Coal Age

MAR 2019

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38 www.coalage.com March 2019 roof bolting continued tion of the rib for pillar 2. The MPRX, however, malfunctioned. Large-scale Model Setup The visual observations and field in- strumentation data collected at each site was used to calibrate the large-scale FLAC3D models. The large-scale FLAC3D model was calibrated based on the following: 1) Comparing the vertical stress from the BPCs inside instrumented pil- lars 1 and 2 at site-1 and site-2 with FLAC3D model results. The calcu- lated vertical stress from the model shows good agreement with the monitored vertical pressure from the BPCs for pillar 1 and pillar 2 at both sites. 2) Comparing the rib sloughing in the FLAC3D model with visual obser- vations. The observed rib sloughing at the instrumented site-1 was less than 1 ft, while for site-2 it did not exceed 2 ft at the side of the pillar. Generally speaking, the FLAC3D models provide acceptable agree- ment with the observed rib slough- ing at site-1 and site-2. 3) Comparing the vertical roof dis- placement in FLAC3D with visual observation and instrumentation results. The visual observation showed small roof deformations at both site-1 and site-2. Based on the instrumentation results, the aver- age vertical movement of the im- mediate roof due to pillar retreat was approximately 1 in. and 0.9 in. at site-1 and site-2, respective- ly. The FLAC3D model shows that the roofline moved approximately 0.22 in. and 0.72 in. at site-1 and site-2, respectively, after deducting the vertical displacement due to development loading. Maximum Roof Shift Currently, the mine is using five 4-ft (1.2-m) No. 6 (6/8-in.), grade 60, ful- ly grouted, nontensioned rebar bolt per row as the primary roof support in areas with a depth of cover greater than 1,000 ft, while in shallower ar- eas the same configuration with No. 5 (5/8-in) rebar is used. The mine is considering using the No. 5 rebar bolt at both deep and shallow cover areas. The first question usually asked about roof bolts is: "What is its ca- pacity?" Two types of capacities are known for roof bolts: yield and ulti- mate. These can be calculated from the diameter of the bolt and the properties of the steel. The yield and ultimate strengths of grade 60 steel is 60,000 psi (413 MPa) and 90,000 psi (620 MPa), respectively. In this study, since there is some uncertainty about the ground conditions/roof proper- ties in the bolted horizon in addition to the uncertainty about the loading conditions due to variable gob caving characteristics, it is prudent to de- sign the capacity of roof bolt systems based on the yield strength rather than the ultimate strength. A small-scale FLAC3D model was generated to calculate the induced axial stresses in the 4-ft No. 5 and 4-ft No. 6 fully grouted rebar bolts when they are subjected to both an axial load of 2.9 tons and a roof shift of 1/8 in. The 2.9-ton vertical load was used to simulate a detached rock-block from the immediate roof of size 3.5- x 3.5 x 3-ft, assuming the unit weight of the rock is 159 lb/ft 3 . The small-scale FLAC3D model consists of two identi- cal rock-blocks of size 4-ft x 4-ft x 4-ft. The model was solved in two steps: in the first step, the bolt was pulled by a 2.9-ton vertical load ap- plied on the bottom block. In the sec- ond step, a horizontal displacement of 1/8 in (3.2 mm) was applied on the lower block to simulate the roof shift- ing. The induced axial stresses after the second stage of loading in the No. 5 (5/8-in) and No. 6 (6/8-in) bolts due to vertical and horizontal movements are obtained from FLAC3D models. Based on the model results, the safety factor based on the yield strength for the No. 6 (6/8-in) rebar bolt is 2.47. Based on the two calibrat- ed small-scale FLAC3D models, the No. 5 rebar should not be used if the depth of cover exceeds 1,000 ft. This article was adapted from a paper delivered at the 2019 Annual Society for Mining, Metallurgy and Exploration meeting, A Case-Study of Roof Support Alternatives for Deep Cover Room- and-Pillar Retreat Mining Using In-situ Monitoring and Numerical Modeling, by G. Rashed, CDC NIOSH, Pittsburgh, Pennsylvania; M. Sears, CDC NIOSH, Pittsburgh; J. Addis, CDC NIOSH, Pitts- burgh; K. Mohamed, CDC NIOSH, Pitts- burgh; and J. Wickline, Coronado Coal, Charleston, West Virginia. Figure 3—Loading conditions and pillar line locations at the two instrumented sites.

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