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 4-way 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 4-way intersections at a 500-ft depth were performed using FLAC3D software. Eight 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 3-way 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 3-way 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 higher than a 3-way intersection (about 7% to 32% along different cross-sections). However, the location of the values for a 4-way 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.
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 off-set 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 shape and extension of failed zones differ slightly. Coal ribs mostly fail due to tensile stress, while roof and floor strata fail primarily due to shear stresses. Rib corners fail due to a combination of shear and tensile stresses.
The peak values for each SCF are located around the edge of the opening or pillar, 0.5-ft into the entry or pillar. The vertical SCF values for a 4-way intersection are higher than a 3-way intersection (about 7% to 32% along different cross-sections) but it occurs deeper into the solid pillar. Both HSCF-X and -Y value for a 3-way intersection are higher than the 4-way intersection. Thus, a 4-way intersection provides better stability for horizontal stress.
The peak VSCF and HSCF values for pillar with 120° angle crosscut occur into the opening that makes this pillar more vulnerable from ground control point of view. Supplementary support might be required near this pillar. Effective pillar off-set distance for 3-way intersection design is critical. It is important to identify the minimum off-set distance value, which would be required to prevent the overlap of stress concentration zones created by the two side-by-side intersections. In this study a 3-way intersection with 45-ft off-set provides better stability and decreases the span of failed zone. Identifying the height of the failed zone is critical for designing an appropriate support system such as cable bolt, truss system, torque-tension bolt system, etc.
For more information on this research program, contact SME (www.smenet.org). Ask for Pre-Print 13-148, Numerical Analyes of Stability of 3-way and 4-way Coal Mine Intersection in Illinois by B. Abbasi, SIU-Carbondale, and Y.P. Chugh, SIU-Carbondale.