Coal Age

MAR 2019

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March 2019 www.coalage.com 37 roof bolting continued larly under deeper cover. No slab cuts were taken from the barrier and a final stump was left unmined to pro- vide roof support during pillar recov- ery. The size of the final stump was a minimum 6 ft (1.8 m) x 6 ft (1.8 m). Six wood posts are spaced 3 ft to 4 ft apart across the entry to provide support before a pillar lift was start- ed. An 8-ft-long (2.4 m) cable bolt was used as a supplemental support in the intersection areas, the spacing be- tween the cable bolts is approximate- ly 6 ft (1.8 m). The mine is using one row of 3-ft-long (0.9 m) No. 5 (5/8-in) fiberglass bolts to support the coal pillar ribs in areas greater than 1,000 ft of overburden. The spacing of rib bolts at site-2 is roughly 8 ft (2.4 m). Visual Observations Underground visual observation was one of the primary techniques used to evaluate the stress and deformation levels at the instrumented sites. Ob- servational data can provide informa- tion about large areas of a mine and, when combined with instrumentation results, can be used to validate large- scale FLAC3D models. When the pillar line was located at the instrumented sites, the immediate roof, floor, ribs, and the extent of roof caving were ob- served in the retreated panel. At both site-1 and site-2, there were no signs of the following: excessive roof sagging, open fractures in the roof, roof cutters near the rib line, or outby roof failure. Also, there were no signs of excessive roof-bolt deformations, such as excessive bolt elongation or severe- ly deformed bolt-plates. The floor was not visibly heaved, and it was in good condition. In other words, the struc- tural integrity of the immediate roof and floor was sufficient to withstand development stresses and retreat min- ing-induced abutment stresses at both site-1 and site-2. The entry between the two instrumented pillars at each site was accessible and the ribs were in good condition with little rib spalling. In addition to the visual observa- tion and evaluation, several test holes were drilled in the roof near the instru- mented sites to evaluate the immedi- ate roof for bedding separations and the lateral displacement/roof shifting. No roof shifting was recorded at either site test holes. However, roof bedding separation was reported at site-1. At site-1, the immediate roof caved right behind the pillar line with no signs of roof overhanging, and the gob generally formed quite rapidly. However, at site-2 prior to the removal of the data logging system, the imme- diate roof did not cave, and was over- hanging for at least one break, approx- imately — 100 ft (30.4 m) inby the in- strumented pillars. The narrowness of the panel width for site-2 and the pres- ence of sandstone and silty shale in the roof may have restricted caving. Instrumentation Plan Instruments have been used increas- ingly in mines to measure deforma- tion, stress, strain and load. The instruments included were one four-point roof extensometer (RXC), two six-point roof extensom- eters (RXL and RXR), one borehole multi-point rib extensometer (MPRX), six borehole pressure cells (BPCs), and three hollow inclusion cells (Hi-cells). The Hi-cells were installed in the roof near the rib at 45° angles (see Figure 2). The pressure change in the BPCs represents the induced abutment load due to only pillar retreat mining. The maximum pressure change in the BPCs in pillar 1 was 2,752 psi (18.9 MPa). The strength and stiffness of the immediate and main roof beams may have inhib- ited gob formation and caving at site-2, which can have a significant impact on stress levels at the pillar line. The RXL and RXR measured the vertical displacement of the imme- diate roof. During retreat mining at site-1, the maximum and minimum anchor displacements were roughly 1.4 in (0.0355 m) and 0.6 in (0.0152 m). The RXR extensometer data was corrupted, as it neither matches the results of the RXL nor the visual obser- vations at the site. At site-2, the maxi- mum and minimum anchor displace- ments were roughly 1.2 in (0.0304 m) and 0.6 in (0.0152 m), respectively. The two main factors that lead to an increase risk of rib falls during retreat mining are thicker coal seams and higher stress levels. The MPRX were used to monitor the deforma- Figure 1—Pillar retreat plan for a) site-1 and b) site-2. Instrumented sites are marked by red circles. Figure 2—Instrument type and location for both site-1 and site-2.

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