PROJ00235

• Contract/PO #: C153633
• Contractor Name: Greeley, City of
• Contract Type: Loan
• Water District: 3
• County: Larimer
• Bill Number: HB 91-1006
• Loan Projects - Doc Type: Feasibility Study

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5.2,3 SUBSURFACE CONDITIONS

The test holes generally showed 11 to 26 feet of embankment material overtying natural soils.
The embankment materials generally consisted of medium dense to dense, slightly silty to silty,
gravelly to very gravelly sands. Laboratory test results show that this material has an intemal
friction angle ~ with no cohesion, has a natural dry density of 116 pc!, and is non-plastic.
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The natural materials below the embankment are glacial deposits which consist predominantly
of loose to medium dense. clean to silty sands with intertayered sandy silt lenses. Other natural
materials in the embankment foundation range from slightly sandy silts and clays to sandy
gravels with cobbles, The stratigraphy of the natural materials does not correlate well between
test holes. The results of triaxial laboratory testing show that the foundation material has an
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internal friction angle Of~h' no cohesion, and has a natural dry density of 125 pc!. Most
of the foundation soil samples tested were found to be non-plastic, Alterberg limits tests on
a sample of sandy silt from the foundation show a liquid limit of 37 and a plasticity Index of
10,

Bedrock was not encountered In any of the test holes to the maximum depth drilled (80 feet).

Free water was measured in the test holes at depths ranging from 19 to 24 feet at the time of
drilling or one day later. Information on long-term fluctuations of water levels in the test holes
is not available; however, it is expected that water levels in the test holes would vary
significantly with the reservoir levels.

5.3.4 STABIUTY OF EXISTING DAM

Our analysis of the stability of the existing dam included consideration of the history of the dam,
exterior slope and dam height. internal water level and subsurface soil conditions we found
within and beneath the dam, We evaluated this information as described below and selected
two idealized study sections for our stability analysis, We assumed soil parameters and Internal
water levels that we believe are reasonably conservative considering the history of the dam and
the variations in subsurface soil conditions found.

5,2.4.1 Study Sections

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