Mining within a weak rock mass - Kencana Underground Mine case study - PT Nusa Halmahera Minerals (Newcrest Mining Ltd), Indonesia

Kencana Underground Mine is located on the remote island of Halmahera, Indonesia in a seismically active region. Kencana uses mechanized underhand cut and fill mining to produce approximately 300 ktpa of ore at a grade of 39 gram/tonne gold. Underground development commenced in July 2005 and over 2 km of 6 m x 5.5 m access ramp and stope accesses were driven within a weak rock mass comprised largely of tuff, conglomerate, mudstone, andesite and volcanoclastic rocks. To ensure overall ground stability, a rigorous quality control and assurance program was implemented. This paper looks at the methods used at Kencana within the development stage which consisted of an arched profile, various support types, a regular pull testing program for friction bolts (grouted and un-grouted), a shotcrete testing program and controlled re-entry based on cure time and strength. These were considered fundamental to understanding the interaction of the ground support with the rock mass and mining activities. The QC and QA program developed a large database which has been used to augment development strategies for poor to very poor ground conditions. It shows a strong interaction between operations, engineering and implementation of a ground control program. This enabled a safe and cost effective strategy used to complete the Kencana access ramp and further provided information for subsequent mining of the ore.