Executive Summary : | The stability of pillars is generally quantified in terms of factor of safety (FOS) which is a basic desideratum to ensure the safety of the underground workplace. In most cases, the FOS of pillars is calculated in a deterministic approach where strength is kept constant for a coal seam. The disadvantage of the FOS approach is that two pillars with identical shape, size and average strength have different failure probabilities for the same FOS. This is due to randomness and heterogeneity in strength. Generally, pillars for long term stability are designed by keeping the FOS greater than 2.0, whereas remnant/rib pillars are designed by keeping the FOS less than 1.0 to facilitate the caving of overlying strata. It becomes difficult for designers to determine the adequate size of pillars/remnants/ribs when the average strength of two different coal seams are same but the variation of strength is less for one coal seam and it is high for another coal seam. As per the available design norms of coal pillars, the pillar size for both cases would be the same due to the limitations of available design norms which do not explicitly consider the variability of strength. All the existing pillar strength formulae are developed where one single value of strength is used. Irrespective of the spatial variation in strength value, the existing formulae give the same strength value of the pillar for all cases. By using the existing pillars strength formula, several case studies reveal that some pillars having FOS greater than 1 failed and some having FOS less than 1 did not fail. The reason is that the variation of the strength value has not been considered to determine the FOS by a deterministic approach. In these cases, there may be chances that the probability of the designed FOS was less. Therefore, the stability of pillars should be quantified in terms of the probability of the designed FOS instead of a single value of FOS. In this project, design norms/procedures of underground coal pillars are to be framed based on a stochastic approach by which uncertainties of rock mass in terms of randomness and spatial variation can be addressed effectively. At first, the spatial variation and randomness in uniaxial compressive strength (UCS) of coal, the most significant parameter, are to be quantified by a semi-variogram model and probability distribution respectively through the field and laboratory testing data. Then, numerical modelling procedures are to be established to incorporate the semi-variogram model and probability distribution for the determination of the strength of coal pillars. After that, the design procedures by considering the variability of UCS of coal will be established to determine the adequate size of coal pillars based on the probability of the designed FOS. This approach helps to increase the recovery of coal vis-à-vis the safety of the underground workplace by optimizing the size of coal pillars with a higher confidence level. |