Vertical stress changes in multi-seam mining under supercritical longwall panels

The aim of this study is to identify the variables that affect stress redistribution in the strata underneath supercritical longwall panels. To do this, Wilson’s equations for the vertical stress distribution in the vicinity of a single longwall panel after it has been mined have been used in conjunction with finite element modelling to evaluate vertical stresses in the underlying strata. Key findings of the study are that the abutment angle has a significantly greater effect on the magnitude of the relative changes in vertical stress in the strata below a pillar than the overburden depth; anisotropic deformation behaviour of the rock mass results in strong ‘columnisation’ of the vertical stresses in the strata below the first mined seam; and the final vertical stress induced into the strata surrounding the second seam is larger in magnitude when narrow pillars are used in the first mined seam in conjunction with a large value of the ratio of overburden depth to interburden depth. For the particular cases considered in this study, increasing the abutment angle by 20° tripled the predicted change in vertical stress at a depth of 25 m below the overlying seam. Furthermore, replacing the isotropic elastic material with a transversely isotropic elastic material increased the vertical stress at the depth of 25 m below the overlying seam by 74%.

► Numerical methods are used to evaluate stresses under supercritical longwalls. ► Abutment angle of first mined seam significantly affects the maximum vertical stress. ► Multi-seam mining stability depends on pillar width as well as the OB/IB ratio. ► Larger vertical stresses at depth are observed for transversely isotropic rocks. ► The coal friction angle and UCS have little effect on the vertical stress changes.