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Construction and development of the Grasberg Block Cave (GBC) mine continues to progress, with undercutting commencing in late 2018. As an experienced operator of block cave mines, PT Freeport Indonesia (PTFI) works to improve the planning and execution of undercutting. The GBC mine, with its large size and complex geotechnical conditions, requires increased focus on initiating and sustaining caving.
For the past two years, PTFI has worked on developing a software tool for detailed undercut planning.
The tool utilises cave management rules, the geologic block model, and ring shapes to produce detailed tonnage and grade schedules. Additionally, further analytical processing of the tool’s output data allows examination of the feasibility and required resources to implement the undercutting plan. As a constraint-based scheduling application, the tool provides a high degree of flexibility in adapting quickly to changing assumptions.
Factors affecting GBC undercutting include high tonnage rates, mine development resources, and variable rock mass conditions. Of primary concern, with production targets between 130,000 and 160,000 tpd, drawbells must open at a sufficient pace to achieve acceptable caving production ramp-up and then maintain throughput targets. Detailed undercut scheduling alleviates those concerns by optimising production block start dates, caving rules, and ring designs, to drive cave production with an achievable drawbelling rate. These detailed schedules also enable vetting of mine development plans, construction plans, and resource requirements.
The GBC geotechnical environment requires undercutting strategies to adapt accordingly. Poor rock mass conditions, large panel boundaries, and high-stress conditions all affect undercut start points, caving directions, and other design decisions. The ability to produce realistic, detailed schedules enables further study of the impacts with 3D numerical modelling. Results from numerical modelling have helped refine not only the caving strategy, but also the overall mine design.
The importance of the undercutting plan for successful caving of the large footprint, high tonnage rate GBC mine remains a priority. In conjunction with the geotechnical engineers and operations personnel, the mine planning team utilises the newly developed undercut planning tool to adapt and improve undercutting plans as more informed data becomes available. The geotechnical and operational challenges to mining the GBC necessitate the methods and approach to planning discussed in this paper.
Keywords: undercutting, scheduling, caving
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