Driving and managing stress in the Deep Mill Level Zone caving mine

The Deep Mill Level Zone (DMLZ) panel cave mine in PT Freeport Indonesia continues to ramp up production since initial undercutting began in 2015. At approximately 1,500 m below surface, the DMLZ is one of the world’s deepest caving operations. The current undercut area has reached 60,000 m2, within a 1,200 m triangular-shaped perimeter. This deep mining environment has given rise to substantial pre-mining and induced loading conditions. Stress management in production areas represents the toughest challenge to date for the operation. In addition to applying lessons learned from the overlying Intermediate Ore Zone (IOZ) and Deep Ore Zone (DOZ) mines, extensive empirical ground response data has been collected from undercut and extraction levels. The understanding of how the various geological, geotechnical and mine design criteria interact has resulted in a much-improved approach towards DMLZ stress management. The most notable successes related to DMLZ stress management include improved understanding of ground response, accelerated cave growth, and increased production rates. This paper summarizes the key learnings regarding stress management in production areas and outlines positive improvements undertaken towards sustained, safe caving in the DMLZ.