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Ken Nelson <br />December 20, 2011 <br />Page 8 <br />A minimum 54-inch diameter pipe is needed based on existing hydraulics and 50 efs capacity. <br />54-inch DR32.5 HDPE butt fused pipe has a 1,662" wall thickness for an equivalent inside diameter <br />(I.D.) of 50.676", and weighs 1.20 pounds per lineal foot. At 50 efs, velocity would be 3.7 feet per <br />second which is sufficient to prevent settling out suspended matter such as silt or mud that would <br />othei-wisebeacoiiceriifollowiiigar,iiiif<i)lcvcjil. The next larger pipe diameter (63" O.D., 59.124" <br />I.D,) would also be acceptable, with a velocity of 16 fps. The larger pipe requires slightly less <br />surcharge at the inlet structure, that may be a benefit in conjunction with tunnel improvements. The <br />next smaller pipe diameter (48" 0,D., 45" LD.) is not compatible with existing system hydraulics, <br />since it would require additional depth of water at the inlet of the siphon, which could overflow the <br />canal bank at the siphon inlet, and also has adverse impacts on tillinel hydraulics. <br />Existing concrete inlet and outlet structures appear to be in good condition, and could remain in <br />place with special details for watertight connections with the new pipe. The new pipe will need to <br />be fitted with a pressure rated operable (train at Tongue Creek, similar to the existing siphon drain <br />assembly. Other details for construction include suitable backfill material within 12" of the pipe to <br />provide a stable pipe foundation, to provide lateral support since FIDPE is a flexible conduit, and to <br />prevent point loadings of oversized rock, The Tongue Creek crossing may also require ballast <br />anchor(s) pending verification of subsurface soil conditions and/or seasonal groundwater elevations <br />to prevent floatation when the pipe is drained. <br />Design and construction of a new siphon pipe will require coordination with the Colorado <br />Department of Transportation (CDOT). CDOT design criteria typically includes bored and encased <br />construction .methods, a minimum 5-foot depth of bury within 1.5 -feet of the asphalt road surface for <br />sign post clearance. A steel casing pipe is typically required the full width of MOT right-of-way, <br />large enough to install the carrier pipe with pipe skids inside, and with sufficient structural strength <br />to push or jack through conventional boring. Open cuts across the asphalt norn-tally are not <br />approved except in special cases where roadway maintenance such as in overlay, patch, or chip seat <br />is plaillied during that Construction season. Open cuts may also be considered in locations where <br />rock or other natural features are identified that may make boring unfeasible based on geotechnical <br />subsurface investigations. <br />Design and construction in the vicinity of Tongue Creek may also require wetland delineation to <br />determine whether the US Army Corps of Engineers (COE) has jurisdiction over the work. If <br />jurisdictional wetland areas exist, construction activities to replace the siphon. pipe must be <br />authorized under Section 1.2 of the Nationwide Permit for utilities. This process typically involves <br />submittal of a formal Wetland Delineation Report. and Preconstruction Notification to the COE, <br />The COE may also request consultation from the Colorado Division of Fish and Wildlife with <br />respect to potential environmental impacts of the proposed construction activity, <br />Tunnel Re&iir and Liner Q)grade. The NDIC conducted )relinlinary investigation,-, for tunnel <br />j I <br />rehabilitation as compared to constructing an entirely new tunnel offset. from the existing alignment <br />prior to this feasibility study, At that preliminary level of analysis, alternative construction methods <br />for offsetting a new tunnel included conventional excavation (100-feet deep) tend new tunnel <br />construction (road header, tunnel borhig iiiachine, micro tunneling or drill/blast). Tunnel <br />