Coal Age

MAR 2019

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36 www.coalage.com March 2019 roof bolting Assessing Roof-bolt Performance in Deep Cover Using field results, NIOSH researchers calibrate models to determine the reaction of different bolt lengths Underground coal operators rely on roof bolts. To establish a good roof control plan, mine engineers and managers need to understand how well roof bolts can be expected to per- form. With years of experience, most mines know what to expect with typ- ical room-and-pillar operations. They also know the roof support principles change with depth. Mining the pillars, or retreat mining, adds another twist. Combining the two creates an inter- esting case study for rock mechanics. To better understand load shed- ding and stress transfer on coal pillars due to retreat mining in deep cover panels, researchers from the National Institute for Occupational Safety and Health (NIOSH) conducted a monitor- ing field study. Two sites, at overburden depths of 1,000 ft (304.8 m) and 1,500 ft (457.3 m), were selected in a room- and-pillar mine in southern West Vir- ginia, operating in the Lower War Ea- gle (LWE) seam. They monitored the deformation and stress changes in the roof and two adjacent pillars at each site during the retreat mining process. The results along with field observa- tions were used to calibrate large-scale models for each site, which were used to assess different sizes of roof bolts. During pillar retreat mining, the likelihood of instabilities increases due to elevated stress levels near the pillar line because of the abutment loading. During the last 10 years, about one-third of the ground fall fa- talities in the United States occurred during retreat mining. One interesting piece of informa- tion about pillar-recovery fatalities was that the victim was nearly always under bolted roof. Sheared and bro- ken No. 5 (5/8-in.), fully grouted re- bar-bolt failure contributed to three of four fatal roof-fall incidents that have occurred in deep-cover retreat mines. This claim raised the following question at the study mine: Is it safe to use a 4-ft-long No. 5 rebar bolt (grade 60) at a depth of cover more than 1,000 ft? To answer that ques- tion and others related to retreat mining in deep cover panels, NIOSH researchers monitored the roof and rib deformation and the change in vertical pressure within two adjacent pillars during pillar recovery. Site-1 represents the monitored site at 1,000 ft of cover, while site-2 represents the monitored site at 1,500 ft of cover. For this study, large-scale FLAC3D models were calibrated based on both field observations and instru- mentation results. FLAC3D is numer- ical modeling software used for the geotechnical analyses of soil, rock, groundwater, constructs and ground support. The calibrated models were then used to compare the ground re- sponse and the induced stresses of two pillars at instrumented sites 1 and 2. The maximum lateral displacement (roof shift) obtained from the calibrat- ed large-scale FLAC3D models was applied to a small-scale FLAC3D bolt- ed models. Then a safety factor for ax- ial stresses of the simulated roof bolts was calculated at the deep cover sites. The typical geology consists mainly of interbedded shale, sandy shale, silty shale and sandstone. Underground ge- otechnical data on the immediate roof rock and the coal bed were collected. The Unconfined Compressive Strength (UCS) of coal was an input parameter in the coal-mass model used to sim- ulate the coal seam. The UCS of the shale, sandy shale, and sandstone were determined and the results were pro- vided by the mine. The average UCS for the shale and sandy shale was 5,677 psi (39 MPa) and 9,455 psi (65 MPa), respectively, while the UCS for sand- stone was 22,145 psi (152.7 MPa). Mining Conditions The mine produces bituminous coal from the LWE seam by the room-and- pillar retreat-mining method. Site-1 and site-2 were located in the No. 6 entry in the third left panel. The pan- el width was subcritical and consists of eight entries and barrier pillars be- tween the subsequent panels. The dimensions of the pillars are approx- imately 53 ft (16.1 m) x 99 ft (30.1 m) rib-to-rib (R-R). The entries and cross- cuts were about 20 ft (6 m) wide at the instrumented sites. Site-1 was located between crosscuts 9 and 10, while site- 2 was located between crosscuts 21 and 22. The mining height is approximately 5.2 ft (1.6 m) at the instrumented sites. There was neither sudden topographic changes nor multiple seam interaction in the third left panel. During retreat mining, two con- tinuous miners (CMs) are used si- multaneously to extract each row of pillars. In this method of pillar ex- traction, mining progresses from the center of the panel outward in a stag- gered fashion. This method results in leave blocks that are left unmined in the center of the panel rather than on the sides. The number of these blocks in the panel corresponds to a panel width providing adequate pil- lar stability for a given depth of cover. In this case, one line of pillars is left unmined in the center of the panel at site-1, while two lines of pillars are left unmined at the center of the panel at site-2. Figure 1a and b show the pil- lar layout and extraction sequence at site-1 and site-2, respectively. Barrier pillars are used to isolate the active panels from the previous gob. Reducing the panel width by leaving unmined pillars at the center of the panel also reduces the elevat- ed stresses of the abutment loading during pillar retreat mining, particu-

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