Engineering & Mining Journal

JUL 2018

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SHOTCRETE JULY 2018 • E&MJ 49 www.e-mj.com tices in terms of efficient underground development and decades of operating ex- cellence. Sika has been involved in both projects for many years and has helped develop efficient underground mining schemes based on an in-cycle shotcrete that allows rapid mine development rates. Over the years, many hurdles have been overcome and Sika explained that today its technologies are being used for fast, early-strength shotcrete liners that allow for the safe re-entry within a short period. Elevated and changing lithological stress levels, induced by deeper extraction lev- els and a migrating cave front, has be- come more and more important over re- cent years, and today efficient shotcrete liners have the capability of taking on de- formations early on, either gradually over time or during sudden seismic events. Kiruna is seen as one of the most mech- anized and efficient mining operations globally and is used as a raw model for many block caves under development. The in-cycle shotcrete requires early strengths of 4 MPa after 4 hours with a minimized two-component binder consumption. Ad- ditionally, the shotcrete mix is designed to reduce rebound of the sprayed concrete to a minimum and interact optimally with the added fiber to cope with seismic events and increased rock strain levels at the deeper installed sublevels. Sika explained that specially designed plasticizers and shotcrete accelerators have been custom- ized for the cement and ambient site con- ditions to achieve this. With an ongoing cost-performance optimization, Kiruna is certainly spearheading underground de- velopment when it comes to in-cycle shot- crete application. From a shotcrete perspective, many of the lessons learned from Kiruna have been implemented at the GBC Project currently being developed and planned for first drawpoint production in 2019. Integrating an effective shotcrete setup in mining operations requires good expertise in concrete production, spray application and the selection of the best performing materials. At the Freeport Grasberg Oper- ation, a team of local shotcrete and con- crete specialists, together with a small group of international experts, are guiding and training the operational teams and main contractors at the mine. The team is there to ensure production and appli- cation quality, application speed, yielding characteristics, durability, and strength of the placed shotcrete and concrete, and to ensure a smooth operation of one of the world's largest underground concrete batching plants. For both Kiruna and Grasberg, the geo- graphical location represents a major chal- lenge. Kiruna endures very harsh climatic conditions during the winter months and Grasberg has a challenging and long sup- ply route. Choosing the right types of ad- mixtures to reduce the amount of cement, for a certain strength requirement, accord- ing to Sika, is critical to keep logistics and costs under control. Especially when large shotcrete volumes are required, there is need for a lean and easy mix design with a limited amount of components for stor- age and handling. This allows efficient and large-volume concrete production in the batch plant and avoids poor mixes, not fulfilling quality control, which may require disposal. Efficient handling of the primary shotcrete support installation has contributed to the GBC footprint develop- ment and it is now on schedule to produce ore from first drawpoints. Sika said it is at the forefront when it comes to supporting technological im- provements in mining and speeding up mine cycle times for projects. Shotcreting Robots for Tunneling and Shaftlining Since 1991, Shotcrete Technologies Inc. (STI) has developed and fabricated Shot- Tech Robotic Arms. A technical break- through for STI came in 2009 when they used a robotic shotcrete system to line the New Afton shaft in British Columbia. The company had been developing the system for years and they decided to use a wet-mix centrifugal sprayed concrete to line a 3.5-m diameter, 290 m-deep raise- bore shaft. Instead of adding air at the nozzle as in wet-mix shotcrete or water at the nozzle as in dry-mix shotcrete, the concrete mixture is pumped to a nozzle that spins at speeds up to 5,000 rpm and sprays the concrete onto the surface. The project was a success and they demonstrated, for the first time in North America, that centrifugal sprayed con- crete technology could be used to line deep shafts effectively and rapidly. A proj- ect that would have taken six weeks for robotically applied dry-mix was complet- ed in 10 days with a minimum amount of waste or rebound. "At the end of the proj- ect, they had two wheelbarrows full of re- bound," said Kristian Loevlie, president, STI. This wet-concrete process generates very little dust." He believes that eventu - ally all shotcreting systems will become wet-mix applications because of the dust and rebound. Today, Loevlie and his team are rou- tinely lining 400-m raise-bore shafts and ore passes with abrasive-resistant shot- crete. The importance of this process is that we can stop the newly opened ground from deteriorating either as a temporary or final lining, Loevlie explained. "We can seal these shafts in a day or two with 6,000 to 7,000 psi fiber-reinforced shot- To rapidly line a shaft, a wet-mix concrete is sprayed through a double-nozzle system that rotates at speeds up to 5,000 rpm. (Photo: Shotcrete Technologies)

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