Stability analysis of underground pillar in the presence of overlying dump: a case study

The stability of pillar and immediate overlying strata is important in the Bord and pillar method of mining. Stable pillars are required to provide support against overlying overburden and dump load. This paper discusses the stability of pillar existent below a dump formed due to opencast mining. The stability of a pillar is determined in terms of factor of safety evaluated from empirical and numerical formulations based on different variants viz. dump dimension, rock quality, and depth of cover. The input parameters for empirical and numerical simulation are taken from two mines named Phularitand mine and Kuya mine which are located in Dhanbad. Subsequently, the parameters (RMR, depth of cover, dump dimension) used in the numerical model are varied to observe the effects of parameters on the stability of the pillar (factor of safety). A comparison between numerical and analytical methods is performed based on the state of stress on a pillar, determined from the numerical and analytical model. The values of the factor of safety from the numerical model were found to be 1.9 and 2.1 while 3.47 and 5.69 from empirical methods for Phularitand and Kuya mine respectively. It is observed that the factor of safety was decreased from 4 to 0.5 with a decrease in the quality of rock (Rock mass rating) from 70 to 40. The factor of safety is also found to be decreased with an increase in the dimension of dump and depth of overlying rock cover due to an increase in load on the pillar.