Laserfiche WebLink
<br />044'~ <br /> <br />Works, from the standpoint that the formation is Imown to have rebound and swelling <br />characteristics, special protective measures during exposure of the siltstone, and <br />special articulation of all concrete structures thereon, will probably be necessary. <br />b. Ri!?;ht Abutment - Slurrv Trench Reach. This reach extends from <br />axis Sta. 278 to 231, a reach in which pervious to semi-pervious overburden alluvium <br />thickens beyond the reasonable capability of performing open cut excavation to the <br />Pierre Formation. The feasibility design calls for a full cutoff in this reach, effected <br />by a 20 ft. deep open trench below which an up to 100 ft. deep by 8 ft. thick slurry <br />trench cutoff to siltstone would be constructed. We concur that such a cutoff can be <br />practicably constructed, may be required, and would be effective. We would recom- <br />mend consideration, however, of an alternative which we consider would be comparably <br />effective and which may be more economical and probably more effectively monitored <br />for quality control during construction. The alternative would consist of allowing a <br />Contractor the option of providing a 4, ft. thick slurry trench with a transit-mixed, <br />gravel-sand-silt-clay-bentonite backfill, the trench being excavated in successive pan- <br />els by a special, broad span, clam bucket on a vertical Kelly bar. Such an excavation <br />procedure has been used successfully at Chatsworth Dam in Southern California, and <br />on many deep building and subway station foundations. <br />In this reach of the dam, we also would recommend conservatism in the require- <br />ments for stripping under the outline of the dam, with particular attention to removal <br />of the loose, aeolian, sand deposits. <br />c. Rig:ht Abutment - No Cutoff Reach. This reach extends from axis <br />Sta. 231 to about Sta. 130, with ground surface elevation varying from a minimum of <br />about 4400 up to about 4450. The feasibility design drawing indicates the possibility of <br />a modest depth open "cutoff" trench in sandy soil. As indicated in a previous section, <br />surface flooding of the reservoir floor adjacent to this reach would occur infrequently <br />and only for short periods for ground elevations above about 4410 (Stas. 220 to 130). <br />Accordingly, conventional seepage analyses for steady seepage in this reach may only <br />apply to the relatively short portion from Sta. 231 to Sta. 220. <br />It has been extensively considered by the USBR, however, that the safety and <br /> <br />11. <br />