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resourceful. naturally. BARR <br />engineering and environmental consultants <br />Memorandum <br />To: Mike Peacock, Kathy Welt <br />From: Paul Swenson, Jed Greenwood <br />Subject: Summary of Additional 2D Stability Modeling for RPE East; Modified Phase 1B Buttress <br />Date: October 12, 2012 <br />Project: 06261003 <br />c: File <br />Introduction <br />Two - dimensional seepage and slope stability modeling has been performed to assess whether the Phase <br />1B buttress will perform satisfactorily with refuse placed to Elevation 6,930 with Phase 1B colluvium <br />buttress in place and a delayed placement of the Phase 1 B rock buttress, referred to in this memorandum <br />as the modified Phase 1B buttress. This modeling exercise has been necessitated due to difficulty in <br />finding sufficient durable rock fill material within the colluvium being moved as part of the project to <br />complete the placement of Phase 1B rock in parallel with colluvium buttress construction. While the rock <br />portion of the Phase 1B buttress will be constructed, Mountain Coal Company requested an evaluation of <br />the impacts of delaying the rock placement until possibly the end of 2013. This construction approach is <br />a refinement to the phased buttress construction described in the refuse pile permit (Exhibit 82, TR127, <br />prepared July 2011 by Barr Engineering Co.). <br />The modeled Phase 1B buttress consists of durable rock fill to Elevation 6,490, above which the face of <br />the buttress consists of compacted colluvium benches to Elevation 6535. Three benches were constructed <br />at heights of approximately 15 feet each with approximately 1.5H:1 V slopes on the downstream faces, <br />and measuring approximately 15 feet wide at the crest. The modeled toe of the refuse slope is at Elevation <br />6531.5. <br />A top of refuse slope elevation of 6,930 was selected for the model based on capacity needed to <br />accommodate a refuse generation rate of 421,000 tons per year for the first two years of operation. The <br />design capacity at this elevation is about 900,000 tons of refuse. <br />Barr Engineering Co. 4700 West 77th Street, Suite 200, Minneapolis, MN 55435 952.832.2600 www.barr.com <br />