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"r------- ;1 ![ II '" Ii :1 t:'"' I !! I I :1 'I it I; ,I ~ I :i I' 'I II :1 I I :1 oo.onz} -16- their crnnparative cost estimates. The subsequent investigations have modified the early geologic concept to considerable extent. Figure 3 shows the location of the drilling investigations at the lower Weld County site and Figure 4 presents a geologic cross-section developed from the results of both drilling and seismic investigations. The explorations show that the bedl'ock under the foundation and lower abutments is Pierre shale and that the upper portions of both abutments are in the sandy, shaly, transitional phase of the Fox Hills formations. The deep ancient channel of the river is offset from the present channel approximately 2,000 feet to the north, has a width of 1,500 to 1,800 feet and a maximum depth of about 160 feet below the ground surface. Descriptive logs of the exploratory borings arc attached to this report. It is the opinion of the writers that the bedrock under the abutments nOl"th of Station 48+00 and south of Station 186+00 is at a sufficiently shallow depth to permit installation of a com- plcte cutoff by ordinary construction methods. The bedrock it- self is, in our opinion, quite satisfactory as a foundation ma- terial and will be essentially impervious to seepage. In the foundation area, the bedrock surface shows an upper and broader flood plain channel approximately 13,600 feet wide, including the 1,800 feet wide inner deep channel. In our opinion, the bearing characteristics of the alluvium are satisfactory for founding an earth embankment structure of the type proposed. Depth of alluvium in the upper flood plain channel ranges from 45 to 90 feet. Drilling samples and percolation tests in- dicate that this sand, silt, gravel and clay material is generally """" .