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• 3. Drain Caoacity • A particle size distribution for the drain rock was determined and is shown as Curve 3 on Figure 5. Using Leps methods (Section 4.2.1.3 of Reference 5) and the approximate gradients in both the West Pit and Section 16 valleys, a flow capacity on the order of 70 to 90 cubic feet per second (cfs) is estimated for the 8 x 24 foot trapezoidal drain section. ~ This is a considerable capacity in relation to any conceivable flow. In comparison, the design surface runoff for the West Pit fill fora 100 year - 24 hour event is on the order of 60 cfs. No springs or seeps were noted within the valleys. Surface water infiltration is the only source of water to the drain. Evaporation exceeds precipitation at this site and infiltration is expected to be very low. We estimate the capacity of the drain is vastly over built and is at least 10 times the maximum conceivable flow. The presence of a very ~ permeable, large diameter highly sorted layer which will form along the entire base of the lift will by itself provide sufficient capacity to drain the fill as evidenced by Streeter Fill. The permeability of the spoil materials was also evaluated. We estimate the permeability of the unsorted, "top of lift" materials to be on the order of 10'' to 10"Z ft(day. The lower, sorted spoil and drain rock section are estimated to have similar permeabilities • on the order of 105 to 106 ft/day. Observations at the Streeter Fill at the Colowyo Mine have shown no development of free water within the spoil over the 15 year life of this spoil (even without a specially placed drain). The proposed spoil piles will have sufficient permeability to pass any infiltration water without developing saturated conditions. ~ 4. Filter Criteria Rule 4.09.2 (2)(c) requires the filter system "shall be designed and constructed using standard geotechnical engineering methods". Filter criteria which can be considered "standard geotechnical engineering methods" were developed for high gradient seepage flows within embankment dams where saturated flow occurs. The low gradient seepage, • lack of saturated conditions, and very large diameter drain rock involved in this project require that engineering judgement be applied to the implementation of these criteria for these conditions. The flow conditions within the unsaturated spoil will not develop seepage forces sufficient to pipe materials into the drain. Therefore, based on standard geotechnical methods no filter is necessary. • Filter design criteria require that the D15 of the filter be 4 to 9 times or more smaller than the DBS of the base (or soil to be protected). This ratio depends on the type of soil, with 4 for coarse, clean soils and 9 for very clayey soils. Details of the procedure are spelled out in Reference 4. To provide adequate permeability D15 of the filter should be five times greater than D15 of the base. Other factors to be considered are permeability, • flow capacity, thickness and cost. • []