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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1940). The steady-state phreatic surface for Peterson lake Dam is shown on
Figure V;3. The location of the estimated steady-state phreatic surface
compares favorably with piezometer readings and the location of visible
seepage on the downstream face, observed when the reservoir water surface was
near the normal pool elevation.

The phreatic surface method can also be used for estimating pore water
pressures under rapi d drawdown condi ti ons. The phreati c surface from the
steady-state seepage condition is modified by conservatively assuming that the
reservoi r water surface, is lowered instantaneously, and that the phreati c
surface coi nci des wi th the upstream face of the embankment. For embankments
with semipervious to pervious materials, partial dissipation of pore pressures
may be assumed. Based on visual and laboratory classification, and laboratory
permeability tests, the materials comprising Peterson lake Dam can be
classified as semipervious to practically impervious, with poor drainage
capabil i ty. However, exploratory bori ngs encountered occasi ona1 c1 ean;
gravelly sand lenses within the embankment, which may provide drainage paths
during reservoir drawdown.

Peterson Reservoir has been restricted to elevation 9498 (G.H. 20), since
August 16, 1982. The steady-state phreatic surface which has developed in the
embankment most likely corresponds to this restricted level and not the normal
pool el evati on of 9510 (G. H. 32). The rapi d drawdown phreati c surface shown
on Figure V.3 was developed considering the normal pool elevation, along with
the potential for embankment drainage through the pervious lenses.

2.3.3 Material Properties

The major soil properties used in stability analyses are density and shear
strength. Field and laboratory soil tests, as well as past experience with
similar materials, were employed to arrive at the Peterson lake Dam embankment
fill material properties shown on Figure V.3.

Densities obtained from relatively undisturbed Shelby tube samples of the
embankment, and fi el d tests usi ng a nuc1 ear density gage, i ndi cate that the

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