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Technical Revision No. 79 -PAR Responses April 18, /997 Page 5 average depth of 12-18 inches on the surface disturbed areas of the general mine site, and 9- 12 inches on the refuse disposal azea on all rough graded, finished surfaces." 18. To the extent practicable, the surface soil layer will be removed at a time when the physical and chemical properties of topsoil can be protected and erosion can be mi~irt+i~ed. MCC is committed to protecting topsoil resources. MCC will have a soils consultant on site to provide field guidance on topsoil removal operations. MCC believes a performance specification for topsoil removal would assist us and will work with the Division staff to determine the scope of that specification. 19. MCC commits to salvaging adequate subsoil for reclamation purposes. However, it is difficult to clarify what subsoils aze designated for stockpile and what subsoils will be used in Sylvester Gulch without knowing the spatial distribution of soil types underlying the RPE. In general, the upper subsoil horizons consist of silt-clay-loam mixtures suitable for reclamation, and it is these soil horizons that will be excavated and salvaged for reclamation. Deeper material consists of sandy gravels which are excellent for construction purposes and will be used in the construction of the Sylvester Gulch facilities. The actual volume required for reclamation is addressed further in the response to question 40 pertaining to the variance to the 4 feet of cover requirement. Coal Mine Waste and Non-Coal Processin¢ Waste 20. Underdrain sizes were calculated using Dazcy's law. A permeability for course rock (10 cm/sec or 20 ft/min) was used for sizing the drains. A factor of safety equal to 2 was used for deriving the recommended drain cross-sectional area. The maximum estimated leachate flow to occur during the life of the RPE is 15 gpm, which is expected to occur after a 100- year, 24-hour storm event during Phase 2 construction. This number was arbitrarily rounded up to 20 gpm and used for sizing each of the main underdrains. In addition, 5 gpm was used for each of the four known springs. for the underdrain sizing, all four known springs plus the entire peak leachate flow were assumed to flow through a single underdrain. In the design layout this flow has been divided into two separate underdrains, effectively creating an additional factor of safety of 2 to the design. The overall factor of safety for the underdrain design is 4, assuming two springs flowing at 5 gpm flow into each underdrain. 21. Figure L-1 has been revised to incorporate a 6-inch layer of crushed stone over the rock drain to minimize the migration of fines into the drain. 22. The rock drains and sand drains were sized using a safety factor of 4, which MCC believes is adequate for expected or unexpected inflows. However, in the unlikely event that higher flows are discovered during construction, MCC has prepared a table to be incorporated in Appendix L for field resizing of the drains, and shows the maximum permissible flow for each rock drain design cross-section area..