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<br />- 4- <br /> <br /> <br />report presents a graph showing the variation in factor of safety with <br />width of buttress. The purpose of our study showing the varying sizes of <br />the toe was to demonstrate the effect of changing size of the toe buttress <br />fill. In our report, after studying the various possibilities, we recom- <br />mended that the buttress fill be constructed 2001 feet in width, in Item 10 <br />on Page 27. I believe this particular item has been essentially disposed <br />of by your agreement to follow the recommendations in our report. <br />Item 18, Page 5. This item was discussed in our meeting with MLRB. <br />We do not intend to extend the underdrain from the toe of the fill to the <br />point of intersection of the fill with natural ground. We have recommended <br />that the fill drains be extended up the natural drainages to near the point <br />of intersection with the natural ground, and that the actual location of the <br />underdrain be determined in the field at the time of the construction of <br />the fill. This is the only practical method of constructing this type of <br />drainage. <br />Memo <br />Eckman Park Hydrology Comments <br />Page 3 <br />The last item is referred to as Eckman Park Hydrology Comments. The <br />request states "what effect upon the stability and hydrology of the excess <br />spoil piles will result if a rising water table develops beneath the piles". <br />This particular problem was addressed in our original report in that we <br />assumed various locations of the water table within the excess spoil pile. <br />As an example, refer to Fig. 12 in our original report. A number of as- <br />sumptions are made with respect to location of the water table as identified <br />in the table presented on Fig. 12. For each possible location of the water <br />table, we have a computed factor of safety. The factor of safety does not <br />significantly drop until the water table is assumed to occur at elevation <br />7310 which, in all likelihood, is not a possible configuration. The spoil <br />will be relatively permeable. We estimate the coefficient of permeability <br />to be on the order of 1x1044 feet per year for the coarse, open graded <br />material. Even if the spoil was similar in performance to a dense clayey <br />or silty gravel the coefficient of permeability would still be relatively <br />high and drainage would occur rapidly through the fill. The risk of develop- <br />ing a water table considering the high permeability and the anticipated high <br />porosity is remote. However, for purposes of analysis, we assumed that the <br />surface drainage and the internal drainage failed. Even under these rather <br />remote conditions, we indicate a factor of safety of 1.37 for the water <br />table at elevation 7310. If all drainage systems were to fail and the spoil <br />disposal site were to fill with water to elevation 7410, which means that <br /> <br />