C150163 Feasibility Study

• Contract/PO #: C150163
• Contractor Name: New Cache La Poudre Irrigating Company
• Contract Type: Loan
• Water District: 3
• County: Weld
• Bill Number: 04-1221
• Loan Projects - Doc Type: Feasibility Study

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bedrock was encountered below the sand with gravel layer. At the Drury and
Cornish Plains sites, sandstone bedrock was encountered below the sand and
gravel layer. The fine sands exhibit low to medium permeability (4.0 x 10-5 to 9.0
x 10-5 cm/s) and can be used to construct dam embankments. The clay material
encountered at the Drury Reservoir site exhibits low permeability (1.4 x 10-6
cm/s) and could be used to construct less permeable cores and cutoff trenches
for the dams. The sand and gravel layer encountered above the bedrock is a
mix of well-graded sand and gravel. Because of the fine sand in this layer, it
exhibits only medium permeability (1.4 x 10-4 cm/s). It is possible that near-
surface gravel layers exist within the proposed reservoir sites that were not
encountered in the borings. If gravel layers exist at or near the surface, they will
need to be cut off to prevent excessive seepage from the reservoirs; or, if
desired, they could be used to facilitate groundwater augmentation.

Dam Embankments

Materials suitable for the construction of dams are available on site. The dams
would consist of fine sand embankments with clay cores or upstream blankets
and clay cutoff trenches along the foundation.

Slope Stabilitv

Slope stability analyses were conducted for the maximum cross-section of
Cornish Reservoir. This cross-section is the most critical section for the three
dams considered in the project. Therefore, the slope stability results can be
conservatively applied to all of the dams. Slopes of 3 (horizontal): 1 (vertical)
were used on the upstream and downstream faces. Based on laboratory testing
of materials sampled from the site, 35 degrees was used as the design value for
the angle of internal friction (phi), and 50 pounds per square foot (psf) was used
for the cohesion. A unit weight of 120 pounds per cubic foot (pcf) was used for
the soil weight. Figures 8 -10 show the results of the analyses for steady-state,
sudden reservoir draw down, and seismic conditions. For the conditions
analyzed, 3:1 slopes on the upstream and downstream faces meet the State
Engineer Office's (SEO's) requirements for dam safety. Table 3 summarizes the
results of the stability analyses.

Condition
Steady State
Sudden Draw Down
Seismic

Table 3 - Slope Stability Analvsis Summary
Reauired Factor of Safety Actual Factor of Safety
1.5 1.8
1.2 1.6
1.0 1.3

New Cache Feasibility Study

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