C150125 Feasibility Study - Laserfiche WebLink

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'Oct-31-2002 11 :32am From- T-599 P,005/008 F-972 Mr. Jim Norfleet, P.E. Mr. Jack Byers, P.E. Page 4 July 11, 2002 Internal Drainage System. The gradation analysis of the existing embankment and abutment material' was used to develop the gradation range for the fitter material in accordance with the filter criteria outlined in Chapter 6.10.(1) of DBS/gn of Small Dams. Based on the results for the filter material, the gradation range for the graded gravel envelope around the drain pipe was developed. The analysis is attached and the gradation range Is presented In the specifications. The filter blanket is proposed to be a 18 inch thIck layer. Using the Darcy equation, the flow capacitY of the filter material is estimated at 1.4 gpm per foot of width. The maximum seepage rate measured for the dam when full is approximately 66 gpm. The filter blanket is over 100 feet wide at the chimney drain and 60 teet wide at the toe drain. Embankment Stability. The existing proposed embankment materials are classIfied as ML eL, GMt and Ge. These materials are stable for unsaturated compacted embankments with slopes at 2.5:1 and flatter. The stabilitY of the dam structure is increased with the addition of internal drainage and more than doubling the downstream embankment sectIon. OUTLET PIPE The existing outlet pipe consists of an 18 inch CMP lined with a grouted 12 ,Inch 10 HOPE pipe. The outlet pipe will need to be extended through the new embankment fill. ,It is proposed to extend the outlet pipe with a matching 12 Inch HOPE pipe connected with a heat fused coupler. In the event a subcontr2lctor cannot be located for the heat fused coupling, a 14 inch 10 Ductile Iron pipe can be slipped on over the HOPE pipe. A 1 foot thick bV 4 foot long grouted collar would be used. A concrete outfall structure and rip rap channel will be constructed. The outfall chBnnel consists of Qo = 12 inch rock rip rap, The rip rap was sized based on the U,blln Drainage and Flood Contl'Ol District Urban DrainagtJ Cdts". Manusl. The existing outlet capacity provides for drawdown of approximately 4.3 feet in 5 days. The proposed additional length of outlet pipe would reduce the drawdown to approximately 4.13 feet in 5 days. A waiver of the outlet capacity requirement was granted for the 12 inch slip line project in 1996 based on en estimated drawdown of 4, feet In 5 .days. DAM CREST The dam crest will be widened approximately 22 feet to aCQommodate the 30 foot wide paved road shifted off of the "damN embankment. The dam portion of the crest will be graded and sloped towards the upstream embankment. The road portion of the crest will be sloped toward the downstream embankment due to the curvature of the :11:1 CJ t::' C n I I I::) ,..... t::