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<br />4-11. End of Construction. The end of construction condition was analY2;ed to . <br />determine if increased pore pressures created by undrained loading during <br />construction will create a critical stability condition in the soil Ioevee. <br />The height of the levee and the granular nature of the fill and foundation <br />soils reduce the potential for instability. Several trial arcs were analyzed <br />for levee toe failure, side slope failure, and foundation failure modes. The <br />minimum factor of safety of 2.00 was obtained for an arc exiting the levee <br />slope. The manual solution confirmed the accuracy of the computer analysis <br />with a value of 1.97. The values are in excess of the required factor of <br />safety of 1.3 for the end of construction analysis. The results of the <br />analyses are presented On Plates 29 and 30. <br /> <br />4-12. Steady See~aEe. For the stability analysis, steady seepage was assumed <br />to occur at the maximum flood stage. The phreatic surface was assumed to be a <br />straight line extending from the intersection of the river slope of the levee <br />and the water surface to the landside toe of the embankment. This condition <br />is considered to be conservative, because only partial saturation of the levee <br />section will occur during the design flood due to the short duration o,f the <br />flood. The minimum factor of safety waS determined among several trial arcs. <br />The value of the minimum factor of safety was 1.67, which compared favo:rably <br />with a value of 1.65 obtained from the manual solution, and is greater than <br />the required value of 1.5. The results are presented on Plates 3l and 32. <br /> <br />4-13. Critical Flood StaEe. Critical flood stage was evaluated to determine <br />the stability of the riverside levee slopes at the critical stage elevation. <br />The critical flood stage condition is the condition whereby some prolonged <br />flood stage saturates the embankment and a condition of steady seepage is <br />established. The condition is evaluated at several stage elevations for each <br />trial arc. The minimum factor of safety is selected for the critical a:rc at <br />the critical flood stage. A horizontal phreatic line passing through the <br />levee was assumed, and is conservative due to the short duration of the design <br />flood. The value of the minimum factor of safety waS 1.76 for a circular arc <br />exiting near the toe of the embankment. The critical stage elevatioll was <br />4,668.2 approximately 5 feet above the toe of the levee. The manual solution <br />factor of safety was 1.76. This value is in excess of the required minimum of <br />1.5 for the critical stage analysis. The results of the analysis are <br />presented on Plates 33 and 34. <br /> <br />. <br /> <br />4-14. Sudden Drawdown. The sudden drawdown analysis was performed for the <br />assumption of a fully developed phreatic surface at the maximum flood s~~age. <br />This assumption is conservative, because the embankment will only be partially <br />saturated during the short duration of the design flood. The solutil)n is <br />based On the one force polygon using bouyant unit weights of the levee <br />materials below the water surface at maximum flood stage. The normal fl)rces <br />acting on the failure arc are determined from the before-drawdown :Coree <br />polygon and used to calculate the after-drawdown factor of safety. The factor <br />of safety as determined from the computer solution was 1.08 for a failure <br />surface passing through the toe of the levee. The manual solution factclr of <br />safety was 1.07. These values are greater than the value of 1.0 required for <br />the sudden drawdown condition. The results of the analyses are presentl!d on <br />Plates 35 and 36. <br /> <br />. <br /> <br />IV-4 <br />