PROJ00170

• Contract/PO #: C153481
• Contractor Name: Colorado River Water Conservation District
• Water District: 0
• Bill Number: XB 99-999
• Loan Projects - Doc Type: Feasibility Study

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2.0 GEOTECHNICAL SUMMARY

The existing dam and reservoir are located on a stretch of Elkhead Creek which
meanders in a southerly direction through gently rolling hills from its SOJ~ce
on the eastern slope of Saddle :-1ountain. The river valley downstream of the
reservoi r is broad and fl at, havi ng been blanketed by recent all uvi a 1
deposits. The stream follows a shallow, constantly shifting channel in the
alluvium. The walls of the valley are relatively steep in the vicinity of the
dam, where the creek has cut through a resistant layer of sandstone. Above
the steeper slopes parallel to the creek, the topography flattens to a series
of hills formed by the weathering of relatively weak claystone bedrock.

Geologically, the area is formed by sedimentary deposits of the Lance and Fort
Union Formations. Both of these units are Tertiary in age and are composed of
light brown, weakly cemented, fine grained sandstone with interbedded layers
of gray claystone. Occasional seams of coal are also found in these
formations. The beds strike northeast and dip about 8 degrees to the
northwest. The bedrock is tightly jointed, and fracturing decreases with
depth. No evidence of major faulting exists in the area.

Vegetation at the site consists of native grass and sagebrush, except along
the flood plain where the ground is covered with grass and occasional clusters
of aspen, willow, and cottonwood trees.

A complete geotechnical investigation of the Elkhead Lake Dam project entitled
"Geology, Materials Investigation and Embankment Design, Elkhead Creek High
Dam" was prepared for the original jesign by Engineering Consultants, Inc.
(19731. Additional investigations by MKE for the dam modifications were
limited to two bore holes on the left abutment in the area of the proposed
auxiliary spillway channel. Bore hole logs and photographs of the core are
incl uded in Appendix B. In addition, several test pits were excavated to
investigate potential sources of filter and drain material and concrete
aggregate.

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