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Ken Nelson <br />December 20, 2011 <br />Page 10 <br />At the maximum known depth, theoretical vertical load on the liner/pipe will be upwards of .1.1,000 <br />pounds per square foot 75 psi). Fill ing the annulus between the tunnel and new liner with grout or <br />other stable and impervious material will not only spread the surcharge load over larger area, it <br />would also prevent further spalls and /car Subsidence which effectively enhances soil arching <br />characteristic,-,. <br />Active ground water seepage is also a concern with respect to soil strength. Saturated soil material <br />is substantially less stable than dry/moist soil. Recontmendationsfor providing a positive means for <br />collecting, conveying and removing ground water seepage are note(( in the geotechnical assessment. <br />Since ground water can theoretically inigrate along the interface of native tunnel excavation and <br />grout fill, a clean sand or pea gravel along the tunnel 11001' Would provide the needed conveyance <br />and could be configured as a horizontal layer of subbase for liner/conduit, or wrapped in filter fabric <br />("burrito" style) and laid end-to-end along the bottom edge of liner/conduit. <br />Alternative materials of (unnel liner conduit include concrete and steel which are highly vulnerable <br />to corrosion, and HDPE which is considerably more resistant to chemical. degradation. Although <br />this feasibility study did not include sampling and laboratory testing of native soil and groundwater, <br />there is a relatively high possibility some level of corrosive conditions exist. In the event concrete <br />or steel pipe structures are preferred alternatives, laboratory analysis should be conducted to verify <br />whether special coatings or thicker structural sections will be necessary to extend the useful service <br />life of final improvements. <br />Tunnel hydraulics are first based on maintaining an equivalent surcharge at the invetted siphon inlet, <br />to maxim ize the available energy gradient. The tunnel conduit may also flow under low pressure as <br />compared to gravity flow, depending on whether inconsistent slope exists in areas not currently <br />accessible for survey. Tunnel hydraulics are also based on aged pipe rather than new pipe <br />conditions, since repeated seasonal cycles of fill and drain of irrigation water tends to roughen the <br />pipe surface and deposit scdinient. Both of these factors degrade pipe capacity over time. <br />Additionally, the tunnel (conduit) is not entirely straight I ine or grade, which also reduce hydraulic <br />capacity. Based on the nominal slope available, and roughness factors for aged material, a <br />minimum 58" inside diameter, smooth wall conduit or equivalent, will be needed through the <br />tunnel. The equivalent corrugated plate is 64". <br />Alternatives for needed hydraulic capacity and structural strength using Milne) liner plate, include <br />circular, arch, and horseshoe (bottomless) shapes, A round tunnel liner has the greatest structural <br />integrity for both vertical and horizontal soil loading, and conforms best with the existing tunnel <br />dimensions. Since the depth of overburden exceeds standard design tables, and because soil <br />mechanics are based on passive earth pressures, a round tunnel liner appears to be the only viable <br />option. Round tunnel liner is available in standard 2" increments of diameter to the neutral axis. In <br />this event, a 64" diameter tunnel liner is recommended. Corrugated plates are also manufactured in <br />2-flange and 4-flange configurations. Joints of 2-flange connections are lapped, with deep, full <br />length corrugations to provide maximurn stiffness and ring compression. Butt (hinged} joints of <br />4-flange connections have less strength and are not suitable for this application. Corrugated liner <br />plate is available in thicknesses from 14 gauge (0.0747") to 3 gauge ((1.2391"), and uncoated black <br />