Abstract: It is essential to assess the stability of coal pillar before mining for the evaluation of its rockburst risk and determination of mining schemes. Based on the geological conditions of a deep coal mine in Shandong province, China, these methods including in-situ case study, theoretical analysis, numerical simulation and engineering practice were adopted to evaluate the stability of the coal pillar with thick conglomerate upper strata. The deformation of conglomerate strata and coal pillar was analysed on the basis of a simplified mechanical model. We investigated the primary source of stress that caused coal pillar deformation and its deformation form, and obtained the stress-strain curve of coal pillar. Then, by considering the stress in coal pillar, surrounding rock stability and deformation, a mechanical criterion of coal pillar failure was proposed. It indicates that the location and size of a coal pillar, as well as the deformation of conglomerate upper strata, have significant impacts on the failure of coal pillar. The deformation of upper strata results in dynamic loading on the coal pillar. The deformation of coal pillar mainly includes collaborative deflection compression deformation induced by the concentration load F and the gravity settlement deformation caused by the concentration load G. The coal pillar failures when the supporting stress (p) is greater than its ultimate strength (Rc). It is found that the coal pillar with 50 m width has a high risk of rockburst, the risk decreased by optimising the mining scheme and achieved good effects. The results of this study are of significance for the analysis of the stability of coal pillar with similar conditions.

Key words: thick rock strata, stability of coal pillar, coordination deformation, instability criterion