Mechanical Study of Shear Failure of Vertical Goaf Drainage Hole

Most of the published research on goaf-hole failures are performed using 2D numerical analyses. However, 2D analyses of the goaf-hole failure mechanism, which is truly a 3D problem, often do not provide a full picture of strata movement and stress change effect. In particular such 2D models completely fail to capture the longitudinal deformation and associated shear component yielding a false sense of goaf-hole stability. This paper uses advanced 3D numerical modelling to understand the fundamentals of goaf-hole shear failure mechanism and conceptualize the transverse, longitudinal and total shears developments during longwall extraction. A mine specific geotechnical model has been developed to compare the conceptualized goaf-hole failure mechanism with the simplified representative model. The effect of geo-mechanical properties and panel widths on shear development has also been investigated. The analysis is based on a case study; however, it is expected that the results presented in this paper will be applicable to other mines with similar geological environment or will at least highlight the need for a 3D modelling to capture the true three-dimensional strata deformation behaviour and goaf-hole shearing mechanism enabling safe and stable vertical drainage hole design. In the modelled scenario, the shear increased as the face advanced, and large shears would be experienced behind 50–100 m of the face. The longitudinal shear was dominant towards the centre of the panel and the transverse shear was dominant towards the panel edges. The layered seam appeared to generate lesser shear compared to the solid seam; this could be attributed to the dissipation of shear in the layer strata of the material.