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<br />,.-\ (' ("; n <br />1..1';... v <br /> <br />.; <br />I <br /> <br />o <br /> <br />intermixed with the several st:=ands of alluvium in the South Platte <br /> <br />River. drainage basin in a complex fashion. More recent blankets of <br /> <br />loe9s cover much of the surface surrounding Ft. Morgan and the <br /> <br />Narrows Dam site. <br /> <br />THE FOUNDA~ION <br /> <br />Pierre Shale is the. bedrock beneath the dam and rese::""loir. <br /> <br />The rock is a densel pre-consolidated siltstone, claystcne. and shale <br /> <br />with smaller amounts of fine sand. In places it consists of lime- <br /> <br />cemented slabs and lenses; in other places gypsum may serve as the <br /> <br />cement. Rock samples exposed at the surface .and taken from drill cores <br /> <br />show considerable variation in particle size, degree of compaction, <br /> <br />color, and permeability. The samples obviously do not come from the <br /> <br />same horizon since there is considerable topographic relief on this <br /> <br />bedrock surface. Horizontal bedding is obscure or lacking in most <br /> <br />samples. <br /> <br />The Pierre Shale has a widespread reputation for swelling under <br /> <br />lo~ loading conditions where excess water is available. It also <br /> <br />wea~~ers to a soft clay which is easily removec by running water. <br /> <br />Hence outlet works on exposed Pierre Shale are subject to rapid erosion <br /> <br />by even low velocity discharges. However, when e."Cca'lfated to unweathered <br /> <br />shale, the rock is remarkably strong and capable of supporting <br /> <br />strUctures and resisting erosion by running water. <br /> <br />It is important that in all c~itical foundation excavations <br /> <br />the cut is carried into unweathered shale. At some places it is ve~ <br /> <br />3 <br /> <br />\,. <br />