The Geotechnical Aspects of a Pillar Recovery Project in the DOZ Cave Mine

Abstract

ABSTRACT The Deep Ore Zone (DOZ) mine is a part of the PT Freeport Indonesia (PTFI) underground cave mining complex. It is the third lift in the Ertsberg East Skarn System (EESS). These mines are located on elevations between 3,000 to 4,500 amsl in an area with very high rainfall of approximately 5,500 mm per year. A significant challenge in the operations of the DOZ mine is the handling and management of wetmuck. As mining progressed since the start of production in 2000, the percentage of wetmuck drawpoints increased dramatically against the dry muck drawpoints. This condition impacted the ore blending strategy in the ore pass systems to mitigate the ore spill hazard in the ore flow systems. To overcome this dry: wet muck ratio problem, an option for pillar recovery to increase the dry muck tonnages was evaluated. Pillar recovery strategies along the major pillar apexes in inactive extraction levels was initiated. This challenging project took place in six panels between 2019 - 2021 and had to deal with a long history of abandoned areas, active wetmuck issues, closed drawpoints and panel instability concerns. This paper aims to provide an overview on the management of the pillar recovery project, geotechnical assessment of pillars and wetmuck to ensure safety and production targets are met. INTRODUCTION The Deep Ore Zone (DOZ) cave mine is a part of the PTFI underground mining complex that implement the block cave mining method between 3,000 to 4,500 amsl. DOZ mine is the third lift in the Ertsberg East Skarn System (EESS) orebody after the Gunung Bijih Timur (GBT) Mine and Intermediate Ore Zone (IOZ) Mine (Figure 1). The topography around these mines is extremely rugged and typical of the Jaya Wijaya Mountain range in West Papua, Indonesia. Wet weather with very high rainfall that averages 5500 mm per year (Widijanto et al., 2012). The very high rainfall combined with the block cave mining methods, that has broken through into the open pits or to surface, is one of the main contributors to the wetmuck at PTFI. Other contributing factors include lithology, fragmentation and draw rates (See Section 4.1 for detail Wetmuck Classification).