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March 26, 1999 <br /> APR 0 8 1999 <br /> Western Mobil - Denver <br /> PO Box 215001 :vision of Minerals&Geology <br /> Denver, Colorado 80221 <br /> Attention: Mr. Dallas Glasser <br /> Resource Manager <br /> Subject: Piping Analysis <br /> Levee at Deepe Farm Pit <br /> Boulder, Colorado <br /> Job No. 25,385 <br /> Gentlemen: <br /> We have completed the requested piping analysis for the levee at the Deepe Farm Pit <br /> Site in Boulder, Colorado. Emphasis was placed on the two locations (station <br /> 111+00 and station 145+00) where seepage was observed and an analysis was <br /> recommended by inspectors from the Division of Minerals and Geology (DMG). Our <br /> analysis showed an exit gradient at station 111+00 of 0.01 and at station 145+00 of <br /> 0.02. The exit gradients are less than one by several tens of times which indicates <br /> no piping. More specific details regarding our analysis <br /> are in the remainder of this letter. <br /> The levee and its foundation predominantly consist of medium permeable, well <br /> graded slightly, clayey to clayey sands and gravels. We previously conducted pin <br /> hole dispersivity tests on samples from the site and results indicated the samples <br /> were nondispersive. We opine the materials that comprise the levee and its <br /> foundation are not subject to piping. <br /> The DMG requested groundwater flow nets be constructed and exit gradients be <br /> calculated at the two locations along the levee where seepage is most apparent. <br /> These locations were identified as station 111+00, near the 900 bend in the south end <br /> of the levee, and station 145+00, near the north end of the levee. <br /> In response to the request by the DMG, flow nets were constructed and are shown <br /> in the attached figures. The soils and bedrock profiles assumed in this analysis were <br /> extrapolated from the conditions disclosed in our exploratory borings and pits. The <br /> levee and its foundation were considered as a single material type underlain by <br /> claystone bedrock. The claystone was assumed to be a "no flow boundary" in our <br /> analysis. The groundwater surface was constructed assuming steady state hydraulic <br /> connection between the water in the irrigation ditch or the 100 year flood elevation, <br /> whichever was higher, and the observed downstream seepage. The flow nets we <br /> calculated using Seep/W finite element software by GeoSlope International, Inc. <br /> CTL/THOMPSON, INC. <br /> CONSULTING ENGINEERS <br /> 1971 WEST 12TH AVENUE 0 DENVER, COLORADO 80204 - (303)825 0777 <br />