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SOIL TYPE MA7iIIvItJM UPSTREAM MAXIMUM DOWNSTREAM <br />SLOPE STEEPNESS SLOPE STEEPNESS <br />GMlSM 3:1 2:1 <br />ML 3.5:1 2.5:1 <br />It is seen that the soil plug 4:1 downstream slope is more conservative than the <br />above recorttntendatians. Haw~'er, it does not consist of a compacted <br />embankment. On the other hand, the phrg could be considered to be relatively safe <br />based on the most likely slope faihrre progression. Tn a somewhat homogeneous <br />embankment, the slope failure surface can be approximated by a circular geometry. <br />Therefore, if a slope failure were to occur, the initial movement would be limited to <br />the downstream slope area itself and would not jeopardize the integrity of the plug. <br />In order for the stability of the entire soil plug io be compromised, a series of slope <br />faaures would be necessary which would progress upstream. This would occur <br />over a long enough period of time that the problem could be identified, the <br />upstream reservoir drawn down and measures taken to stabilize the plug. <br />b. Stidietg - Sliding is not norcnaity a critical consideration for embankment <br />dams because fire soil shear friction along the base of the cmbankment is typically <br />quite high. The sliding mechanism for the soil plug would be for the upstream <br />hydrostatic pressure to force We soil plug to move downstream as a single mass. <br />Pore pressures along the sliding surface were assumed to vary linearly from the <br />upstream face of the soil phtg to the downstream toe of the slope. The attached <br />calculation sheets show safety factors against sliding based on the stretlgth <br />characteristics previously described. It is seen that the safety factors are very high <br />with the minimum safely factor of 6.4 at the maximum upstream water depth of 9 <br />feet. It should be noted that the safety factor calculations are conservative since <br />they don't include the effects of friction along the sides of the soil phtg. Sliding <br />safety factors substantially increase as the upstream water depth decreases. The <br />minimum acceptable safety factor for sliding ie generally considered to be 3.0. <br />c. Internal Erosion -Internal erosion occurs when seepage through the <br />embankment, foundation or along the etnbankment/foundation contact creates <br />sufficient forces to initiate migration of soil particles along open paths or neaz ffie <br />soil surface, The most likely paths along which internal erosion would occur are <br />the contact between the bottom of the soil plug and the mine edit foundation or the <br />contact between rite outlet pipe and the soil plug material. Lane's Weighted Creep <br />Ratio is considered a generalized procedure for estimating the potential for internal <br />erosion. This method calculates an effective erosional path length (weighted creep <br />distance) as the sum of the seepage distances which lie at an angle of 45 degrees or <br />more from the overall seepage Bath direction plus one third of the distance in the <br />overall seepage path direction. Therefore, features which force the seepage water <br />to change directions at an angle of 45 degrees or more are considered to be three <br />times as effective in preventing internal erosion as the path in the general seepage <br />