Laserfiche WebLink
SEC110NSEVEN seeuaoe Anatysis <br />For the purpose of modeling seepage from the river during periods of reservoir drawdown, the <br />river was assumed to be upgradient. On the upgradient side of the cutoff wall, a total head <br />(pressure head plus elevation head) boundary condition was assigned at the interface between the <br />river and underlying sand and gravel layer with a water elevation of 4740 ft. in the South Platte <br />River. On the downgradient (gravel pit) side of the cutoff wall, a total head boundary condition <br />was assigned one foot above the bottom of the gravel pit at elevation 4705 ft. <br />7.2 SEEPAGE ANALYSIS RESULTS <br />The results of the seepage analysis are presented in Figure 7-1. Flow vector arrows illustrate the <br />relative magnitude and direction of flow. As expected flows through the sand and gravel layer <br />were significantly retarded by the cutoffwall and forced into the underlying Iower permeability <br />weathered siltstone/claystone and siltstone/claystone layer. <br />A flux section was placed along the centerline of the cutoff wall to allow SEEP/W to calculate <br />flux from the river into the pit. The model calculated flux of 1.4518 ft.3/day/ft is posted next to <br />the flux section. This flux value was multiplied by the perimeter length of the cutoff wall <br />(approximately 12,000 ft.) which results in a total estimated seepage rate of about 90 gpm into <br />the pit proposed reservoir through the sidewalls and bottom. <br />The SEO Guidelines for Lining Criteria for Gravel Pits include a Design Standard and a <br />Performance Standard. The Design Standard states that "The intent of the reservoir lining design <br />is to achieve ground water inflow (leakage rate) into the reservoir that is not greater than 0.03 <br />ft3/day/ftz multiplied by the average vertical depth of the perimeter wall as measured from the <br />ground surface to the pit bottom along the toe of the pit slope, plus 0.0015 ft3/day/ftZ multiplied <br />by the azea of the bottom of the liner system or natural bedrock bounded by the perimeter wall." <br />The SEO's Design Standard maximum allowable seepage values of 0.03 ft3/day/ftZ and <br />0.OO15ft3/day/ft2 for pit sidewalls and pit floors, respectively, were converted to a maximum <br />combined seepage rate approximately 140 gpm for the proposed reservoir having an <br />approximately 12,000 ft. long perimeter wall with a depth of 34 ft. (average depth to bedrock) <br />encompassing an area of approximately 9,900,000 ftZ. As can be seen, the model estimated <br />inflow of approximately 90 gpm is less than the SEO's Design Standazd maximum allowable <br />flow of 140 gpm. Note that the 90 gpm model estimate is conservative in that the modeled pit <br />depth and cutoff wall geometry along the entire perimeter are based on the deepest bedrock <br />encountered and conditions adjacent to the South Platte River. <br />N:\PROJECTS\22238240 PLATTE SAND GRAVEL\SUB 00\12.0 WORD PROC\PIATTE SANDB GRAVEL GEOTECHNILAL DESIGN REPORT 4.O.DOCIISNOV-05\\ 7-2 <br />T - - - - - - <br />