Coal Age

NOV 2012

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ground control continued (15.24-30.48 m) from the gob line of the Pittsburgh No. 8 seam (Figure 7). Within the stress influence zone of the identified remnant pillars, intensified roof support with more cable bolts and tensioned pri- mary bolts are required. Figure 3: Remnant pillars in the Pittsburgh No. 8 seam. Roof Instability Rating Analysis When experiencing multiple adverse fac- tors affecting entry stability, it would be greatly instructive to predict and delin- eate roof conditions based on identified factors contributing to the roof instabili- ty. The Roof Instability Rating (RIR) anal- ysis is an effective methodology that integrates factors pertaining to entry sta- bility with different weighting factors, geological data and roof failure history, and it has been successfully applied at many mining operations. Figure 4: Vertical stress distribution across the model. decades of under-mining, rock layers within the fracture and caved zones might be compact, reaching new stress equilibrium and retaining certain residu- al tension around the remnant pillars. When upper-mining in the Sewickley seam is in the vicinity of stress concen- tration around the remnant pillar, severe damage on the floor, rib and roof can be expected. To minimize the influence of the rem- nant pillar, a stress influence zone around a single remnant pillar and/or gob line was generally defined as zone (a) 100-200 ft (30.48-60.96 m) around the perimeter of the pillar(s) and (b) 50-100 ft At Four West, the identified factors con- tributing to the roof instability and used in the RIR analysis are sandstone thickness and interval, overburden depth, stream val- ley effect, interburden thickness between the Sewickley and Pittsburgh No. 8 seams, and large underlying pillar. The RIR rating is calculated by the following equation: The RIR can be approximately divided into three groups based on roof fall history cali- bration: • RIR < 30: When the RIR is less than 30, the roof condition can be considered good; • 60 > RIR > =30: When the RIR is less than 60 and greater than or equal to 30, the roof condition can be considered as requiring more intensive support; and • RIR > = 60: When the RIR is greater than or equal to 60, the roof condition should be considered as critical and requiring the most intensified support. The mine history and experience has shown that the RIR distribution is very accurate in predicting ground condi- tions. However, the RIR does not predict localized geologic anomalies such as slips, slickensides, horsebacks, etc. Further, in terms of predicting seam interaction from mining below, the RIR is highly dependent on the accuracy of the undermining maps. As an aid to mine personnel, the RIR is updated on a regu- lar basis as mining proceeds and new drill hole data becomes available. The mapping (Figure 8) is then superimposed on the mine map for mine personnel to review and become aware of upcoming potential changes in conditions. Where, w1, w2, w3, w4 and w5 are the weighting factors of SR (sandstone rat- ing), SVR (stream valley rating), ODR (overburden depth rating), ITR (inter- burden thickness rating), and RPR (rem- nant pillar rating) respectively. The corresponding weighting value for each factor is calibrated and assigned in terms of mine roof failure historical cases and geological analysis. The RIR ranges from 0 - 100. The higher the RIR value, the more unstable the roof. The RIR analysis distribution across the current mine property is shown in Figure 8. Optimization of the Ground Control The strategy of optimizing the roof sup- port system is to choose/modify the opti- mal bolt type based on duly identified roof stability factors. Historically, the sig- nificant improvement in ground control at Four West was changing from non- tensioned to tensioned bolting systems (primary and supplemental). The major advantages of the tensioned bolting sys- tem are the enhanced beaming effect/self-support competence and pro- viding a much higher stiffness and resis- tance to the initial roof movement. The Figure 5: Vertical stress distribution along the Sewickley seam roof line. 40 www.coalage.com Figure 6: Vertical stress distribution from the surface to the Pittsburgh No. 8 seam floor. November 2012

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