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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />. <br /> <br />I <br />I <br />f <br />I <br />I <br />I <br />I <br /> <br />2.3.5 <br /> <br />Results <br /> <br />For the purpose of slope stability analysis, the factor of safety is defined <br />as the ratio of total available she~r strength of the soil to shear stress <br />required to maintain equilibrium along a potential surface of sliding. The <br />factor of safety indicates a relative measure of stability for various <br />conditions, but does not precisely indicate the actual margin of safety. <br /> <br />The safety factors for the upstream and downstream slope stability of Peterson <br />lake Dam are given on Figure V.3. The upstream slope is fairly steep at 2H to <br />1 V, consi deri ng the possl b1 e weak 1 enses in the embankment. The margi n of <br />safety for thi s slope duri ng rapi d r~servoi r drawdown is low, as ref1 ected by <br />safety factors of 1.05 and 1.17. It appears the critical failure mode may be <br />governed by a non-ci rcu1 ar sl i p surface through a weak 1 ayer in the dam. The <br />downstream slope is modest at 3H to 1 V. However, the margi n of safety for <br />this slope, under existing steadY-state seepage conditions, is low to <br />moderate. Safety factors range from 1.15 to 1.32. The analysis demonstrates <br />that when the phreatic surface is modeled as being coincidental with the slope <br />face, smaller failure surfaces near the toe have a lower margin of safety than <br />larger surfaces encompassing the entire slope. <br /> <br />In the past, small fai 1 ures in the form of surface sl oughi ng have occurred <br />near the downstream toe of Peterson Lake Dam as a resul t of the embankment <br />seepage. Because slope failures are typically initiated at the toe, <br />progressi ve fail ure of the downstream slope cou1 d resu1 t from these small er <br />fail ures. <br /> <br />2.4 Existing Seepage Conditions <br /> <br />Seepage data col1 ected from June 4 to August 27, 1984, whll e the reservoi r <br />water surface was fluctuating around the restriction elevation of 9498 <br />(G.H. 20), were analyzed to determin~ major sources of seepage. Four V-notch <br />weirs are utilized to measure seepage at Peterson lake Dam. Weirs 1 and 2 are <br />located on the downstream right abutment-embankment contact, with weir 1 being <br />above weir 2. Weir 3 is located on the downstream face over the outlet works. <br /> <br />-55- <br />