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direction Since there are rto such featums along the critical seepage path for the <br />soil plug, the weighted creep distance is one third of the distance from upstream <br />face of the soil plug to its downstream tce, or about 25 feet. The weighted creep <br />ratio is the weighted creep distance divided by the total head difference between the <br />upstream and downstream ends of the critical seepage path, or about 2.8 at the <br />maximum head of 9 feet. Following era the recommended mtnimum creep ratios <br />for di$erertt sort types along with the maximum upstream head on the soil pfig <br />which would produce a weighted creep ratio within the recommended minimum <br />valises: <br />SOIL TYPE NIINIMUM RECOMMENDED MAJ~IMUM <br />CREEP RATIO ALLOWABLE <br />BEAD <br />GM 2.5 9.0 <br />SM 6.0 4.2 <br />ML 8.5 3.0 <br />It is seen that the above values suggest a potential for internal erosion at certain <br />potential upstream water depths if the sor7 plug material contains sufficient Eater <br />material. It must be noted that the weighted creep ratio method is considered to be <br />somewhat a rule of thumb and does not indicate erasion potential with arty degree <br />of certainty. The concern is that farlures due to internal erosion can occur suddenly <br />and catastrophically with fittle advance indications, Reduction of erosion potential <br />can be accomplished by instaUin$ cutoff features such as wails or trenches or by <br />installing seepage interception filters. <br />d. Overtopping -Methods used far construction of the soil plug along with <br />consolidation of the soil plug material would result in a void between the top of the <br />plug and the roof of the mine alit. Therefore, overtoppung failure is a possibt7ity <br />and must be prevented by assuring that the upstream water level remains safely <br />below the top of the soil plug. <br />CONCRETE DAM: <br />1. The concrete dam consists of stacked bags of sack Crete which were placed, <br />perforated and wetted. The Back-Crete dam was then covered with a duee inch <br />thick layer of gunite reinforced with fencing wire mesh. Based on the number of <br />baggy of sack-Crete used, the total weight of the wncrete dam was estimated by you <br />to be 18,840 lbs. Using the cross-sectional area of the concrete dam shown on the <br />attached calculation sheets and the dam length of 16 feet, the average density of the <br />sack-Crete dam is estimated to be 65.41bs/cu R. Assurrdng a density of 1301bs/cu <br />tt for loose, partially hydrated bags of sack-Crete results in an estimated proportion <br />