A geotechnical review of the transition from narrow incline undercut to w-undercut design at the Deep Mill Level Zone mine, PT Freeport Indonesia

Authors: Putra, F; Perdana, A; Hamman, J; Campbell, R

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DOI https://doi.org/10.36487/ACG_repo/2465_87

Cite As:
Putra, F, Perdana, A, Hamman, J & Campbell, R 2024, 'A geotechnical review of the transition from narrow incline undercut to w-undercut design at the Deep Mill Level Zone mine, PT Freeport Indonesia', in P Andrieux & D Cumming-Potvin (eds), Deep Mining 2024: Proceedings of the 10th International Conference on Deep and High Stress Mining, Australian Centre for Geomechanics, Perth, pp. 1319-1334, https://doi.org/10.36487/ACG_repo/2465_87

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Abstract:
The Deep Mill Level Zone (DMLZ) panel cave mine is located at PT Freeport Indonesia’s operations in Papua, Indonesia. Originally designed and constructed with a narrow incline undercut design with 15 m pillar spacing between parallel drill drives, the DMLZ mine experienced significant issues related to seismicity, damages and rockbursts in undercut and extraction levels during undercutting and production. One of the mitigations identified was to transition the undercut design to a wide-undercut (w-undercut). A w-undercut design doubles the spacing between drill drives to 30 m, which reduces extraction ratios and increases the size of the major apex. A transition from the narrow incline undercut design to a w-undercut layout was initiated through slot blasting towards the existing cave. A critical challenge associated with the transition included the initiating slots which were located near high-stress and rockburst-prone ground. Following the interconnection, a systematic investigation of the ground response in the first four drill drifts of the wundercut layout was undertaken. A substantial improvement in the rock mass behaviour has since been observed in the undercut level, including reduced rockburst frequency and severity as well as an overall decrease in excavation deformation and ground support rehabilitation requirements. This paper discusses the geotechnical review of the narrow incline undercut and w-undercut layout in terms of the ground response due to stress along the cave abutment, seismicity and rockbursts, as well preventative support maintenance.

Keywords: w-undercut, displacement, transition, rockburst, major apex

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