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<br />. <br /> <br />. <br /> <br />. <br />" <br />-'" <br /> <br />4-09. Slope stability analyses were performed in accordance with criteria <br />stated in EM 1110-2-1913, Design and Construction of Levees. The conditions <br />analyzed include the end of construction, steady seepage, critical flood <br />stage, and sudden drawdown. Solutions were calculated using the slope <br />stability program WES/LIB, 1009 available On the Honeywell 635 computer. The <br />analyses were based on the modified Swedish method of slices. A minimum <br />factor of safety was determined for each case, and a manual solution was <br />performed for each of the critical arcs. <br /> <br />4-10. Soil StrenEth Parameters. Materials that affect the stability of the <br />soil levee s lopes may be divided into four categories: foundation, existing <br />levee fill, new levee fill, and rip rap slope protection. The foundation <br />strengths were estimated from the relative density of the sand materials <br />encountered beneath the natural ground in areas of the proposed levees. <br />Relative densities in the area were correlated to the standard penetration <br />blow count with the relationships developed by Gibbs and Holtz as presented in <br />U.S. Navy Design Manual, DM7, 1982. Standard penetration blow counts were <br />variable along the center line of the levee. Low values were recorded in <br />silty sand layers approximately 10 to 15 feet below the ground surface. The <br />minimum value recorded was 3 blows per foot and the average was 11 blows per <br />foot. These values correspond to a relative density between 40 and 60 percent <br />at a vertical effective stress of 1 kip per square foot. Using a correlation <br />from the US Navy Des ign Manual, the strength of a medium grained sand at 40 <br />and 60 percent relative density should possess an angle of internal friction <br />of 30 to 34 degrees. A value of 30 degrees was selected for the drained <br />friction angle and was used in stability analyses for the end of construction, <br />steady seepage, and critical flood stage analyses. Due to the uncertainty in <br />the determination of the strength of the soil in undrained loading, the <br />friction angle for the foundation was reduced to 25 degrees for the sudden <br />drawdoWD case. Unit weights used for the analyses were a moist unit weight <br />of 115 pounds per cubic foot and a saturated unit weight equal to 125 pounds <br />per cubic foot. Strength of the existing levee soils were similarly <br />correlated to the standard penetration blow count. Values of standard <br />penetration blow count for the near-surface silty and clayey sands of which <br />the existing levees are constructed, indicated that the materials were <br />generally medium dense to dense and correlated to an angle of internal <br />friction of 35 degrees or greater. The materials available for new levee <br />construction are silty sand and some sandy silt. The sand is generally poorly <br />graded and ranges from fine to medium grained. When placed in lifts and <br />compacted, these soils will possess an angle of internal friction of 35 <br />degrees or greater-. Since the s,oils are generally granular in composition, an <br />angle of internal friction for a drained condition was assumed for all <br />conditions analyzed. The unit weights for both the existing levee and the new <br />construction were assumed to be 126 pounds per cubic foot for s moist material <br />and 134 pounds per cubic foot for a saturated material. Rock riprap will be <br />placed as slope protection on the proposed levees. A drained angle of <br />internal friction of 40 degrees and a moist unit weight of 130 pounds per <br />cubic foot and a saturated unit weight of 140 pounds per cubic foot were <br />assumed for the rock riprap for the analyses. <br /> <br />IV-3 <br />