situations where blind orebodies lie deep. Rio Tinto and BHP Billiton's Resolu- tion project in Arizona comes to mind in this context. The orebody lies at depths of between 1,500 m and 2,130 m (5,000–7,000 ft), and panel caving is being proposed with a nominal production rate of 120,000 mt/d, and perhaps up to 25% higher. Coincidentally, the porphyry resource tonnage at Resolution is compa- rable to that at Chuquicamata, at around JULY 2014 • E&MJ; 33 www.e-mj.com B LO C K C A V I N G The operator control station for a Sandvik Automine LHD installation. The operator loads by teleremote; the machine hauls, dumps and returns autonomously. EMJ_pg32-37_EMJ_pg32-37 7/2/14 8:35 AM Page 33

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