Stability analysis of pastefill used for backfill material at Kencana Underground Gold Mine, North Halmahera, Indonesia

摘要

PT Nusa Halmahera Minerals (NHM) is a gold mining company which located in North Halmahera, North Maluku Province, Indonesia. One of NHM mine site is Kencana Underground Gold Mine which applying Underhand Cut and Fill (UCF) mining method. Backfill material used in this side is pastefill type. Stability analysis of pastefill block needs to be carried out due to the reason that it will act as back (roof) when underneath stope is opened. This research is carried out in Kencana 2 Sub 12 C undercut 3 Ore Drive (OD) 1 North. The stability of pastefill block is analyzed by applying numerical analysis using 3DEC software Ver. 2 from ITASCA and analytical analysis using limit equilibrium method. Mechanical and physical characteristics of rock is obtained from NHM while the one of pastefill is obtained from laboratory testing. From the result of numerical analysis, pastefill with 14% cement content and 7 or 28 days curing time will collapse through caving failure mechanism on 6 m span. This result is in a good agreement with limit equilibrium analysis where caving failure mechanism will occur at 6 m span. To overcome this instability, reinforcement materials consist of chain link mesh and stand up bolt are installed at the floor of mined out stope before pastefill placing is conducted. Result of numerical modelling shows that reinforced pastefill after 28 days curing time is in stable condition. Therefore it can be suggested that mining operation underneath pastefill with 14% of cement content can be carried out after 28 days of pastefill placing.