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Mr. Lee Sampson <br /> April 23, 2021 <br /> Page 3 <br /> Previously, AAI suggested leaving a safety pillar below structures that Colowyo would like to <br /> protect from subsidence damage should HWM pillar failure occur. In particular, a transmission <br /> line passes through the expanded South Taylor target area. The extent of the safety pillar below a <br /> protected structure,where no HWM would occur,can be determined following a method suggested <br /> by Peng.7 This method includes a fixed 30-foot (ft) offset, continues at an outward angle of 45' <br /> from vertical through any soil, spoil, or unconsolidated material,and continues at an outward angle <br /> of 20' from vertical to the seam of interest (the G7 in the case of South Taylor). For example, the <br /> distance around all points of a transmission line support,underlain by 100 ft of spoil and 200 ft of <br /> bedrock would be: <br /> 30 + 100 tan 45 +200 tan 20= 203 ft <br /> For areas where the depth to bedrock is unknown and/or difficult to determine, AAI recommends <br /> an offset based on a fixed 50-ft offset plus an angle of 25°, resulting in an offset of 190 ft for the <br /> above example. This alternative is more conservative for thinner depths to bedrock, but less <br /> conservative for thicker depths. <br /> Thank you for the continued opportunity to assist with HWM planning at Colowyo. Please let me <br /> know if you have any comments or questions. <br /> Best regards, <br /> Tom Vandergrift <br /> Vice President and Principal <br /> tomvdakagapito.com <br /> TLV:klg <br /> Attachments: Figures 1-6 <br /> Transmitted as a PDF via e-mail to lsampsonAtristategt.org <br /> Peng, S. S. (1992), "Surface Subsidence Engineering," Society for Mining, Metallurgy, and Exploration, Inc., <br /> Littleton,Colorado,pp. 78-79. <br /> Agapito Associates, Inc. <br />