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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Due to the poor foundation and existing embankment ronditions (i.e.,
saturated, medium consistency, sandy silty clay) the proposed embankment
includes a more sophisticated internal drainage system. The embankment
is a modified homogeneous earthfill on a fine grained saturated foundation.
The reconnnendations in "Design of Small Dams" (Reference-5) were used as
guidelines during the preliminary design.

To increase the embankment stability, a toe drain, drainage blanket,

and chimney drain were included at the maximum section (Figure-V). The

chimney drain will also help during earthquakes and other earth movements

since the drain provides a positive path for seepage from the potential

cracks during earth movement. In addition, granular soils are "self

healing" when movement occurs (i.e., the soil reconsolidates rather than

maintaining cracks, typical of cohesive soils).

To verify the design, a slope stability analysis was also performed and
the results are shown on Figure-VIII (page 59). The factor of safety of slid-
1ng was found to be greater than 2.0. The critical slip surface shown indi-
cates that any failure potential 1S a deep foundation failure. This condition
is more critical than a shallow surface slough due to the relatively flat
embankment slopes. In addition, the analysis assumes a slip surface (i.e.,
zero cohesion) when the circle intercepts the bedrock and therefore the
sliding resistance is minimized.

3 - Enlargement
Due to the site constraints, the amount of dam enlargement is limited
by the elevation of the abutment area (see Figure-VI, page 57). By raising
the dam from an elevation of 5320 to 5325, an add i t ional storage of 176 AF
(see Area Capacity Curve on Figure-II) can be obtained, or an additional
storage of 360 acre-feet above the present storage restriction. The required
improvements for this alternative are shown in Figures VI and VII (page 58).
They consist of constructing a new embankment (140,000 Cy) on the downstream
portion of the existing embankment, replacing the emergency spillway (240 CY),
replacement of the outlet works, and other miscellaneous items (see also Table-
6 (pages 45 and 46). The required land acquisition (approximately 32 acres) is

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