PROJ00318

• Contract/PO #: C153618
• Contractor Name: Greeley, City of
• Contract Type: Loan
• Water District: 3
• County: Larimer
• Bill Number: SB 87-15
• Loan Projects - Doc Type: Feasibility Study

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The stability of the existing embankment was checked using the J.lodlfied Bishop
method for the downstream slope under steady-state conditions, and the
upstream slope during operational con<lltlons. Pore pressures were estimated
based on the steady-state phreatic Surface derived from piezometric data.
J.laterl al properties were assigned based on laboratory test results and
experl ence with slmll ar materl als. The results of the stabll Ity analysl s
showed the upstream slope to have a factor of safety as low as 1.05 r1urlng
rapid drawdown, and the downstream Slope 1.15 under steady-state conrlltlons.
These safety factors do not meet ml nlmum accepted standards of 1.2 and 1.5
respectl vely. The analysl s demonstrated that small er fall ure surfaces near
the' toe have an even lower factor of safety when the phreatic surface Is
coincidental with the slope.

Seepage data, collected when the reservoir was at the restricted elevation of
9498, was analyzed and suggested blo ll1ajor sources of seepage, one along the
outlet works conduit and another at the contact between the dam and the right
abutment foundation. Although the measured seepage is relatively small, Its
apparent 1 ocat f on Is crltl cal to the (jam's safety. Even though thl s seepage
has occurred for many years, It shoul d not be taken for grante<l or assumed to
be an acceptable condition.

Ilased on the site inspections, geotechnical Investigation, stability an"
seepage analysis, and review of the dam's performance record, It was concluded
that Peterson lake Reservoir can be safely operated on an Interim basis.
However, due to the danger posed by excessl ve seepage' to the downstream toe of
the dam, It Is recommended that storage be limited to elevatlol1 9493, or gage
height 15.0 feet. This would represent a storage volume of 394 acre-feet.

In order to remove the restriction and restore the reservoir storage capacity
to Its decreed volume, and to operate the reservoir safely in the future, a
number of deflcfencles would have to be repaired. These deflcfencles are
11 sted as follows, In order of importance to the safety of the structure.

o Deterioration and reduced capacity of the drop Inlet structure of the
outlet works and uncertainty regarding the voids In the backfill.

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