The Balaghat mine have all service roadways as well as shaft driven in hanging wall, having a weak layer of phyllitic zone in contact with the orebody. On exposure its bulging in the stopes is observed. In few cross-cuts, hangwall collapse is also observed near this contact. To adjudge its stability and corresponding changes in stope parameters when level interval is increased from 30 m to 45 m between 12th level (12L) to 13.5th level (13.5L), numerical modeling analysis was performed. 2D elastic models were solved for stope back stability and crown pillar design, while 3D elastic models were solved for post pillar design and cross-cut stability analysis. Simulation was performed by using FLAC3D software. It was found that, stope can be worked safely to a maximum span of 15 m without post pillars, beyond which post pillars of size 5 m × 5 m is required at different intervals. Result of detailed analysis is discussed in this paper.
The orebody dipping approximately at 60° varies in width from 9 m to 20 m. Footwall contact is of Felspathic quartzite. Hangwall contact is of phyllite and its thickness varies from 0 to 10 m. It is thicker at northern and southern end while quite thin near central zone of the orebody. On exposure its bulging in the stopes is observed even though a thin layer of ore, 0.5 m thick, is left against the hanging wall contact. Some cracks in cross-cuts and hangwall drivage were also observed following stope settlement in higher levels. In few cross-cuts, hangwall collapse is also observed near the contact. Stope distance from the haulage roadway is generally 30 m. In the stope, an opening of 3 m is maintained between stope back and fill material. Present practice is to use cable bolt of 12 m length at an interval of 2 m, to stabilize the immediate roof along with rock bolts of 1.5 m length. Up to 12L, which is at a depth of 320 m, 30 m Stope height is maintained with a crown pillar of height 5 m. A representative cross-section showing working levels is shown in fig.1. However, to improve the production and productivity, it was decided to increase the level interval to 45 m. Subsequently, numerical modelling studies was undertaken to adjudge safe stope dimensions along with changes in support pattern.
A review of all the plans and sections from the mine reveals that the orebody thickness, depth of cover, orebody dip and thickness of phyllite in the hangwall are varying from north side to south side of the deposit. The physico-mechanical properties utilized in numerical models are as shown in table 1. Since measured horizontal in situ stresses are not available at any of the underground manganese mine site, a theoretical equation for estimating the mean horizontal stresses proposed by Sheorey  given in equation (1) is used to evaluate mean horizontal stress for Bhalaghat Mine and is given in equation (2).