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SEVEN Seepage Analysis <br />7.1 SEEPIW MODEL DEVELOPMENT <br />The SEEP/W (Version 4.22) computer program was used for the seepage analysis. It is a two- <br />dimensional finite element model that calculates flux (seepage per unit width), given user- <br />defined geometry and hydraulic conductivity (k) values. <br />7.1.1 Model Geometry and Hydraulic Conductivity (k) Values <br />The geometry developed for the SEEP/W model represents pertinent site features and idealized <br />stratigraphy and is based on information obtained from our field investigation. The stratigraphy <br />surrounding the cutoff wall from top down are topsoil, sand and gravel, and asiltstone/claystone <br />bedrock unit which transitions from weathered at the surface to competent bedrock <br />approximately 3 feet below the surface. The geometry (relative position, thickness, and extent) <br />and permeability of these units were idealized for the model. A discussion on the development <br />of the geometry and hydraulic conductivity (k) values of these units is presented below. The <br />idealized cross section conservatively places the ground surface at Elevation 4750 feet and the <br />reservoir pit bottom at Elevation 4704 feet (the deepest bedrock encountered in the test borings). <br />The soil-bentonite cutoff wall in the model extends 49 feet from the ground surface to a depth of <br />3 feet below the top of the bedrock. The idealized geometry is illustrated in Figure 7-l. <br />The hydraulic conductivity values (k) assigned to the topsoil and sand and gravel units for input <br />into the SEEP/W model are 1.00 x 105 cm/sec and 1.00 x10.2 cm/sec, respectively. These k <br />values reflect typical permeability values for clayey topsoil and natural sand and grave] deposits <br />at the site. Isotropic conditions (kh/k\, = 1) were assumed for the topsoil and sand and gravel <br />units. The cutoff wall was modeled under isotropic conditions, with a conservative k value of <br />1.0 x 10-~ cm/sec (Note that permeability tests on the laboratory backfill design mixes resulted in <br />permeabilities less than 2.0 x 10_s cm/sec). <br />Based on analysis of the bedrock stratigraphy encountered during our investigation, anisotropic <br />conditions (kh/k„ = 10) were assumed for the siltstone/claystone bedrock unit. The more <br />competent lower bedrock zone was assigned a kh value of 3.60 x 10 _5 cm/sec, which is the <br />average permeability from the packer test results. The uppermost weathered bedrock zone was <br />assigned a kh value of 1.00 x 104 cm/sec, which is approximately three times the permeability of <br />the lower bedrock zone to account for weathering. <br />The entire perimeter of the gravel pit was conservatively modeled with a pit depth of 46 feet and <br />conditions similaz to the southwest side of the pit, which is adjacent to the South Platte River. <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 />7.1.2 Boundary Conditions <br />The entire perimeter of the gravel pit was conservatively modeled with a pit depth of 46 feet and <br />conditions similar to the southwest side of the pit, which is adjacent to the South Platte River. <br />' N*:\PR~OJECCTS@2238240_PLATTE_SAND_GRAVELISUS_00112.0_WORD_PROC\PLATTESANDBGRAVELGEOTECNNICAL DESIGN REPORT d.O.DOC\1SNOV-051\ 7-I <br />V iW <br />