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<br />. <br /> <br />that.~he standards would be violated and raising the probability that. <br />000.) <br />primary contact recreation would be permitted. <br /> <br />4. <br /> <br />Safety of Dam/Seepage Issue <br /> <br />Discussion: Firm foundation for the proposed llarrows Dam varies in <br /> <br />depth from the natural ground surface from about lO feet at the left <br /> <br />or north abutment and under the South Platte River Channel to depths <br /> <br />greater than 100 feet as it slopes downward to the south. The south <br /> <br />or the right abutment is composed of permeable sand and gravels. This <br /> <br />configuration requires special consideration for the dam cutoff design. <br /> <br />To control seepage, preliminary designs anticipate an excavated cutoff <br /> <br />trench backfilled with impervious material to the firm foundation at <br /> <br />the left abutment and transitioning to a cutoff formed by slurry <br /> <br />trench to firm foundation extending southward under the active <br /> <br />conservation pool. Maximum depth of the slurry trench cutoff is <br /> <br />about l30 feet. The proposed incomplete cutoff at the south abutment <br /> <br />is not unusual in dam construction provided adequate design consideration <br /> <br />for drainage facilities or other protective features are incorporated <br /> <br />in the dam design. <br /> <br />The slurry trench technique for seepage control is a well tested <br /> <br />.. <br /> <br />concept used most extensively in Europe and Japan. According to Boye~/ <br /> <br />the first development occurred in 1948 by Veder and ~~rconi. At <br /> <br />Maniconagan 3 Dam in Canada an extraordinary record depth for slurry <br /> <br />l/ R.G.H. Boyes, Structural and Cut-off Diaphragm Walls <br />(New York, John Wiley and Sons, 1975) <br /> <br />lO <br /> <br />- <br /> <br />'., <br /> <br />-. <br /> <br />-- <br /> <br />- <br /> <br />.. <br />~., . .' <br /> <br />I . <br />I <br />, ., <br /> <br />I. <br />! <br /> <br />, <br />, <br />c. <br />I <br />I <br />t <br />. <br /> <br />- <br /> <br />'. <br />,. <br /> <br />-- <br />