Longwall face stability analysis from a discontinuum-Discrete Fracture Network modelling

This paper presents the longwall face stability analysis under a realistic representation of geological structures and caving behaviours within coal seam. Face spall and its accompanying top coal fall, previously studied separately from each other, are jointly investigated by using a discontinuum-based code with a built-in Discrete Fracture Network (DFN) generator. The study provides further evidence to demonstrate that longwall mining forms increasing relief zones of vertical and horizontal stresses in vertically and horizontally elliptical shapes, respectively. Well ahead of face line intact blocks and fractures predominantly fail in shear while near face line fractures predominantly fail in tension. Both coal face spall and top coal fall ahead of shied support are found to occur in gravity-induced caving mode, and the spall is most severe at the top portion of face line. The study reveals that a longwall face is least stable when its fractures plunge into mined-out area and make an angle of 45 degrees with the overburden loading direction. The face is more stable when the fractures plunge into unmined area rather than when plunge into mined-out area. The stability of longwall face is found to be directly proportional to fracture strength. An increase in fracture stiffness decreases coal face spall and top coal fall rates but at the same time it increases fracture failure rate to some extent. The findings from this study can assist engineers in better identifying longwall face instability risks and accordingly improving face control for safe and productive underground longwall mining.