C150008 Feasibility Study

• Contract/PO #: C150008
• Contractor Name: Leroux Creek Water Users Association
• Contract Type: Loan
• Water District: 40
• County: Delta
• Bill Number: HB 95-1155
• Loan Projects - Doc Type: Feasibility Study

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Seepage. The seepage analyses were utilized to determine the need for internal drainage, the
location of the required control, and the capacity required. The location of the phreatic surface
from the seepage studies were used in the stability analyses. Varying the location of the drains
and thus the phreatic level in the darn was found to be a major consideration as to overall stability,

The analysis of the existing embankment predicted the phreatic line would exist the dam face at
a location approximately 15 feet above the darn toe as shown on Figure 5. If this is the case, the
darn would not be expected to be stable under long term steady state seepage conditions. The fact,
that the reservoir was normally not full for long periods, most likely is the reason it did function
acceptably for 50 years prior to the failure. .

Due to the high phreatic surface predicted in the dam, a drainage blanket and toe drain are
considered essential to the long term stability of the darn when it is rebuilt, A dam lower in height
may only need a toe drain, but further analysis will have to be conducted to determine this.

Figure 6 through Figure 8 show the results of the seepage analyses and the location of the phreatic
surface for each alternative. The location of the phreatic surface for Alternative 2 with only a toe
drain is marginal, as can be seen, and if this alternative were chosen additional studies would have
to be conducted.

Figure 7 and Figure 8 for alternatives 3 and 4 indicate a drainage blanket in conjunction with a
toe drain will effectively handle the seepage and lower the phreatic surface within the darn to
acceptable levels.

Slope Stability. The slope stability analyses were conducted utilizing effective stress shear
strength parameters that were assumed as shown in Table 3. FinaI design strengths will be based
on drained triaxial shear tests and drained direct shear tests.

The results of the analyses for the long term steady state conditions are shown in the Table 3 and
on Figures 9 through 12. The embankment slopes, for the alternatives evaluated, are believed to
be stable with factors of safety near that required by the State Engineer. Minor changes in the
dam configuration and final shear strength data would be expected to produce adequate factors of
safety against sliding, The shear strengths used in the analysis are assumed for the dam
embankment and darn foundation and we would expect somewhat higher values, especially in the
glaciated foundation clays, based on testing of the insitu materials.

The long term seepage condition is the critical case and is presented in Table 3. Sudden
drawdown and steady state seepage with earthquake were also analyzed but were not the
controlling conditions and are not presented herein.

Carl Smith Dam Feasibility

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