PROJ00504

• Contract/PO #: C153334
• Contractor Name: Henry Waneka Mutual Reservoir Company c/o City of
• Contract Type: Loan
• Water District: 6
• County: Boulder
• Bill Number: FSA
• Loan Projects - Doc Type: Feasibility Study

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4 - Embankment Analysis
For the embankment analysis, two aspects were investigated: the slope
stability and the potential seepage quantity. The purpose of these analyses
was to determine the stability of the existing embankment, which affects
the design of any proposed improvements.

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A. Slope Stability
For the slope stability, a computer program SLOPE-II (Reference-4)
was used to investigate the maximum dam section. SLOPE-II utilizes
the limit equilibium theory to solve for the factor of safety of an
earth slope. The Simplified Bishop Method of slices was selected
since the method includes the inters lice forces (by assuming the
resultant acts horizontally) and therefore the method is less conserva-
tive than the Ordinary or Modified Fellenius Method. Excerpts from
the Users Manual and sample output (Enlargement Alternative) are presented
in Appendix-E.

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The typical section and data assumptions for the stability ana-
lysis are shown in Figure-III. The embankment is basically a homo-
geneous sandy, silty clay material. The maximum steady state reservoir
condition was used to estimate the phraetic line by the Cassagrande
Method. The soft zone, identified by the test drilling program, was
modelled by using a separate embankment layer with no cohesion, and
a lower 0 angle. Due to the inferred fault line identified by the Ref-
erence-2 report (see Figure-II), the stability analysis was performed
with and without the effects of earthquake, by including a horizontal
force acting on the slice (0.1 g acceleration times the slice mass).
A search pattern was used to identify the critical slip circle, by
varying the circle radius, and radius point location.

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The results of the analysis indicate a factor of safety of 1.43

without earthquake and 1.15 with earthquake effects. The critical rad-

lUS point and radius were the same for both cases. The suggested mini-

mum factors of safety for non-seismic end of construction, steady

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