ABSTRACT
A rock mechanics research including rock mass displacement measurements at the surrounding area of a tunnel using an extensometer, a convergencemeter and a borehole camera was conducted in the Pongkor Underground Gold Mine, West Java, Indonesia. Rock mass observation at the research area and rock mechanics laboratory test on rock samples obtained were analyzed to predict the rock mass rating (Bieniawski, 1989) and yielded an RMR of 60 which was classified as fair rock – class III. The wall condition of the underground opening was considered stable due to the fact that convergence rate measurements for horizontal, left, and right convergences were 0.0022 mm, 0.0059 mm, and 0.0078 mm daily respectively. These value were less than the critical displacement rate criteria proposed by Cording (Bieniawski, 1973) 0.05 mm daily. Blasting activities conducted at a distance of about 10 m from the point of monitoring apparently influenced the back to moving upward (negative displacement). The picture from the borehole camera at a depth of 0.75 m below the collar, snowed a crack with a gauge of 2.64 mm and a dip of 45°.
