Abstract. Jim Walter Resources No. 4 mine produces coal through retreat longwall mining at 610 to 670 m (2000 to 2200 ft) of depth. A three- entry, total yield pillar, longwall development experimental region was established for three adjacent longwall panels utilizing 6.1 m (20 ft) wide gate section pillars. Presently, the first longwall panel is completed with roadway deformational behavior monitored for a ground control assessment of the mining system. Data illustrate that the total yield pillar gate section design at No. 4 mine is presently comparable to gate section designs having large equal size chain pillars. Ground conditions will continue to be monitored during the mining of subsequent longwall panels in order to determine the ground control implications from multiple panel extraction.
Introduction
No. 4 mine is situated approximately 64 km (40 mi) southwest of Birmingham, Alabamanand extracts coal from the Black Warrior coal basin a depth of 610 to 670 m (2000 to 2200 ft). Continuous miner units excavate multiple entry developments to form main air courses and to isolate production panels for retreat longwall mining.
Mine Geology
The mineable geologic seam consists of the Mary Lee coal, a rock layer termed the "Middleman" and the Blue Creek coal seam as shown in Figure 1. In the experimental area, the majority of the continuous miner excavation involves the extraction of the Blue Creek coal leaving the Middleman rock and Mary Lee coal as the immediate roof. This extraction of only the Blue Creek is designated as single seam mining. When Middleman thickness is not excessive, continuous miners extract the Mary Lee, Middleman rock, and Blue Creek which is designated as twin seam mining. Longwall mining only extracts the Blue Creek coal seam. Approximately 10.7 m (35 ft) of shaley sandstone overlies the Mary Lee seam. The Mary Lee seam is approximately 0.5 m (1.5 ft) thick and the Blue Creek seam ranges from 1.2 to 2.1 m (4 to 7 ft) in thickness. Both seams are a bituminous coal.
Figure 1. Stratagraphic Column in the vicinity of the Mary Lee and Blue Creek coal seams(available in full paper)
Middleman rock lithology varies from a carbonaceous shale to a shaley sandstone and can range in thickness from 0.3 to 1.5 m (1 to 5 ft). A fire clay / shaley sandstone floor underlies the Blue Creek seam. Single seam mining heights range from 2.4 to 3.4 m (8 to 11 ft). A 6.1 m (20 ft) mining width is used at No.4 mine.
Longwall Development Section Pillar Design
Ideatly, longwall gate section pillar design should allow for maximum development productivity, maximize section advance rate, enhance resource recovery, as well as provide for effective ground control during longwall mining. Presently, No. 4 mine longwall and main entry developments are designed upon a yield-abutment-yield pillar principle as illustrated in Figure 2. These designs have been discussed in previous publications where yield-abutment-yield pillar configurations were shown to offer improved roadway stability and improved resource recovery when compared to designs employing equal size chain pillars (Gauna, Price, and Martin, 1985).
