2014-04-17_REVISION - C1981019 - Laserfiche WebLink

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Interoffice Memorandum To: Rob Zuber From: Zach Trujillo Subject: Colowyo – Second TR-103 Review of Geotechnical Analysis Date: 4/17/2014 Rob – With TR-103, Colowyo Coal Company L.P. proposes to revise Colowyo Mine’s mine plan to include highwall mining within the E seam in the South Taylor Pit along the southern wall. After my review of Agapito Associates, Inc. (AA) Geotechnical Design and Operational Considerations for Highwall Mining, South Taylor Pit G78 and E/D2 Seams, and West Pit F, E, D2, and C Sea ms (the Report), I have the following comments: 1.When comparing Figure 7. South Taylor Pit, E Seam Floor Elevation in AA’s Report to surface elevations provided on Map 23A, it is observed that as penetration depth of the highwall miner increases, the amount of cover above the E seam decreases within the proposed highwall mining area along the southern wall in the South Taylor Pit. As the depth of cover decreases, the potential for subsidence increases. Within the Report, AA does not include minimum cover depth analysis and states that engineering judgment should be used in determining minimum cover of depth. Additionally, it appears to the Division that some areas within the highwall mining limit have little to no cover when reviewing Figure 10. South Taylor Pit, E Seam Cover. If maximum penetration from the highwall miner occurs, the potential exists for the highwall miner to mine through the surface of the slope opposite to the entries. In AA’s Report, it states that actual penetration distance is likely to be lower based on multiple variables including the burned and oxidized coal zone which runs through the highwall mining section in question. However, Colowyo has provided us with maps detailing the possible extent of highwall mining which should be assumed is the target penetration depth regardless of potential limiting factors unless otherwise stated by Colowyo.