Laserfiche WebLink
<br />I <br />I <br />I <br />1 <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />The safety factors for the upstream and downstream slope stabi 1 ity of Twi n <br />Lakes embankment are given on Figure V.2. Both factors of safety appear <br />reasonabl e based on past long-term behavi or and vi sual observati on of the <br />embankment. The upstream slope is relatively flat (3.5H to lVl and has not <br />exhibited any signs of distress. The margin of safety for this slope; even <br />under severe rapid drawdown loading conditions, appears to be high, as <br />refl ected by a safety factor of 1. 62. The downstream slope is fai rly steep, <br />averagi ng about 1. 5H to 1 V. The breaks in slope and masonry wall supporti ng <br />the toe suggest that a small rotational slump may have occurred sometime in <br />the past. The margi n of safety for the enti re downstream slope is low, as <br />reflected by a safety factor of 1.07. Smaller slip surfaces near the <br />downstream toe may be very near failure under steady-state seepage conditions. <br />Because slope failures are typically initiated at the toe, progressive failure <br />of the downstream slope could result from these smaller failures. <br /> <br />2.4 Existing Seepage Conditions <br /> <br />Seepage data collected from June 4 to July 19, 1984, while the reservoir water <br />surface was near normal pool elevation 9295 (G.H.20l, was studied in an <br />attempt to determine major sources of seepage. The maximum seepage; as <br />measured by two V-notch weirs on the left side, downstream of the embankment <br />toe, approached 100 gpm. <br /> <br /> <br />An approximate seepage analysis of the embankment was performed using <br />1 aboratory permeabil i ty results and the hi gh-l evel steady-state phreati c <br />surface shown on Figure V.2. A seepage estimate of roughly 1 gpm was obtained <br />using an embankment permeability of 5 x 10-5 cm/sec. (50 ft/yrl. Thi s <br />represents only 1 percent of the total measured seepage. <br /> <br />It is difficult to obtain an accurate estimate of seepage because of the wide <br />range of permeability possible for the weathered bedrock material comprising <br />the dam. For exampl e, usi ng a typi cal permeabil i ty range of 10-3 cm/sec <br />(1000 ft/yrl to 10-7 cm/sec (0.10 ft/yrl for weathered bedrock, a seepage <br />estimate ranging from 0.01 to 100 percent of the measured seepage was <br />calculated. <br /> <br />-55- <br />