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I I I I I I I I I I I I I I I I I I I CHAPTER III GEOLOGIC AND GEOTECHNICAL INVESTIGATIONS A. INTRODUCTION The purpose of the engineering geological investigations include 1) delineating the geologic units in the area of the proposed Hardin Dam and Reservoir, 2) evaluating the suitability of the dam foundation and abutments, and 3) determining the availability and characteristics of potential construc- tion materials. These investigations provide a basis for the appraisal-level engineering design of the dam and for the associated cost estimate for dam construction. The geological and geotechnical conditions at the proposed dam site were to be based on previous work, primarily by Woodward-Clyde-Sherard (1964). An extensive search of files at Woodward-Clyde Consultants, at the CWCB and at the U.S. Bureau of Reclamation (USBR) revealed, however, neither site-specific soils laboratory test data nor detailed boring logs. It was concluded that this deficiency in information had to be remedied. The scope of work was modified to include a limited field investigation and related laboratory testing. Therefore, these investigations were based on published regional geologic reports, the cross-section in the Woodward-Clyde-Sherard report, extensive air photo interpretation, a brief geologic reconnaissance of the proj ect area and the field exploration program which formed the backbone of this study. B. GEOLOGIC RECONNAISSANCE 1. Physiography The proposed Hardin Dam site is located near the eastern edge of the Colorado Piedmont physiographic province. This province provides a transition between the High Plains to the east and the Front Range of the Rocky Mountains to the west. The gently rolling topography of the Piedmont slopes to the east and is drained by the South Platte River and its tributaries. Major tributaries in the Hardin Dam site vicinity include the Cache la Poudre River, Box Elder Creek, Lone Tree Creek and Crow Creek. III-l