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<br />.-..n.... n <br />Li;Jj0 <br /> <br />(Area X) would in our judgment qu31ify as being either pervious or reasonably resistact <br />to piping erosion. <br />6. In view of the reported considera.ble thickness (50 to 70 ft.) of fine alluvIum <br />above tb,e present ground water table for over 3.5 mI. along the rIght abutment axIs, <br />and considering the reported alluvial fan mode of deposition of that alluvium, It seems <br />probable that signIficantly large total fOl.mdation settlements and p:>tentlaUy damagIng <br />differential settlements should be anticipated along that reach of the dam upon construc- <br />tion of the dam, and most Importantly upon first pondlD.g of the reservoir against that <br />reach of the dam. <br />7. The extensive river bottom area for several miles upstream from the dam- <br />sIte appeared to have substant1al potential as a source of sand and gravel, recoverable <br />by dredging. Subsequent review of exploratory data showed the riverbed sand to be <br />from 10 to 30 ft. thick, to be fairly clean, and to average 16% coarser than a No.4 <br /> <br />screeD. <br /> <br />8. No source of clean. directly usable gravel was sho'WD. to us, although some of <br />the geologIc reports lndlcate the presence of several gr3.\"el deposits wlthl.n 10 mIles of <br />the damslte. It Is our view that a substantial volume of clean, very pervIol1s gravel <br />should be ca1.led for in the final design of the embankment. and hence further efforts to <br />define and inventory gravel deposits, both as to quantity and nature. seem highly JustL- <br />fied. <br /> <br />~. There appe:u-ed to be practically l.m.Umited quantities of Impervious core <br />borrow along the broad bench north of the ri\"er and upstream from the damsite. Like- <br />wise, there appeared to be practically unlimited quantities of random, fine, silty sandy <br />borrow on the fiats south of the dver. The latter borrow, however, appeared to us to <br />be of such a nature that it would be relatively brittle when normally compacted. and <br />relatively sensItive to pipIng. Hence, it should be utilized cautiously J In locations of <br />the dam cross section where It can be fully protected against pateDtial piping. <br /> <br />GENERAL COMMENTS <br /> <br />In its simplest form, the questIon beIng raised at this time is whether, at the <br />2.'1 proposed Narrows damsite. a safe foundation exists and whether a safe earthfill <br /> <br />6. <br /> <br />- , <br />