POST-PILLAR BEHAVIOR AT DEEP LEVELS IN A COPPER MINE

Mosaboni mine is one of the copper mines in the Singhbhum Thrust Belt in Bihar, India. The post-pillar stoping method has been employed to e xploit a shallow dipping wider orebody at deeper levels (950m) in the mine. In this method, slender pillars are left at regular intervals in stopes to support the immediate stope back. The main rock mechanics a spect of the method is that post-pillars (PPs) should yield under the fill and be in a post-failure condition, most of the superincumbent lo ad being transferred to the abutments. Numerical simulation of post-pi llars (4 x 4 m square) with clear room of 9 x 13 m between them is sim ulated using FLAC software. The model analysis showed that the PPs wer e in a post-failure state since their formation. However, if they are provided with confinement by applying support to them and are prevente d from unraveling, they will be able to support the stope back. It is further observed in the numerical modelling that once the yielding por tion of the pillar was buried under the fill, it becomes stabilized. I n actual mining practice there are PPs larger in size and closely spac ed. Fractures appear on their sides followed by spalling of fractured rock. The lateral deformation in one of the pillars was monitored to s tudy its stability. It was found that the lateral deformations induced by stresses were controlled by foliation and joint planes. A 2 x 2 m core of the pillar was stable and movement was restricted to a 1.5 m z one from the periphery of the pillar.