2017-02-21_REVISION - M1988112 (3)

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the bank to the reclaimed area of the tailing impoundment. This estimate is based on the weir equation below. When flow in the ditch below Basins A and B is 1,125 cfs, the water surface elevation above the catchment will be 8625.5 ft, which is the top of the bank in South Diversion Ditch overflow section. Q=CL°t where, Q = Discharge, cfs C = Weir coefficient, 2.64 (Brater and King) L = Weir length, 500 ft H = Height of water over the weir crest, 1.24 ft 4.0 DROP STRUCTURE STRUCTURAL DESIGN 4.1 Lateral Wall Loading 4.1.1 Lateral Earth Pressures The lateral earth pressures used for design of the spillway walls were based on the expected reaction of the walls when loaded. The structural walls were designed using active, passive, and at rest soil pressures. Equivalent fluid pressures for each stress case were developed using stress ratios equal to or greater than those presented in the geotechnical report (Engineering Analytics, 2015). Table 4.1 Lateral Earth Pressures Stress Case Equivalent Fluid Pressure Ibslft3 Active 36 Passive 446 At Rest 65 4.1.2 Vehicle Loading In addition to the Active earth pressure loads, the spillway walls were also designed for the short term lateral loads produced by an H-20 vertical live load in accordance with the recommendations by AASHTO (American Association of State Highway and Transportation Officials) 2012 LRFD Bridge Design Specifications. AASHTO loading conditions for bridge abutments where traffic is parallel to the wall and remain a distance of 1 foot or greater away from the wall are shown in Table 4.2. Table 4.2 Equivalent Height of Soil for Vehicle Loading From ASHTO LFRD Bridge Wall Height (feet) Equivalent Soil Height heq (feet) 5.0 2.0 10.0 2.0 >-20 2.0 San Luis Project - South Diversion Ditch Drop Structure - Final Design Report 24