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Scutt Fit <br />10253•COM v itudelecws• <br />lL4'3UOK L `? r..i...,?rr:ylrr <br />znp.-cts of Flood Cycles <br />Based upon information provided to 1.113ET, Schmueser Gordon Meyer, Inc. expressed some <br />concern about excess pore water pressures that inny develop in the gravel and cobble soils during <br />the rcecding liinb of the flood hydrograph where bead differences develop between the lake <br />water elevation in the mined pits and the river water elevation. In addition, Schniucscr Gordon <br />'Meyer, ]nc. expressed coneern about liquefaction of the soils resulting from the excess pore <br />water pressures. <br />In general, excess pore water pressures can develop in soils along rivers and streams cluHrig € load <br />events. 1'lowcver, the magnitude of excess pore water pressures, head differences, seepage <br />forces, etc. is dependent upon the hydraulic conductivity of the soil. A?t one end (if the spectrum, <br />where tight clay soils are present, significant head diff'erence's cart develop. However, excess <br />pore water pressures and seepage forces are negligible due to the very low hydraulic conductivity <br />of these materials. At ilia other end of the spectrum, where clean gravels are present, the <br />hydraulic conductivity is so higli that excess pore water pressures and head differences do not <br />develop. <br />In order to evaluate the hydraulic conductivity of the native gravel and cobble soils at the site, <br />,in the test pits excavated in February 2008. Tlie percolation <br />HBET conducted percolation testing <br />testing indicated percolation rates III'between 1.6 and 1.9 ininutcs-per-inch. This corresponds to <br />hydraulic conductivity values of between approximately 0.3 and 0.65 centinietcry per-second <br />(crnts). Percolation test results are included in Appendix A. <br />In addition to the percolation testing, the hydraulic conductivity oFthe native gravel and cobble <br />soils was estimated using the Hazen equation. The Hazen equation estimates hydraulic <br />conductivity based upon the min size distribution of the soils. Using the Hazen equation in <br />conjunction with the grant-size data provided by United Companies, the hydraulic conductivity <br />of the gravel and cobble soils was cstintatcd to be 0.36 cm/s. <br />In general, the native gravel and cobble soils were deterinitied to be highly permeable. This is <br />consistent with the brain size analysis results shinning [fiat less than 1'%a of the soil was in the silt <br />and clay size range, While the percolation it-sting and Hazen equation provide only a sitirnates of <br />the hydraulic conductivity, they do provide a meaningful approximation of the order-of- <br />magnitude of the conductivity. <br />Based upon the estimated hydraulic conductivity of the native grlvcl and cobble soils, HBET <br />does not believe; that any significant head differences, excess pore water pressures, or seepage <br />forces will develop it [lie gravel and cobble soils during flood events. Although HBET has not <br />observed the flood hydrograph for the applicable section of the Colorado River, it is highly <br />unlikely that the river level will rise and fall faster than the water can move tlirouJt the soil at <br />this site. In general, the water levels in the, pit lakes and river will rise and fall together at nearly <br />the same time and at nearly the same rate. <br />11 CENI& AM. PROJUMIAWO -U11itrJCarttp.miec-,a?356.9%$I Sant Pit'--%V -Geri (V3t(ilJ.?OI L-tet?;31a sax ?