A Numerical Study to Assist Assessment of the Stability of Shallow Coal Mine Goafs

Coal mine goafs are distributed widely across many parts of the world and their stability is a major cause for concern, particularly when designing new infrastructure. To reduce risk, the coal mine goafs are often stabilized using cement grout and such operations can be very expensive and difficult to verify. This paper uses the finite element method to examine the relative influence of a number of key factors controlling the stability and surface deformations of shallow, horizontal coal mine goafs overlain by sedimentary rock. Representative ranges in the stiffness and strength characteristics of coal and rock are examined and each material is assumed to satisfy the Hoek–Brown failure criterion. The analyses show that, for a typical maximum coal extraction rate of 40%, the critical goaf span varies linearly with the depth of the coal seam (for the maximum depth investigated of 45 m) and increases with the competency of the overlying rock. A relationship combining the rock quality and the ratio of the critical goaf span to the depth of the coal seam is proposed to enable assessment of abandoned coal mines with marginal stability. This relationship is shown to be consistent with observations made in two coal mining case histories.