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roof control Understanding the Illinois Intersection Research sheds more light on why and how roof falls occur at intersections in underground coal mines At the 2013 Society for Mining, Metallurgy and Exploration (SME) conference, which was held during February in Denver, researchers from Southern Illinois University presented new findings about roof control at intersections in coal mines. Roof falls typically occur in or around intersections. Their research looked at stress distribution and the associated failure behavior around 3- and 4-way intersections. They performed 3-D numerical analyses to determine factors that influence intersection stability. Failure zones around 3-way and 4-way intersections were modeled. Mining engineers know that the intersection span and horizontal stress have a major influence on roof stability. At a 4way intersection, pillar corners across the intersection fail first and lead to progressive failure of immediate roof and floor layers. The failure mechanism is similar for the 3-way entry but the shape and extension of failed zones differ. Coal ribs mostly fail due to tensile stress, while roof and floor strata fail due to shear stresses. Rib corners fail due to a combination of shear and tensile stresses. In addition to stress distribution, safety factor contours analyses were performed to assess stability of intersections. About 70% of falls occur at intersections. More than 80% of falls occur at intersections in Illinois. With underground coal production growing rapidly in the Illinois Basin, there is now a pressing need to improve the stability of intersections. By analyzing stress distribution and instability around intersections in the region, researchers hope to identify more appropriate supports for improving their stability. Model Development & Analysis Techniques Numerical analyses for typical 3- and 4way intersections at a 500-ft depth were performed using FLAC3D software. Eight 50 www.coalage.com models were analyzed. The model extended 50-ft above and 50-ft below the coal seam. The coal seam thickness was 6 ft. Engineering properties of roof, coal and floor lithologies were based on previous exploration studies. The effect of pre-mining horizontal stress and width of entry span was analyzed. Linear elastic and non-linear failure analyses using the Hoek-Brown failure criterion for rock masses were used. Failure criterion parameters were developed from rock mass classification data. Progressive failure and failed zones of the rock mass were developed. Analyses were used to perform stress distribution and sensitivity analyses for selected variables. A 550-psi vertical stress was applied at the top of the model. Different lateral stress ratios were applied. Field estimates of in-situ stresses (1,100-psi in the E-W direction and 650-psi in the N-S direction) were uniformly applied to the entire model prior to excavation of intersection. The immediate roof strata above the coal seam was replicated as black shale (6.5 ft), gray shale (2 ft), weak limestone (2 ft), weak shale (3 ft), competent limestone (4 ft) and thick shale (22 ft). The strata below the coal seam consisted of gray shale (3.3 ft), weak limestone (3.3 ft) and thick shale (33 ft) below the weak limestone. The model allowed slippage along layers interface. The cohesion and friction between different layers was assumed to be zero to simulate the unbonded layers. Models simulated the 3-way intersection with a 60° x-cut angle and 4-way intersection linearly and nonlinearly. Pillar off-set distances of 25-ft, 35-ft and 45-ft were selected for assessing stability. The rock mass properties used for modeling correlated with field measured values of roof-floor convergence. Analyses were done in a sequential manner that involved the application of pre- mining stress and excavation of the opening. Stress Distribution Around a Typical 3-way Intersection After evaluating stress concentration factors (SCFs) for 3-way intersections with a different off-set distances, the results show that an insufficient stagger interval creates a large intersection span. Moreover, due to the shape of the pillar, the peak SCF occurs outby the pillar toward the opening, which may cause instability. Thus, alternative 3way intersection geometries should be considered. Safety factor contours (SF) were plotted for 3- and 4-way intersections (See Figure 1). The lowest SF of 0.65 for both models was observed at the pillar ribs. However, SF at the corner of a 4-way intersection is 0.7 while this value for a 3way intersection is 0.9. Comparison of Alternative Intersection Geometry It is important to consider mining cycle times in re-designing the 3-way intersection. From a stability point of view, choosing two right angle pillars might be a very effective solution. However, it increases mining cycle times. Therefore, intersection off-set distances were considered for angled cross-cuts. Analysis showed the need to increase the distance between two adjacent intersections (off-set). To find the minimum off-set value, which would be required to prevent the overlap of stress concentration zones created by the two side-by-side intersections, analyses were performed for three values of off-set distances (25 ft, 35-ft and 45-ft). The peak vertical SCF value for a 3-way intersection can occur in immediate roof outby the pillar toward the opening. This may cause roof control problems. The SCF values for a 4-way intersection are symmetric and the peak VSCF values are high- April 2013