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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Test resul ts of a potenti a1 impervi ous materi al source at the extreme south
end of the reservoir revealed a sandy silt (ML), inferior to soils from the
other borrow area as an impervi ous bl anket materi al. Further detai 1 s on
1 aboratory test resul ts for soil samp1 es from Peterson Dam and Reservoi r are
included in Appendix C.

2.3 Slope Stability of Existing Dam

2.3.1 Procedure

Static stabi 1i ty analyses of the exi sti ng embankment dam were performed usi ng
the Modi fi ed Bi shop 1 imi t equil ibri urn procedure. Two different embankment
loading conditions were analyzed. These loading conditions were:

A. Steady-State Seepage Conditions - The stability of the
downstream slope was analyzed wi th the reservoi r at
normal pool elevation 9510 (G.H. 32). This is the
reservoir level that will control the development of the
steady-state phreatic surface in the embankment.

B. Operational Conditions - The stability of the upstream
slope was analyzed for rapid reservoir drawdown
conditions from the normal pool elevation to the outlet
works inlet elevation. Rapid drawdown represents
emergency evacuation of the reservoir, or normal drainage
operation if the drawdown rate is on the order of 1 to 2
feet per day.

2.3.2 Pore Pressures

Pore water pressures for steady-state seepage loading conditions can be
estimated as hydrostati c pressures below the steady-state phreati c surface.
The location of this phreatic surface can be estimated for homogeneous dams on
re1 ati ve1y impervi ous foundati ons by fol1 owi ng graphi ca1 procedures developed
by Casagrande ("Seepage Through Dams," Boston Society of Civil Engineers,

~50-