As the Grasberg open pit is completed in 2019, the underlying Grasberg Block Cave (GBC) will rapidly mature as a primary source of production at PT Freeport Indonesia operations, supplementing the Deep Ore Zone (DOZ) and Deep Mill Level Zone (DMLZ) caves. After years of planning, development and revisions to the extraction strategy, early information on footprint rock mass response to undercutting and the nature of caving is now available to confirm or guide adjustments to strategy. Despite the early stage of mining, much has been learned regarding cave propagation, fragmentation, stress states, pillar stability and ground support performance. This paper provides a timely rock mechanics focused update from one of the largest planned caving mines in the world.

At PT Freeport Indonesia's Grasberg mining complex, the Grasberg open pit is scheduled to cease production in early 2019. Mining of the Grasberg deposit will then transition to the underlying Grasberg Block Cave (GBC) mine, with initial undercutting currently planned for late 2018. The GBC is projected to become the district's flagship underground mine for the next two decades. Early-stage production areas at the GBC are hosted within a heterogeneous rock mass. The footprint will be subjected to variable abutment and cave loading conditions. Mine design and sequencing adjustments, combined with customised ground support options, continually evolve to improve footprint reliability and permit a favourable transition between open pit and underground operations. This paper provides a snapshot of strategies employed to promote the successful caving operation at the GBC mine.

Monitoring of fracture limits, cave backs and drawpoint fragmentation at both the Grasberg Block Cave (GBC) and Deep Mill Level Zone (DMLZ) caves at PT Freeport Indonesia's operations suggest that Cave Propagation Factor (CPF), weak and persistent vein intensity and hydrofracture spacing exert a strong control on the caving rate and primary fragmentation within the cave back. CPF in this case is defined as the ratio of cave back stress to defected rock strength and can vary significantly across the cave back, both due to variability in vein mineralogy and intensity and complex cave back geometries. Cave back stress is understood through 3D elastic modelling of anticipated cave shapes (fracture limits) while point load indices, weak and persistent vein intensity and Synthetic Rock Mass (SRM) testing are used to understand the variability in strength and fragmentation potential of the sparsely jointed, massive to heavily veined rock mass domains. When coupled with predictive relations for secondary fragmentation, these ultimately led to the development of a methodology for forecasting of caving rate and primary fragmentation in the cave back, the fragmentation (including oversize and fines) reporting to the drawpoints and the associated hangup frequency and productivity. These forecasts are presented in the form of block models, forecast maps and drawpoint-based charts. When calibrated against monitored cave back and drawpoint performance these can be used to assist in more local cave back shaping and preconditioning (function of caving rate), recovery estimation and secondary breakage planning (function of productivity) and management of wet muck and mill throughput (function of fines entry).