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To: Susan Burgmaier, DRMS August 31, 2011 <br />From: Jill Carlson, CGS <br />Subject: Landslide repairs at Yoast Mine <br />I visited Yoast with you, Brian Len of NWCC, and representatives of the mine operator, Peabody <br />Energy /Seneca Coal Company, on July 6, 2011. The purpose of the field meeting was to view and <br />discuss a number of new and recurring landslides within and adjacent to reclaimed areas at the Yoast <br />Mine, and to determine what actions, if any, should be taken to repair any slides, slumps, and unstable <br />areas within the mine permit boundary. Table 1 summarizes the areas discussed. <br />The Yoast Mine is underlain by Residuum on the Mesaverde Group (sand, silt, and clay soils), derived <br />from and grading down to weathered, interbedded shale, sandstone and coal beds of the underlying <br />Williams Fork Formation. The layers that compose the Williams Fork Formation were deposited during <br />the late Cretaceous in a fluctuating shoreline environment. The problematic shales and clay soils <br />associated with landslides in this area were deposited as low- energy marine sediments. Landslide <br />deposits, potentially unstable slopes, and mudflows are mapped along the western edge of the Yoast Mine <br />boundary, corresponding to the Neck Pit/Haul Road B, Pond 12, and Pond 13 areas, in the vicinity of <br />drainages YPM -1 and YPM -6, and on most of the slopes surrounding the North Pit. <br />Spring of 2011 was a very active season for landslides in Routt County. Infiltration from an unusually <br />large snowpack and heavy precipitation caused soil and weathered rock mass strengths to decrease, and <br />caused increased instability. Numerous new and reactivated landslides, soil slumps and mudflows were <br />observed during routine inspections by DRMS beginning in May 2011. <br />It is important to note that the Yoast mine permit boundary contains many potentially unstable slopes, <br />mudflow and rockfall hazard areas identified during geologic hazard mapping of Routt County by Allen <br />Miller in approximately 1977. These mapped features predate the mine. Landslides and unstable slopes <br />are a relatively common feature of the natural landscape in the area within and surrounding this mine. <br />During our site visit, we evaluated all of the landslides that you identified within the context of the <br />following questions: <br />1) Would further movement threaten structures such as roads, ponds, stock tanks and drainages that are <br />to remain as permanent features after final bond release? <br />2) Is there evidence of excessive surface or subsurface water, contributing to instability, that can be <br />corrected with a reasonable level of effort? <br />3) Would repairs, such as surface regrading, create more disturbance and surface compaction, thereby <br />reducing the potential effectiveness of post- repair revegetation efforts, than is justified by the current <br />(temporary) unsightliness of any bare landslide surfaces? <br />Please note that the repairs and stabilization measures discussed below have not been analyzed to <br />determine the degree of stability (factor of safety) that will be achieved. Improving surface and <br />subsurface drainage should help reduce, but will not eliminate, the risk of continued or future movement <br />in the event of another high snowfall or high precipitation year. If the state, mine operator or property <br />owner requires greater confidence in the potential effectiveness of the repair /stabilization measures <br />discussed below, further investigation, landslide characterization, analysis and design will be needed. <br />