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In this study, proper design of coal pillar has ever remained a difficult and challenging task because of two conflicting objectives, namely the conservation of coal on one hand that requires coal pillars to be of minimum size and stability of pillars, and on the other hand that ensures safety of working. The complex loading conditions, the occurrence of seams in close succession and geological abnormalities present in seams make the design problems of coal even more complex and complicated. An attempt has been made in this paper to enlist and describe briefly factors affecting pillar strength and empirical strength equations/formulae developed by different researchers in different parts around the globe. Further, a comparative study has been made for a given set of geo-mining conditions to know the suitability of these equations in terms of their being either over-estimators or under-estimators.

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The study investigates the effect of various soil mixtures, mostly found in the overburden material of coal mines, on the strength parameters and compaction properties. Samples were prepared in laboratory to represent both the soil-type and the rock-type waste dump material by mixing shale, sandstone and clay in varying proportions at an interval of 10% each. Samples from two coal mines located at different geographical locations in the eastern part of India were collected and the shear strength (cohesion and friction angle) and compaction parameters (Maximum dry density, MDD and Optimum moisture content, OMC) of the specimens were determined from laboratory study at moisture content around their OMC and analysed. The results so establish that the shear strength parameters for mixtures containing shale are less sensitive to changes and follows rather a flat curve when plotted against their respective material content in the mixture when compared to that containing clay. However, the compaction parameters were found to follow different trends when plotted against the constituent material (shale, sandstone and clay) content of the mixture.

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