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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2.0 GEOTECHNICAL EVALUATION

2.1 Field Investigation

2.1.1 General

The geotechni ca1 i nvesti gati ons at Peterson Lake Dam were carri ed out to
supplement work performed previously in 1982 and 1985. The objective was to
characterize the embankment dam and its foundation, investigate sources of
potential construction material, and to gather soil samples for laboratory
testing. The field investigations consisted of surficial geologic mapping and
subsurface exploration by means of bore holes, test pits, and seismic
refraction surveys. Multi-level standpipe piezometers were installed in each
bore hole following completion. The work was performed during the months of
August and September, 1986, di rectly by, or under the supervi si on of, MKE
personne 1 .

2.1.2 Geologic Mapping

A surficial geology map of Peterson lake Reservoir is included in Figure V.2.
The dam foundati on and much of the reservoi r are in Precambri an metamorphi c
and igneous bedrock. Dark gray gneiss, schist, granite, and white pegmatite
of the Idaho Spri ngs Formation outcrop on both abutments and throughout the
northern one-half of the reservoir. Foliation and sChistocity are subvertical
and wavy; and strike approximately N70oE. Princ;"pal joint sets are parallel
to foliation and schistocity, N250W, 700NE and N20oE, 4SoNW. The western side
and southern end of the reservoir, including the area of the auxiliary
spillway, are covered with glacial till of Pleistocene age. The till is a
1 i ght brown, c1 ayey sand and gravel wi th 1 arge subrounded bou1 ders to four
feet in diameter.

2.1.3 Subsurface Investigation

Peterson lake Dam and Reservoi r were i nvesti gated usi ng four rotary bore
holes, 13 test pits, and 11S0 linear feet of seismic refraction survey lines.

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