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To: Susan Burgmaier, DRMS December 19, 2011 <br />From: Jill Carlson, CGS <br />Subject: Review of TR -51, landslide repairs at Yoast, and notes from September 28, <br />2011 site visit <br />I have reviewed Attachment 20 -2, "Summary Report Identifying Unstable Areas within the Yoast <br />Disturbance Boundary" of technical revision TR -51 (Seneca Coal Company, November 2011). SCC <br />states (page 20 -2.1) "This proposed plan is based on a report prepared by Jill Carlson (CGS)." Our Yoast <br />landslide work during this past summer involved three site visits to observe surface conditions and <br />discuss which of the landslides may impact structures or areas outside permit boundaries or have long- <br />term impacts on final reclamation. <br />Our field observations and field discussions with the mine operator and geotechnical consultant (July 6 -7, <br />2011) were oriented toward improving drainage and revegetation, thereby improving short term stability. <br />However, CGS did not perform any quantitative investigations or analyses and recommends that such <br />work is needed to ensure long -term stability. The technical revision did not include this information, <br />which includes: <br />• Characterization of slope geometry and failure surface(s) <br />• Quantitative analysis of driving or shearing forces acting on the slide mass due to water and <br />gravity <br />• Quantitative determination of shear strength values (resisting forces due to cohesion and friction) <br />along the failure surface <br />• Analysis of various stabilization strategies to determine what measures, or combination of <br />measures, would be required to achieve the desired long -term factor of safety. <br />Based on the information submitted, I have the following comments: <br />Neck Pit Slide. Remediation plan outlined in TR -51 is consistent with field discussions. I would be <br />happy to discuss this slide and the surrounding geology with BLM. <br />Pond 012 Slide Area. As we discussed in the field on September 28, this slope will be difficult to <br />vegetate. Erosion and sloughing of the shale surface will likely continue. More resistant sandstone lenses <br />or layers may provide natural terracing as the slope approaches equilibrium. Monitor (visually). <br />Pond 013 Slide Area. Remediation plan outlined in TR -51 is consistent with field discussions. <br />