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ground control Mining the Sewickley Coal Seam: A Study in Mine Ground Control Understanding influences above and below the seam allows a mine operator to anticipate ground control problems BY ERIC GRIMM, DR. JOHN C. STANKUS, DR. QUANZHONG GU AND DR. XIAOTING LI The Sewickley is a rich coal seam that lies 70- 130 ft (21.34-39.62 m) above the Pittsburgh coal seam in Southwestern Pennsylvania. From a ground control perspective, the seam has proven very difficult to mine because of the complex and varying ground control issues that are present. These issues have been identified to include: (1) seam interac- tion stress induced by remnant pillars and gob areas left below in the Pittsburgh seam; (2) localized horizontal stress generated by steep surface stream valleys; (3) rapid changes in overburden depth; (4) sandstone in the immediate roof that changes in thick- ness and proximity to the coal seam; and (5) interburden thickness changes between the Sewickley and Pittsburgh seams. This article will detail the ground control issues and control methods that have been implemented including use of the "Roof Instability Rating (RIR)" system. The RIR system has accurately predicted and mapped potential unstable ground condi- tion areas allowing for the implementation of appropriate support, improving safety and reducing costs. Entry stability is paramount in ensuring mine safety and efficient operation. Due to the increasing energy demand, more and more mines have to operate under adverse conditions due to deeper overburden and multiple-seam influences. Dana Mining Co's Four West mine has been successively mining the Sewickley seam for more than six years while experi- encing significant ground control problems, such as roof falls and/or roof cutter, rib sloughage and floor heave. Because of the natural geological deposition and moun- tainous topography, Four West has persis- tently mined through complex and ever-changing mining conditions such as multiple-seam interaction from undermin- ing, sandstone intrusion and localized hori- zontal stress. The turning point of the roof support effectiveness was the change from non-tensioned to a tensioned primary and supplemental bolting system. The Four West roof support history indicates that tensioned 38 www.coalage.com primary and supplemental support is con- sistently reliable and more effective in stabi- lizing the roof. The major factors contributing to Four West's success in mining the Sewickley seam are: (1) Geological abnormality awareness, iden- tification and analysis; (2) Prediction of unstable ground conditions utilizing the Roof Instability Rating (RIR) analysis and calibrated with historical ground control problems; and (3) Optimization of the roof support system. Major Factors Adversely Influencing Entry Stability The mining height at Four West is 42-72 in. (1.07-1.83 m) with an average of 60 in. (1.524 m). The entry width varies from 16-20 ft (4.88- 6.1 m) with an average of 18 ft (5.49 m). Pillars in the Mains typically are 90 x 90 ft (27.43 x 27.43 m) while panel pillars are 100 x 70 ft (30.48 x 21.34 m). The major factors adversely influenc- ing ground stability have been identified as: underground roof strata deposition including sandstone intrusions, stream valley influence, overburden depth, and the dominant factor being the seam inter- action caused by remnant pillars/gob in the Pittsburgh No. 8 seam. Underground Roof Strata Deposition: A typical stratigraphic column of Four West is shown in Figure 1. The first 30 ft (9.14 m) of Sewickley seam roof consists mainly of var- ied thicknesses of laminated shale and sandstone. The roof fall and support history suggests that a tensioned bolting system could successfully control this type of roof condition. Underground examination and video- scope data indicates that discontinuities, such as naturally irregular joints and/or slickensides (Figures 3 and 4), adversely intersect the roof and reduce the action area of a single bolt, thus weakening the roof's integrity. When the roof experiences a stress concentration caused either by localized horizontal stress and/or seam interaction, roof failure might occur. Figure 1: Typical stratigraphic column. Stream Valley and Overburden Depth: The three-dimensional surface topogra- phy as generated from the surface con- tours is shown in Figure 2. As shown, the mine is under a mountainous area with numerous steep stream/valleys, various overburden depths, and potential high localized horizontal stress zones. The average differential between the moun- tain summit and valley bottom is approxi- mately 300 ft (91.44 m). Past mining experience indicates that entries under- neath the bottom of a v-shaped valley are usually adversely influenced by localized horizontal stress. The roof fall history indi- cates that when the overburden is greater than 500 ft (152.4 m), stress concentra- tions would occur at the v-shaped valley bottom thus inducing roof falls if suffi- cient roof support was not provided. Seam Interaction: As stated, the seam interaction generated from previous mining below in the Pittsburgh No. 8 seam is the dominant factor affecting ground stability. November 2012