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Introduction <br />Sediment Pond 012A was constructed in the SWIA NE1/4, Section 19, T5N, R87W, 6d' P.M. The primary <br />purpose of this pond and embankment will be to control runoff and sediment from the reclaimed mining <br />disturbance area to the south of the pond. Construction was initiated in July 2008 and completed in August <br />2008. This sedimentation structure will be maintained until reclamation of this structure is approved by the <br />Colorado Division of Reclamation, Mining and Safety (CDRMS). <br />Embankment and Excavation <br />The embankment is a homogeneous earth fill structure ranging from 10 to 13 feet in height. The top width <br />of the completed embankment is a minimum of 15 feet with upstream slope being constructed to a 4 <br />(Horizontal) to 1 (Vertical) slope configuration and downstream slope being constructed to a 8 (Horizontal) <br />to 1 (Vertical) slope configuration. The principal spillway consists of a 24-inch CMP riser which drops <br />into a 24-inch CMP outlet pipe; the emergency spillway is a trapezoidal channel 15 feet in width. The <br />keyway was constructed in the embankment at a depth of 10 feet and approximately 15 feet in width. A <br />portion of the pond capacity was the result of excavating spoil material below the recontoured ground level. <br />The material used in the embankment construction came from suitable spoil material obtained from the <br />project site. <br />Design Analysis <br />The stability of Sedimentation Pond 012A was originally evaluated by our firm in a report dated August 20, <br />2007. The slope stability evaluation was completed under the assumptions that the embankment would be <br />constructed of the on-site spoils and that the dam would be 12 feet in height and have a crest width of 20 <br />feet. The embankment was also analyzed assuming the downstream and upstream faces of the <br />embankment would be constructed with slope configurations on the order of 3 (Horizontal) to 1 (Vertical). <br />The slope stability analyses were conducted assuming that a failure of the downstream face of the <br />embankment would occur with the pond at its peak water storage, as well assuming that a failure of the <br />upstream slope would occur after a rapid drawdown of the impounded water. The stability analyses <br />indicate that the subject embankment exhibited a minimum theoretical factor of safety for a downstream <br />slope failure of 2.84 and a minimum factor of safety for an upstream slope failure of 3.19. Both of these <br />minimum factors of safety exceed the minimum required factor of safety of 1.5 typically required for these <br />types of embankment structures. The Slope Stability Evaluation report completed for this project is <br />provided in Attachment A. <br />Job Number: 07-7600 NWCC, Inc. Page i