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<br />-~- <br /> <br />," <br /> <br />(/ <br /> <br />Colorado Department of Corrections <br />April 10, 2003 <br />Page 5 <br /> <br />value pTior to construction. Although the originally estimated value would have <br />been adequate to maintain seepage losses below that allowed to meet <br />augmentation requirements, we estimate that actual seepage losses, and the <br />corresponding range of permeability for the liner, are exceeding the maximum <br />allowable values by about 50% to oveT 300%. <br /> <br />Possible causes of the excessive seepage losses, based on our review of the <br />available infoTmation TegaTding the geotechnical engineeTing studies, and the <br />TeseTvoir liner design and constTuction, are presented below: <br /> <br />. <br /> <br />In our opinion, the excessive seepage in the original, natural liner system <br />appeaTS to be caused pTimarily by desiccation cracking in the 12-inch <br />liner, possibly combined with the presence of relatively permeable sand <br />of gravel deposits exposed in the reservoir bottom and/or side slopes. It <br />is also possible that fine-gTained soils in the lineT system may have <br />eroded or piped into adjacent gravel deposits exposed in the reservoir <br />excavation at some locations, although there is no information regaTding <br />the gradation properties of gravel deposits exposed in the excavation at <br />the time of construction. The Temoval of fine-gTained soils caused by <br />piping would act to increase seepage flows through the liner at that <br />location. Leakage through the liner at only a few points can have the <br />effect of significantly increasing the overall average permeability. <br /> <br />. <br /> <br />Although a large percentage of bentonite was added to the on-site clayey <br />soils for the bentonite-amended liner, dTY bentonite was appaTently mixed <br />with the soil prior to moisture-conditioning this material. Based on our <br />experience with bentonite-amended soils, we believe that the bentonite <br />should be fully hydrated by mixing it :with wateT prior to mixing it with <br />em-site soils. It appears possible that the bentonite was not fully , <br />'hydrated at the time of mixing, Teducing it's effectiveness in Teducing the <br />peTmeability of the liner system. Also, a 12-inch protective layer would <br />not necessarily protect tlie liner system from frost-related heaving that <br />, could also act to crack or degTade the liner. <br /> <br />,. <br /> <br />It should be recognized that, given possible vaTiations in material pToperties, the <br />actual peTmeability fOT a given Ii neT system can be significantly greater than that <br />estimated based on laboTatory testing and seepage modeling. Differences between <br />peTmeability values on materials obtained fTom laboTatory testing compared to back- <br />calculated values obtained from field performance information in the range of one <br />order of magnitude or greater are not uncommon. Therefore, we believe that it is <br />important to provide an adequate safety factor fOT the liner system with respect to <br />the permeability of the system. <br /> <br />Based on the above considerations, we recommend that a modified liner system <br />have a permeability ,of at least 10 times. that required to satisfy the allowable <br />seepage rate requiTements. discussedabov8. Possible liner systems that could <br />satisfy these requirements include a composite clay geomembTane liner system <br /> <br />Kumar & Associates, Inc. <br /> <br />C:\My Documents\MJ\Letters\03-1-21 O.gjm.doc <br />