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llec 19'17 Srrsna Cao/Cmnpar~~ ~ 1 o2v llure SrdrmenAr[rorr Puud 01? S/.dvLg~.9rml~;,~r ~ 1 1.0 INTRODUCTION • 1.1 PURPOSE: The purpose of this stability analysis is to calculate the static safety factor (SF) for the designed Sedimentation Pond 012 embankment in order to document design compliance with applicable regulatory standards under Rule 3.05.6(1 I)(j t or (k). 1.2 METHODOLOGY: The static SF for Sedimentation Pond 012 was calculated using a basic, one-dimensional equilibrium analysis program called XSTABL (v5.2). The calculated SF is the ratio of those resistin. forces that would prevent embankment movement or failure to the cortespondine drivine forces. Resisting forces include the internal strength parameters for both the soil materials used in the construction of the embankment and the underlying base materials on which the facility will be constructed. Potential drivine forces include gravitational forces on the embankment and foundation materials and the gravitational and lateral forces associated with the water volume that could be impounded behind the embankment. For the analysis, a circular failure mode was assumed and the static SF was calculated usin¢ the Bishop method. 1.3 PROCEDURE: Using the proposed design configuration for Sedimentauon Pond 012 as shown on Exhibit 13-12.2 of the 7"oast Permit. (Leidich. 19991, the typical embankment cross-section shown in Figure I, Pond 012 Stability Cross-Section, was developed to evaluate embankment stability. For the stability analysis, a water surface elevation behind the embankment of 7,259 feet above mean sea level (amsl) was assumed. This water surface elevation corresponds to the calculated maximum pond stage, which would result from the runoff and peak flows from a 25-year. 24-hour storm even[ over the contributing watershed. This peak flow • condition assumes that the pond is discharging through the emergency spillway and that a minimum of one foot of freeboard is maintained between the maximum pond stage and the crest of the embankment. The embankment is designed with upstream and downstream slopes not steeper than 3H:1 V. The design width of the embankment crest is IS feet.'fhe upstream height (measured from the invert of the emergency spillway to the original upstream channel elevation) is approximately 19 feet. The emergency spillway invert is 3 feet below the embankment crest and the pond basin is incised an additional 8 feet below [he original channel elevation into native material. Total height from bottom of pond to top of embankment is 30 feet with an effective upstream height for the stability analysis of 22 feet (top of embankment to original upstream channel elevation er natural ground surface). As designed, the base of the embankment slopes vradually from the upstream embankment toe to the downstream embankment toe at approximately 0.7 percent. The potential phreatic surface for the stabtlity analysis is based on a water surface elevation of 7152 Ff- amsl corresponding to the lowest open dewatering orifice in the pond discharge riser structure. It is assumed that the peak stage of 7,259 ft-amsl would only occur for a brief time period following high spring runoff or a major thunderstorm and that cortesponding high water levels would not be maintained for a long enough duration to result in a substantial change to the phreatic surface through the embankment. The phreatic surface through the embankment was estimated based on Darcy's law and Casagrande's procedure as outlined by Huang (1983). The resulting surface is shown on Figure I, Pond 12 Stability Cross-Section. Based on soil surveys performed by Jim Nyenhuis (1993 - 1996) and as indicated in Table I. Sedimentauon Pond 012 Soils Data, the soil and subsoil materials in the Sedimentation Pond 012 area are predominantly slightly sandy to sandy clays. These materials are of a similar type to the soils materials previously collected and analyzed for Sedimentation Pond O10 based on soils mapping information and NRCS soils characterizations. Given reasonable comparability based on soil type, the soil physical parameters previously determined For Sedimentation Pond O10 have been used in the stability analysis for Sedimentation Pond 012. .1lom[gomep~ 1G ohou,-Immrat, Inr. lGS Swdh Unrou BauBerard Sullr d 10 * Lakewood, Co%rodo SO?=S * (IOJJ %G7-5140 f:\Si i\nFur/\(rondl7.l~biGh Oi/1i/I9 nh