WSP02924 - Laserfiche WebLink

Home Browse Search

043G (Area X) would in our jlldgment qualify as being either pervious or reasonably resistant to piping erosion. 6. In view of the reported considerable thickness (50 to 70 ft.) of fine alluvium above the present ground water table for over 3.5 mi. along the right abutment a."ds, and considering the reported alluvial fan mode of deposition of that alluvium, it seems probable that significantly large total foundation settlements and potentially damaging differential settlements should be anticipated along that reach of the dam upon construc- tion of the dam, and most importantly upon first ponding of the reservoir against that reach of the dam. 7. The extensive river bottom area for several miles upstream from the dam- site appeared to have substantial potential as a source of sand and gravel, recoverable by dredging. Subsequent review of exploratory data showed the riverbed sand to be from 10 to 30 ft. thick, to be fairly clean, and to average 16% coarser than a No, 4 screen. 8. No source of clean, directly usable gravel was shown to us, although some of the geologic reports indicate the presence of several gravel deposits within 10 miles of the damsite. It is our view that a substantial volume of clean, very pervious gravel should be called for in the final design of the embankment, and hence further efforts to define and inventory gravel deposits, both as to quantity and nature, seem highly justi- fied. 9. There appeared to be practically unlimited quantities of impervious core borrow along the broad bench north of the rhoer and upstream from the damsite. Like- wise, there appeared to be practiCally unlimited quantities of random, fine, silty sandy borrow on the flats south of the river. The latter borrow, however, appeared to us to be of such a nature that it would be relatively brittle when normally compacted, and relatively sensitive to piping. Hence, it should be utilized cautiously, in locations of the dam cross section where it can be fully protected against potential piping. GENERAL COMMENTS In its simplest form, the question being raised at this time is whether, at the proposed Narrows damsite, a safe foundation exists and whether a safe earthfill 6.