Horizontal stress under supercritical longwall panels

• Numerical methods are used to evaluate stresses under supercritical longwalls.
• Maximum change in horizontal stress was approximately 11% for isotropic material.
• Percentage change in horizontal stress was independent of initial insitu stress ratio.
• Stratum anisotropy increases horizontal stresses only at shallow depths below seam.
• Clay layer beneath coal seam did not alter the maximum horizontal stress changes.

This study identifies the relative magnitude of horizontal stress change below a series of parallel longwall panels as a consequence of multi-seam mining. The investigation method yields an upper bound to the horizontal stress change by assuming that the fractured and caved overburden above the supercritical longwall panels is not able to support any lateral load. Finite element modelling was conducted to consider the effects of the following variables on the horizontal stress redistribution: the ratio of the equivalent width of the extracted area to the overburden depth, the ratio of the original horizontal to vertical in situ stress components, and anisotropy in the rock mass behaviour.The key findings of this research are that the maximum horizontal stress induced as a consequence of extracting multiple supercritical longwall panels will be within approximately 10% of the original in situ horizontal stress for the case of isotropic rock strata. The magnitude of this horizontal stress may be larger in a stiff stratum if the overall sequence of strata consists predominantly of softer rocks. Anisotropy in the strata underlying the extracted longwall panels also causes an increase in the maximum horizontal stress induced into the immediate floor of the first extracted seam, and an increase in the rate of dissipation of horizontal stress change with depth. These findings indicate that a significant increase in horizontal stress relative to the original in situ horizontal stress should not be expected in multi-seam mining situations. The results from this study need to be considered in conjunction with predicted vertical stress distributions in order to accurately predict the overall stress environment likely to be encountered when conducting multi-seam mining.