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West E!k Mine <br />This pond has a capacity of ten acre-feet, an embankment height of 19.5 feet, and a surface azea of <br />• I.10 acres. The pond is located on the natural bench above the valley floor at an approximate <br />elevation of 6,290 feet. <br />Normal surface runoff has been kept from entering Freshwater Pond- I (F W-1) by providing a ditch <br />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 designed <br />to pass many overland flows that may overtop this ditch and enter the pond. <br />Spillway Designs <br />Spillways have been sized using the broad crested weir equation, Q = CLF' S A coefficient "C", <br />of 2.67 was assumed as this is the low end of the practical range. Spillways are constructed of <br />riprap or gabions. The design head for each spillway has been limited to 2.50 ft. Freshwater <br />Pond-1 has a service spillway to ensure that small inadvertent flows entering the ponds are <br />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 Ordinary <br />• (Fellenius) Method and the Simplified Bishop Methods were used to compute stability factors of <br />safety for each embankment's maximum cross section. <br />Both methodologies use the method of slices and utilize the limit equilibrium theory to solve the <br />factor of safety. The principal difference between the two methods is the treatment of interstice <br />forces. The Ordinary Method assumes that interstice forces can be ignored,while the Simplified <br />Bishop Method assumes that the interstice forces act horizontally. Merrick and Company's results <br />indicated that the Ordinary Method yielded lower factors of safety in all cases. Consequently, for all <br />subsequent trials on the given dam, only the Ordinary Method 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 order to <br />minimize cost, a coazse grid was initially established. The grid was then successively refined to <br />converge on the minimum factor of safety. In most cases three to five interactions 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 circulaz or non-circular <br />failure. <br />• <br />2.05-34 Revised tan l996 R,Y03: Revised Nov. 1996 PR07; Revised Jan. l99%TRi9: Revised Jun. /990 TR90 <br />