REV99478 - Laserfiche WebLink

Home Browse Search

Q = AV„n and A= Q/V„n • Where Q is the required flow through the rock underdrain (cfs) Vv is average water velocity through rockfill voids (ft/sec) n is drain porosity (dependent on the size consist of the rock material, porosity may vary from approximately 0.8 to 1.0, use 1.0 to maximize drain size) A = (8.6 ft3/sec)/(2.03 ft/sec x 1.0) = 4.2 ft2 A 4' x 10' underdrain is the minimum specified size for a sandstone underdrain for a fill of <1 MM cy (Rule 4.09.2(2)(d)). A 4' high by 10'wide triangular drain has across-sectional area 20 ft2. All underdrain calculations are based on very . conservative assumptions, so even applying a 2.0 multiplier as a Factor of Safety, a 4' x 8' triangular underdrain configuration (16 ft2) would be more than adequate (1.9 x required drain size with 2.0 Factor of Safety) to safely pass the maximum anticipated infiltration flows. It is, therefore, recommended that underdrain design, approval, and construction be based on a 4' x 8' triangular configuration with 1H:1V sideslopes. UNDERDRAIN CONSTRUCTION AND INSPECTION The underdrain will be constructed ofnaturally-occurring sandstone rock and cobbles • recovered during foundation preparation activities for the coal refuse facility. These materials will be separated from soil materials and temporarily stockpiled along the inside edge of the perimeter drains for use in construction. Underdrain construction will be initiated following completion of foundation preparation activities, including clearing, grubbing, soil material removal, and minor grading to establish the triangular drain configuration. Underdrain construction will involve placement of 12 to 14 ounce/squaze yard non-woven geo-textile material along the exposed portion of the underdrain alignment, placement of suitable rock drain materials on top of the geo-textile to establish a triangular drain configuration at ]east 4 feet deep and 8 feet wide, and wrapping or placement of additional geo-textile over the completed rock drain to form an enclosed drainage envelope. Underdrain configuration is graphically illustrated by Figure 1, Underdrain Design Configuration. No more than 10 percent of the suitable rock drain material will be less than 12-inches in diameter and maximum rock size will be 30 inches in diameter. Geo-textile materials will have a minimum overlap of 12 inches at any joint and will be anchored with metal stakes. Underdrain construction will be supervised and monitored by a Registered Professional Engineer. Monitoring will include grading of the design drainage configuration, placement and anchoring of geo-textile materials, selection and placement of rock drain materials, and completion of the geo-textile over-wrap. Underdrain construction activities will be photo-documented and the inspecting engineer will prepare and submit a certified inspection report to the CDMG documenting that underdrain construction was • completed consistent with the approved designs.