Laserfiche WebLink
Mr. Lee Sampson <br />December 7, 2021 <br />Page 3 <br /> <br />Agapito Associates, Inc. <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 G78 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 /> <br />30 + 100 tan 45 + 200 tan 20 = 203 ft <br /> <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 /> <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 /> <br />Best regards, <br /> <br />Tom Vandergrift <br />Vice President and Principal <br />tomvdg@agapito.com <br /> <br />TLV:klg <br /> <br />Attachments: Figures 1–6 <br /> <br />Transmitted as a PDF via e-mail to lsampson@tristategt.org <br /> <br /> <br /> <br />7 Peng, S. S. (1992), “Surface Subsidence Engineering,” Society for Mining, Metallurgy, and Exploration, Inc., <br />Littleton, Colorado, pp. 78–79.