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Geotechnical Evaluation of Mine-Induced Seismicity on Monument Dam <br />Dorsey 8 Whitney, LLP <br />Section 1 -Introduction <br />1.1 Purpose and Objectives <br />The purposes of this report are to present: a) the results of engineering analyses of mine- <br />induced seismicity on Monument Dam and on a landslide that forms the left (looking <br />downstream) abutment of Monument Dam, b) our opinions regarding possible impacts to <br />Monument Dam and the landslide from mine-induced seismicity, and c) identify and analyze <br />solutions to prevent impacts from mine-induced seismicity. <br />The intent of the analyses is to evaluate the likely impacts ofmine-induced seismicity on the <br />earthfill dam and landslide. The locations of the earthfill dam and landslide are shown on <br />Figure 1.2. Analyses to evaluate the stability and safety of the earthfill dam and landslide, <br />and other appurtenances from other loading conditions and from tectonically-induced <br />earthquakes, are not part of this work. <br />1.2 Background <br />• The original Monument Dam was constructed in about 1899 to an unknown height, though <br />smaller than its current height. Monument Dam was re-classified by the Office of the State <br />Engineer (SEO) from a U.S. Army Corps of Engineers Class I (high hazard) dam to a Class <br />II (moderate hazard) dam. Monument Dam and Reservoir are owned and operated by <br />Minnesota Ditch and Reservoir Company of Paonia, Colorado. <br />As part of Mountain Coal Company's (MCC) submittal for Permit Revision No.10 to the <br />Colorado Division of Minerals and Geology (DMG), MCC requested GEI evaluate the <br />probable geotechnical response of Monument Dam to mine-induced seismicity. The <br />proposed E Seam mine is overlain by gently rolling, mountainous terrain. The overburden <br />depth above the E Seam generally increases from about 300 to 600 feet where the E Seam is <br />near its closest approach to Monument Dam to about 1,400 feet at a horizontal distance of <br />about 12,000 feet from the dam. <br />Dorsey & Whitney, LLP, retained GEI Consultants, Inc. (GEI) in January 2006 to perform <br />engineering evaluations to identify the probable response of the dam and landslide to mine- <br />induced seismic events. <br />• ~ <br />GEI <br />0603500603-16 Monumm~ Ovn Gmteclwcsl Evd <br />March 2006 <br />