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Mr. Lee Sampson
<br />December 7, 2021
<br />Page 2
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<br />Agapito Associates, Inc.
<br />original studies included compilation of site-specific rock physical properties for both pits,
<br />comparison with previous data for other HWM areas of the Colowyo Mine, and detailed analytic
<br />and numerical modeling analyses. AAI also reviewed documents related to the filing of the Ground
<br />Control addenda for the South Taylor G78 Seam3 and the Collom B1/B2, C3/C5, D1/D2, and E2
<br />Seams4 with the Mine Safety and Health Administration (MSHA). The Ground Control addenda
<br />referenced different versions of AAI’s work for South Taylor5 and Collom,6 but the approved
<br />design criteria meet or exceed AAI’s recommendations.1,2
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<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/expanded areas indicated on Figure 1. The geometric details
<br />of the seams in the new areas are slightly different than those discussed in our previous reports,1,2,6
<br />primarily with regard to planned mining height. These slight differences were not to the extent that
<br />would require reevaluation or reconsideration of the design parameters established. A summary of
<br />seam characteristics in the current target areas is given in Table 1. Small differences in seam height
<br />and/or design cover depth required that the approved web and barrier pillar design curves for the
<br />target seams be expanded to include a wider range of these inputs. The updated design curves are
<br />shown in Figures 2 through 6. AAI recommends the use of these curves in conjunction with other
<br />recommendations from our reports1,2 to accomplish the mining envisioned for the
<br />additional/expanded areas of Figure 1.
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<br />Table 1. Geometric Characteristics of the Target Seams in Additional Areas
<br />Pit Parameter Value
<br />(ft)
<br />South Taylor G78 Cover 30–355
<br />G78 Thickness 4–9.5
<br />Collom
<br />B2/B3 Cover 90–205
<br />B2/B3 Thickness 5–8
<br />B3 to C3 Interburden 20–55
<br />C3/C5 Cover 170–265
<br />C3/C5 Thickness 8–10
<br />C5 to D1 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
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<br />3 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, F5/F6, 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.
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