Laserfiche WebLink
West Elk Mine <br />Normal surface runoff has been kept from entering Freshwater Pond-1 (FW-1) by providing a <br />ditch upstream of the pond to intercept and carry the Probable Maximum Precipitation (PMP) <br />assumption "B" storm flow to MB-1. This ditch keeps all nuisance flows from entering and <br />combining with the raw water stored in the fresh water pond. A spillway section has been <br />designed to pass many overland flows that may overtop this ditch and enter the pond. <br />?pillway Designs <br />Spillways have been sized using the broad crested weir equation, Q = CLF`'. A coefficient <br />"C", of 2.67 was assumed, as this is the low end of the practical range. Spillways are <br />constructed of riprap or gabions. The design head for each spillway has been limited to 2.50 <br />ft. FW-1 Pond has a service spillway to assure that small inadvertent flows entering the ponds <br />are passed without relying on the emergency spillway. The individual plan sets in Exhibit 43, <br />Exhibit 47, and Exhibit 66 includes all pertinent hydrologic and spillway data. <br />Stability Program Description <br />The stability analysis for the dams was performed using a slope stability computer program <br />(SLOPE II) developed by Geo Slope Programming, Ltd., Regina, Saskatchewan. The <br />Ordinary (Fellenius) Method and the Simplified Bishop Methods were used to compute <br />stability factors of safety for each embankment's maximum cross-section. <br />Both methodologies use the method of slices and utilize the limit equilibrium theory to solve <br />the factor of safety. The principal difference between the two methods is the treatment of <br />interslice forces. The Ordinary Method assumes that interslice forces can be ignored, while <br />the Simplified Bishop Method assumes that the interslice forces act horizontally. Merrick and <br />Company's results indicated that the Ordinary Method yielded lower factors of safety in all <br />cases. Consequently, for all subsequent trials on the given dam, only the Ordinary Method <br />was used. <br />The computer program determines the factor of safety for slip circles of varying radii about <br />specified points but does not automatically converge on the minimum factor of safety. In <br />order to minimize cost, a coarse grid was initially established. The grid was then successively <br />refined to converge on the minimum factor of safety. In most cases three to five interactions <br />were required. <br />Stability analysis for SG-1 was performed using a slope stability computer program entitled <br />XSTABL5.105. The program utilizes Bishops and/or Janbus method for circular or non- <br />circular failure. <br />Stability Analysis <br />Various soil states were investigated to determine the most critical condition for each <br />embankment. The unconsolidated undrained condition, the consolidated undrained condition, <br />and the consolidated drained condition were considered for all embankments. In the case of <br />2.05-35 Revised June 2005 PRIO; Rev. January 2006; Rev. March 2006; Rev. April 2006 PRIG, May 2006 PRIO; Rev. January 2009 MR-350