Support Technical Evaluation Using Q-System Method in Development Area of Grasberg Block Cave (GBC) Mine PT Freeport Indonesia

PT Freeport Indonesia (PTFI) applies two mining systems, open pit mining in Grasberg and underground mining. This study was carried out in the Grasberg Block Cave (GBC) underground in extraction level at panel area, one of the underground mines located just below the Grasberg open pit mine by the block cave method. At present, GBC is in the development process, one of the cycles in development process is supporting installation. Supporting installation is the last cycle which include two operations, namely primary support installation and secondary support installation. The study was conducted on primary support. Primary support is the operation after excavation has been completed and aims to strengthen the rock mass (reinforcement). The use of primary support material actually experiences a significant increase so a technical study is needed to determine the difference between material requirements based on the design and actual conditions. After that, an evaluation is carried out by redesigned the support system based on rock mass classification of the Q-System to obtain a safe support recommendation. The Q value for Panel 16 is 0.7 including to the poor rock mass. The Q value for Panel 14 is 2.9 including to the fair rock mass. According to this study, recommendations are obtained using a split set length of 2.1 meters and 1.3 meters spacing and resin bolt with a length 2.4 meters and 1.2 meters spacing for panel areas with poor and fair conditions. Numerical modelling with finite element method is done with the help of Phase2 software version 8.0 determine the strength factor of the recommendations obtained analytically. Based on the results of the modelling, it is found that the smallest strength factor is 1.3.