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Mr. Lee Sampson
<br /> April 23, 2021
<br /> Page 2
<br /> AAI reviewed the relevant South Taylor' and Collom2 HWM reports and the updated seam models
<br /> supplied by Colowyo for the target seams in the additional areas shown on Figure 1. The original
<br /> studies included compilation of site-specific rock physical properties for both pits, comparison
<br /> with previous data for other HWM areas of the Colowyo Mine, and detailed analytic and numerical
<br /> modeling analyses. AAI also reviewed documents related to the filing of the Ground Control
<br /> addenda for the South Taylor G78 Seam3 and the Collom 131/132, C3/C5, D1/D2, and E2 Seams4
<br /> with the Mine Safety and Health Administration (MSHA). The Ground Control addenda
<br /> referenced different versions of AAI's work for South Taylors and Collom,6 but the approved
<br /> design criteria meet or exceed AAI's recommendations.1,2
<br /> In all, AAI has completed more than 10 HWM studies for Colowyo. Based on our experience on
<br /> the property over the last 18 years, it is our opinion that the approved design criteria are applicable
<br /> to the target seams in the additional areas indicated on Figure 1. The geometric details of the seams
<br /> in the new areas are slightly different than those discussed in our previous reports.1,2,6 A summary
<br /> of seam characteristics in the additional target areas is given in Table 1. Small differences in seam
<br /> height and/or design cover depth required that the approved web and barrier pillar design curves
<br /> for the target seams be expanded to include a wider range of these inputs. The updated design
<br /> curves are shown in Figures 2 through 6. AAI recommends the use of these curves in conjunction
<br /> with other recommendations from our reports',2 to accomplish the mining envisioned for the
<br /> additional areas of Figure 1.
<br /> Table 1. Geometric Characteristics of the Target Seams in Additional Areas
<br /> Pit Parameter Value
<br /> (ft)
<br /> South Taylor G7 Cover 30-355
<br /> G7 Thickness 6-9.5
<br /> 132/133 Cover 90-205
<br /> 132/133 Thickness 5-8
<br /> B3 to C3 Interburden 20-55
<br /> C3/C5 Cover 170-265
<br /> C3/C5 Thickness 8-10
<br /> Collom C5 to DI Interburden 30-105
<br /> D1/D2 Cover 30-305
<br /> D1/D2 Thickness 7.5-12.5
<br /> D2 to E2 Interburden 17—55
<br /> E2 Cover 50-355
<br /> E2 Thickness 4.5-7.5
<br /> s Phillipson, S. E. (2013), "Evaluation of Pillar Stability in the South Taylor Pit, G78 Seams, at Colowyo Coal Co,
<br /> LP's Colowyo Mine,I.D. 05-02962,"MSHA memorandum 13BA142a, September 30,2 pp.
<br /> 4 Lemons, M. L. (2020), "Colowyo Mine ID No. 05-02962 Ground Control Plan Highwall Mining Addendum for:
<br /> Collom Pit X3/X4, B2/B3, C3/C5, D1/D2,E2, 175/176, FA/FB, G8/G9 and GB Coal Seams," letter to Colowyo
<br /> Coal Company,LP,May 14,44 pp.
<br /> 5 Agapito Associates,Inc. (2013),"Highwall Mining Design Curves Based on the ARMPS-HWM Program,"letter to
<br /> Dr. Sandin Phillipson,MSHA Approval and Certification Center, September 27, 16 pp.
<br /> 6 Agapito Associates, Inc (2019b), "Geotechnical Design and Operational Considerations for Highwall Mining,
<br /> Collom Box Cut,"report to Colowyo Coal Company,August 5, 143 pp.
<br /> Agapito Associates, Inc.
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