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<br />01,1\3 <br /> <br />. <br /> <br />with the drain zone modification as in a. above, is acceptable. A much thinner slurry <br />trench, as suggested in an earlier section, would be acceptable. <br />If the concept is accepted by USBR designers that reservoir levels above about <br />Elev. 4410 are so infrequent, so transient and so brief that they do not constitute a <br />realistic cause of significant under seepage , consideration could be given to extending <br />the slurry trench to a location where the natural ground surface is at that elevation <br />(4410). This would be at about Sta. 210, involving an extension of 2100 Un. ft. The <br />extension would of course require deepening the open trench portion of the cutoff, pre- <br />suming slurry trench construction equipment to be used would be limited to an excava- <br />tion depth of 100 ft. <br />c. Ril<ht Abutment - No Cutoff Reach. The feasibility design, with <br />the drain zone modification as in a. above, is acceptable. The drain zone and its filters <br />could be minimized in thiclmess for this rarely-in-service reach of the dam. <br />d. Right Abutment - Supplementary Dike. In our review, assuming <br />severely pessimistic conditions could theoretically prevail at the right end of the slurry <br />trench cutoff as shown in the feasibility design, and as suggested by the 11/8/77 USBR <br />seepage analysis, we have made a brief study to determine whether the slurry trench <br />could be positioned in a more favorable location. The possibly obvious location would <br />be at a position where the Pierre siltstone is at shallow depth and a supplementary sur- <br />face dike could be built at low cost. We believe that the following revised layout at <br />least partially satisfies this concept and probably deserves consideration by USBR. <br />We suggest consideration of the additional construction of a low dike to Elev. <br />4430, beginning at the Dam at about axis Sta. 270, running due West to the center of <br />Section 14 and angling slighUy Southwest to the 4430 contour in Section 15. We would <br />provide a slurry trench cutoff to siltstone under the full length of this 13,500 ft. long <br />dike. The Main Dam would be built southwesterly for its full intended length, to provide <br />containment against a Probable Ma.'l:imum Flood, but its cross section would be simp- <br />lified to eliminate the slurry trench beyond Sta. 270 and to eliminate all sophistication, <br />to the extent of possibly constructing an homogeneous section. This would be justified <br />since the probability of the Main Dam serving to impound water would be reduced by the <br /> <br />16. <br />