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March 26, 1999 <br />Western Mobil - Denver <br />PO Box 215001 <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 Igo. 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+ 0 and station 145+00) where seepage was observed and an analysis was <br />recommended by inspectors from the Division of Minerals and G eology ( DM G). 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 o f this lever. <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 di per ivity tests on samples from the site and results indicated the samples <br />were nondi per ive. 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 0" bend in the south end <br />of the levee, and station 145 +, near the north end of the levee. <br />In response to the request by the D G, 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 />clay tone bedrock. The clay tone 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 SeepNV finite element software by Geo lope International, Inc. <br />CTL /THOMPSON, INC. <br />CONSULTING ENGINEERS <br />1971 WEST 12TH AVENUE • DENVER, COLORADO 80204 • (303) 825 -0777 <br />