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Ken Nelson <br />December 20, 2011 <br />Page 11 <br />steel, galvanized steel an(] asphalt coated, A minimum 5 gauge is needed for structural reasons, and <br />asphalt coating is highly recommended For corrosion protection to extend the useftil service life, <br />Coated 3 gauge corrugated plate would dicoretically increase the service life. However, regardless <br />of how well protected the metal plate niaterial is, :manufacturers do not claim service life greater <br />than 50 years. <br />A second alternative for slip-lining the tunnel utilizes HDPE pipe. HDPE has a higher resistance to <br />COI•O.SiOrl than coated steel in most cases, pending soil and water pH and resistivity. A minimum <br />63" 0,D, DR 17 HDPE provides needed flow conveyance capacity to match existing hydraulics. <br />Installation of HDPE pipe would require staging pipe at one end of the tunnel, and heat fusing <br />joints. As pipe is assembled at the staging end, it is pulled/pushed through the tunnel. Assembled <br />pipe could theoretically clear tunnel timbers at the most restrictive locations (within 1" to 2"), but <br />may be very difficult to position at the center of restrictions without some physical entry and labor. , <br />Needed pipe strength is based oil the most demanding of either stresses during installation, or <br />ultimate potential soil hearing pressure. During installation, die estimated maxinnum pulling force <br />will be 944,400 pounds (472 ton). Properly fused pipe has SUffiCient strength to be pulled a greater <br />distance than the tunnel length, Pulling fused pipe through the tunnel will likely require some form <br />of bottom roller systern. Tile void between tunnel shell and DR I? HDPE pipe will need to be filled <br />with amaterial having at least 500 psi compressive strength for needed long-term resistence to <br />potential soil pressure. Suitable fill material includes flowable grout or fly ash slurry, installed <br />either by pumping through a horizontal hose, or vertical through the existing and/or supplemental <br />vertical shafts, Bulkheads with ail- Vents Would need to be Constructed at each end of the tunnel as a <br />stop forflowable backfill material. Estimated volume of the void area is approximately 2,300 cubic <br />yards (470,000 gallons). 1JDPE pipe is generally more resistant to corrosion and many chemical <br />compounds than coated metal, yet manufacturers do not have sufficient historical data to claim <br />service life greater than 50-years, <br />The NDIC requested hydraulic evaluation based on new pipe characteristics as a comparison to aged <br />pipe to ascertain whether the next smaller HDPE pipe size would have sufficient capacity upon <br />installation. Hydraulic analysis of new 54" OR DR 17 HDPE, installed with nominal horizontal or <br />vertical offsets under the same available slope and pressure head, indicated that new pipe would <br />have sufficient capacity to convey 50 cfs. Tile smaller diameter pipe tiny be considered viable with <br />extreme caution, since its performance over time is expected to degrade to the extent flow capacity <br />diminishes well below the needed capacity for downstream users, If the timely availability, capital <br />cost, or other factor(s) for the recommended 63" 11DPE pipe size render the smaller 54" HDPE more <br />desirable, the N.DIC will need toinonitor the canal system downstream from the tunnel to ensure <br />adequate now capacity is maintained. This may require additional routine maintenance and <br />cleaning. It may also elevate the need to confirm evaporation and seepage loss downstream from <br />the tunnel and is siphon in the event loss of tunnel flow capacity could be offset in the future <br />by reducing natural losses by piping open ditches downgradient. <br />Because the tunnel has at least one high point along its leiigth that may become completely <br />submerged when irrigation water is conveyed, ail air vent may be needed for all alternative <br />line• /conduit alternatives. Although file NDIC has no known historic problems with entrapped air, <br />