2011-07-01_PERMIT FILE - C1982057A (3)

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• If possible a bench will be left at the base of the slope, above the trench. If the final slope is shallower than 0.5H:1 V a bench may not be possible. The trench will be constructed to collect the water and convey it along the trench to a down drain that will discharge to Pond 016. The trench bottom will be approximately ten (10) feet wide with a six (6) inch perforated pipe placed along the bottom and covered with a minimum of one (1) foot of 4" minus gravel. The trench will be a minimum five (5) feet deep. The down drain will be approximately five (5) feet wide with a six (6) inch perforated pipe placed along the bottom and covered with a minimum of one (1) foot of 4" minus gravel. A portion of the trench will remain as an open channel while the down drain will be covered. See Exhibit 20 -D.2, Pond 016 Area Remediation Plan and Post Mine Cross Sections, for conceptual locations and design details of the overall plan. It should be noted that the final trench location will be determined in the field as conditions dictate. Shrub Plot Slump Remediation All monitoring and study indicates that the instability results from saturation of overburden material in a final pit and piping as the water surfaces at the lip of the final pit along a possible pit ramp. Channel PM -2 will be relocated to capture the flow at the point of origin. Once the flow is captured and directed into constructed channels the area is expected to stabilize. The slumped area will be regraded and seeded. See Exhibit 20- D.2, Pond 016 Area Remediation Plan and Post Mine Cross Sections, for conceptual locations and design details of the overall plan. It should be noted that the final channel • locations will be determined in the field as conditions dictate. Pond 016A Removal A portion of the embankment in existing Pond 016A will be removed to allow the impounded water to drain and to allow the construction of Channel PM -2. The remainder of the original embankment will be left inplace. The pond area will be backfilled using material excavated from the slide area. Once the pond is drained, an under drain will be installed through the bottom of the pond. The dryer excavated material will be mixed with the wetter material in the bottom of the pond to improve stability. Once the dryer material is placed to the level of the top of the under drain geotextile will be placed across the fill and then the remainder of the area will backfilled to the approximate proposed configuration. Pond 016 Capacity Verification SEDCAD4 was run on the current conditions occurring in the watershed above Pond 01 6 to ensure that the current capacity is sufficient to retain the 10 year /24 hour runoff event. The existing conditions indicate that the total runoff volume would result in 3.9 ac -ft. of runoff. The current pond survey indicates that the existing Pond 016 configuration contains sufficient capacity (3.9 ac -ft) to retain/handle runoff from future 10 year /24 hour storm events at the principal spillway and 10.0 ac -ft at the emergency spillway. The SEDCAD • results are attached. TR -73 2 6111