Laserfiche WebLink
st Colorado Consultants. Inc. <br />Geotechnical /Environmental Engineering • Materials Testing <br />September 11, 2008 <br />Twentymile Coal Company <br />29515 RCR #27 <br />Oak Creek, CO 80467 <br />Attn: Mr. Jerry Nettleton <br />Job Number: 07-7556 <br />Subject: Supplemental Slope Stability Evaluation, <br />Proposed 6MN Mine Water Storage Reservoir, Foidel <br />Creek Mine, Routt County, Colorado. <br />Ladies and Gentlemen: <br />As requested, NWCC, Inc. has completed a Supplemental Slope Stability Evaluation of the proposed embankment <br />slopes to be constructed for the proposed 6MN Water Storage Reservoir to be constructed at the Foidel Creek Mine in <br />Routt County, Colorado. Our fuTn previously completed a Slope Stability Evaluation for the proposed embankments <br />that was prepared under our job number 07-7556 and dated August 6, 2007. This supplemental slope stability <br />evaluation has been completed under the assumption that the reservoir level has been lowered to an elevation of <br />6,714.9' (from 6715.5') and the embankment configuration has not changed. <br />The stability of proposed embankment structure was assessed by computing theoretical factors of safety (against slope <br />failure) for the proposed slope configurations. A factor of safety in this case is calculated by summing the forces <br />resisting slope movement (i.e. soil strength generated along a theoretical failure plane) and dividing that sum by the <br />driving forces of the slope (i.e. the weight or mass of the soil and groundwater). A minimum factor of safety of 1.5 is <br />required for Class B or C dams that are full and under steady state seepage saturation conditions. Whereas, the <br />minimum factor of safety for the dam under seismic conditions is 1.2. <br />By inputting slope geometry and soil strength values into the slope stability program SB-SLOPE, strength and <br />geometry values can be easily varied to help assess the factors of safety for the proposed slopes under varying <br />conditions. This program is a comprehensive slope stability program for microcomputers and uses the "Simplified <br />Bishop Method of Slices" for factor of safety calculations. The slope configurations used in our analysis were obtained <br />from the cross section drawing provided by the client. The slope stability analyses and data are presented in Appendix <br />A and a discussion and summary of the results are discussed below. <br />The analyses were completed on the maximum embankment section that will be constructed with an upstream slope <br />configuration of 3(H) to 1(V) and a downstream slope configuration of 2'/z (H) to 1(V). The first case that was <br />analyzed was with the reservoir full and water at the emergency spillway elevation. A phreatic surface has also been <br />added to this model to analyze the embankment under a full reservoir condition with steady state seepage. The second <br />case illustrated represents the same cross section with a horizontal seismic coefficient of 0.05. In both cases, the slopes <br />were analyzed for what was considered to be the most likely failure mode, which is a failure of the downstream <br />embankment slope during peak water storage. These analyses were conducted using conservative strength estimates <br />(s7o) sus-uses • FaX (s~o) 8~s-~sai <br />2580 Copper Ridge Drive • Steamboat Springs, CO 80487 <br />