Impact of Discrete Fracture Characteristics on Longwall Top Coal Stability

Abstract

Discrete fractures may exist in thick coal seam and significantly impact the top coal stability in the Longwall Top Coal Caving method (LTCC) both ahead of shield support (top coal fall) and behind shield support (top coal caving). The top coal stability in such conditions is not well understood in the literature and has been studied from either fall or caving behaviour. In this paper, a discontinuum-based numerical program is used to study longwall top coal stability when discrete fractures exist in coal seam and vary in characteristics (i.e., orientation, density, stiffness, strength, and intersecting fractures). The study demonstrates that the existence of discrete fractures decreases the top coal stability ahead of shield support, particularly in initial face extraction. The parametric study finds that when the fracture orientation makes an angle of 90 degrees to the positive x-axis, it has the least impact on top coal fall. When the fractures plunge into the mined-out area, they facilitate top coal caving and vice versa when they plunge into the unmined area. The study reveals that the fracture density is directly proportional to top coal fall and top coal caving. Meanwhile, the fracture stiffness and strength are inversely proportional to both top coal fall and caving. The study also demonstrates the important role of coal seam characteristics (strength, elastic modulus, and depth) in top coal fall. The findings from this paper can assist engineers in improving panel geometry design and roof control for efficient underground mining when discrete fractures exist and vary in a coal seam.