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12. Geology of the Dam Site. - Geological studies of the site were initiated in July 1946, and the reconnaissance geological report submitted in September 1946. Pre- liminary drilling was started in December 1946. A preliminary geological report sub- mitted in March 1947 was based on information obtained from nine drill holes that had been drilled along the proposed axis of the dam. Addi.tional investigations were inau- gurated in the fall of 1947. A total of 135 cable and core drill holes, totaling approxi- mately 5,500 linear feet, was drilled; and five 6- by 6-foot test pits and two trenches were excavated. Numerous Denison-type drive-core, sack, jar, and undisturbed block samples were extracted for design studies. , Geologic features at the dam site consist of the following: (I) Valley-fill alluvium, (2) Valley sides of 50ft Ogallala sandstone, (3) A loess mantle over the high land. (4) Fine sand located at the right portion of the dam. (5) Pierre shale bedrock under the deep portion of the valley. (6) Talus, forming a narrow strip along each side of the valley. An examination of the geologic profiles indicated a valley fill of alluvium to a depth of 15 to 30 feet overlying 30 to 40 feet of Ogallala formation which, in turn, is overlying Pierre shale (appendix A). The right side of the valley consisted of fine sand with some silt overlying the Ogallala formation. At an elevation slightly above the valley floor, the right abutment was covered with a loess mantle. The river valley rose more abruptly on the left abutment, and the Ogallala beds were covered less deeply. Along the left valley side, a talus strip of loess and Ogallala sediments was formed. Higher up, the abutment was covered with a loess mantle. Stripping of the dam foundation was specified in order to remove undesirable or unsuit- able materials and to obtain good embankment contact. In May of 1948 a Theim-type permeability test was made in the right valley floor about three-eights of a mile from the river to determine the average permeability rate for the foundation material overlying the shale. One a-inch well and nine observa- tion holes were drilled for that test. Five observation holes were drilled to within a short distance of the underlying shale, or a depth of about 90 feet; the remaining obser- vation holes were drilled to shallower depths. The percolation rate, as determined on the basis of a constant discharge of 40.24 gallons per minute, was 36,890 feet per year. Permeability was determined by gravity tests at various depths in numerous drill holes located along the proposed axis of the dam. Leakage studies indicated that the alluvium overlying the Ogallala formation was relatively impermeable. The water tests at depths below the alluvium revealed a more permeable stratum. In view of these findings, it was considered desirable to excavate the cutoff trench to the top of the Ogal- lala formation. Flow net studies of the foundation with this cutoff trench indicated that an estimated seepage loss of about 5 cubic feet per second could be anticipated at normal water surface, elevation 3672. ~ Tests in the laboratory revealed that considerable consolidation upon saturation could be anticipated in the loess mantle of the right abutment. Provisions were made for preconsolidation of the right abutment by inundating the foundation area above ground level, elevation 3660, for 60 days prior to placement of embankment. This operation was accomplished by progressively flooding separate sections of the dam foundation through basins and ditches constructed in the abutment. 13. Earth Materials. - Early reconnaissance investigations indicated that ample quantities of impervious material for the construction of the dam would be available within a reasonable hauling distance. Two areas with similar loessial deposits were 12