In spite of significant research on coal mine ground control in the world, falls of roof, face and sides still account for most of the fatal and nonfatal injuries in coal mines. Over 70% of these falls occur at intersections and this percentage has remained relatively unchanged in the last two decades. Therefore, there is a significant need to develop alternate approaches to stability of coal mine intersections. Toward this goal, the authors have performed 3-D finite element modeling studies and have developed a displacement-based analysis approach that is more intuitive and delineates stability and artificial support requirements better than stress based approach typically used in the past. The two approaches should be used together to understand the stability problems around an intersection. The authors present here the displacement-based approach with analysis of the effects of a few important variables.
Spearing and Mueller (2008), based on an analysis of rock falls related to fatal and non-fatal injuries in the USA for the period 2002 to 2007, indicated that about 70% of the rock falls occur at intersections even though they represent only about 20–25 % of the area developed for coal extraction in a typical coal mine. Thus, there is a significant need for technical studies to improve stability of coal mine intersections. Toward this goal, the authors attempt to develop a better understanding of stability issues around an intersection and evaluate the effect of several important variables such as width of entry, thickness of weak floor strata immediately below the coal seam, in-situ lateral stress field, and laminations in the immediate roof. A displacement-based approach, similar to analysis of mine subsidence, is developed that seems to more clearly delineate the stability issues around a coal mine intersection. The authors suggest that the stress-based and displacement-based approaches should be used together to better analyze stability problems around an intersection. The analyses above were performed using 3-D finite element analysis (FEA) of an intersection for typical geologic conditions associated with Herrin (No 6) coal seam in Illinois. Analyses were performed in elastic, layered media using approximate rock mass properties for different lithologies based on previous modeling experience. Analyses were initially performed for
layered, but with no slippage or separation between layers (bonded),
no weak floor strata,
zero tectonically-induced lateral stress field, and
entry width of 6.0 m. Then the effects of one weak floor strata thickness, one set of values for horizontal stress field typically reported for Illinois, and layered media with ability to slip and separate (un-bonded) were evaluated. Finally, the effect of variable weak floor thickness across an entry and variable horizontal stress in different layers was considered.
The coal seam is overlain by Black Shale, weak shaley-limestone, gray shale, and limestone to a height of about 8.3 m above the coal seam. The immediate floor strata consist of claystone, limey-gray shale, and limestone down to a similar depth below the coal seam.