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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We evaluated two idealized study sections : the maximum section at about Station
3 + 00, and at the seepage area at about Station 115 + 00, The upstream slope for the
main embankment averaged about 3:1 (horizontal to vertical), and the downstream slope
ranged from about 2.4:1 to about 4:1 on the lower portion of the slope. The exterior
slope of the study section taken at about Station 115 + 00 ranged from about 3: 1 on the
upstream slope to about 2:1 on the downstream slope. The dam height ranged from
42 feet at the maximum section to 13 feet at about Station 115+00, Phreatic levels in
the idealized study sections were based on knowledge of the history of the dam and
the assumption that the reservoir is returned to full storage capacity (gage height 34,5
feet), The study sections used in the stability analysis are shown on Figure 11,

5,3.4,2 Stability Analysis

We considered standard penetration test results, triaxial test results and our experience
for selecting soil parameters for the stability analysis of the existing embankment.
Earthquake loading was simulated by psudo.static analysis using a seismic coefficient
of 0.10, The parameters used for the stability analysis are presented on Figure 11. The
results of our stability analysis, summarized on Figure 11, indicate a minimum factor of
safety of 1,6 for static conditions and 1,4 with earthquake loading, thus the stability of
the existing embankment appears to be adequate,

Considerable experience-based judgement must be used in applying the results of
theoretical stability analyses, since the analyses involve simplifying assumptions, as well
as the use of assumed geometry and soil and seepage parameters, which in themselves
are not amenable to accurate determination.

5,3,5 SEEPAGE CONDITIONS

A crude but Informative test of seepage loss from Hourglass reservoir was made by the City
on September 11, through September 13, 1990. Both the inflow and the outflow to the reservoir
were cut off and the reservoir level was observed, Based on the reservoir area-capacity data,
the apparent reservoir loss was calculated to be approximately 7 acre.feet per day at gage
height 24 feet. Data is not availabie to extrapolate reservoir losses at other gage heights,

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