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,- Colorado Department of Corrections April 10, 2003 Page 2 peTformed by GTG Geotechnical Services, L TD (ApTil 15, 1997), subsurface conditions encounteTed in borings drilled along the dam and in the TeseTvoir area generally consisted of sandy to silty clay with seams of gravelly sand and other scattered gravel zones to depths ranging from 1 6 to 31 feet. A zone of gravel-sized shale pieces was Teported in one boring from 16 to 27 feet, and silty sand was encountered below these deposits at depths ranging from 27 to 31 feet, and was reportedly underlain by sandy clay mateTial to the explored depths in the borings. f7 \ Significant reservoir losses were noted based on inflow/outflow measurements I, . during the initial reseTvoir filling. No flows were observed exiting the toe drain to date, suggesting that the Teservoir losses were caused primarily by seepage through the reservoir lining Tather than through the embankment. During a subsequent site visit conducted to observe the lineT, GTG noted the pTesence of closely spaced desiccation cracks over most of the liner area. In their letter dated January 13, 1998, the DOC indicated that corrective action would be required to reduce the amount of seepage losses such that the DOC could meet their water delivery ("augmentation") TequiTements, assuming that "any corrective action [wouldl achieve at least the lower end of [permeability] established in the GTG report for the native material, i.e., 1.6 x 10-7 em/sec." Thereafter, GTG developed a corrective measure, which included excavating to a depth of 24 inches in cut areas within the reservoir, and to a depth of 18 inches at the bottom of the reservoir. Following excavation, the lower 12 inches of the remaining subgrade was backfilled with moisture-conditioned, compacted soil from the excavations mixed with 12% to 15% bentonite (by dry unit weight). The bentonite-amended lineT was then reportedly capped with 12 inches of on-site soil in cut aTeas, and 6 inches of on-site soil on the Teservoir bottom. The capping layer was intended to protect the liner against desiccation cTacks caused by drying or shrinkage. GTG estimated a permeability of about 1 x 10-7 cm/sec for the bentonite-amended liner, and a corresponding estimated se~page loss of about 0.02 acre-feet per day. The bentonite-amended liner was completed in the summer of 1998. However, excessive seepage losses have TepoTtedly occurTed since that time. Based on inflow/outflow information, Bishop-Brogden Associates, Inc., has estimated aveTage seepage losses ranging up to 0.66 acre-feet per day at the higher reservoiT levels. As a Tesult, the DOC is cUTrently not able to meet the required augmentation needs, particularly in the low-flow or drought years. If the construction history or conditions vary significantly from that describe above or depicted in this report, we should be notified to reevaluate the analyses and recommendations provided in this Teport. SEEPAGE EVALUATION Seepage analyses were peTfoTmed to estimate the amount of seepage flow through the original and bentonite-amended liner systems, and to compare that to estimated, actual permeability of the current liner system. Analyses were also peTfoTmed to evaluate the maximum allowable, uniform permeability that would allow the DOC to meet augmentation requirements for the reservoir. C:\My Documents\MJ\leners\03-1-21 O.gjm.doc Kumar ~ Associates, Inc.