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roof control continued er than a 3-way intersection (about 7% to 32% along different cross-sections). However, the location of the values for a 4way intersection is deeper into the pillar than for a 3-way intersection. Horizontal Stress Analysis Horizontal stress is an important factor for roof stability especially in shallow mines. It must be considered in both X and Y directions.High horizontal SCF-X for a 3-way intersection was observed. This may cause cutter-roof failure. The peak HSCF-X value for a 4-way intersection is significantly lower than 3-way intersections. A 4-way intersection is better for HSCF-X. The 3-way intersection with 45-ft off-set shows slightly better HSCF-X (about 3% to 5% lower) than 25-ft and 35-ft intersections. The maximum HSCF-Y for a 4-way intersection is located into the pillar. However, the peak HSCF-Y for a 3-way intersection occurs into the opening. This may cause tensile cracks in this zone and ground control problems. The peak HSCF-Y value for a 4-way intersection is lower than a 3-way intersection. Shear Stress Analysis In stratified deposits such as coal, the roof failure is due to a combination of shear and tensile stresses. It has been shown that among the three independent shear stresses (SSCF-XY, SSCF-XZ and SSCF-YZ), the SSCF-XY is most critical. Researchers analyzed all three components and confirmed that the SSCF-XY affects stability significantly. The results of SSCF-XY only were plotted and interpreted. However, this may not be the case for all mining depths. The peak SSCF-XY values for 3-way intersections were located in the pillar and they were about 0.25 for all models. Analysis of Failed Zones The failure zone develops gradually around an intersection. Failure initiation results in stress redistribution into areas that are capable of sustaining higher stresses. The failed zone may develop residual engineering strength properties that are much lower than the in-situ rock mass properties. If the failed zone is not adequately supported, it extends gradually and large displacements associated with the failed rock mass may result in roof fall. April 2013 Figure 1: Safety factor contours for 3-way and 4-way intersections. Extension of the failed zone into the floor may result in floor heave and pillar failures. Large absolute displacements of failed rock mass can also result in large differential displacements that can impose additional stresses in the intact and failed rock mass to progressively increase the failed zones and develop new failed zones. The 3-way intersection with 25-ft offset has the larger failed zone as compared to 35- and 45-ft offset distances. Moreover, for the 25-ft off-set, the failed zone at two adjacent intersections intersects and develops a large span of failed zone (Figure 2). This mechanism can be seen in 3-way intersections with 35-ft off-set as well, but it is not present in a 3-way intersection with 45-ft off-set. The height of the failed zone for a 4-way intersection is about 3 ft more than the 3-way intersection. The failed zones described here represent failures of rock mass in both shear and tension. For the 4-way intersection, pillar corners across the intersection fail first and lead to progressive failure of immediate roof and floor layers. The mechanisms of failure are similar for the 3-way intersection but the www.coalage.com 51