PROJ00345

• Contract/PO #: C153330
• Contractor Name: Upper Yampa Water Conservancy District
• Contract Type: Loan
• Water District: 0
• County: Routt
• Bill Number: SB 77-35
• Loan Projects - Doc Type: Feasibility Study

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With the exception of these modifications to insure abutment stability, no

shortcomings were noted in the design of the dam, Although a detailed check

of the stability analysis was beyond the scope of the review, the safety factors

presented are considered adequately conservative in view of the limited design

data available.

Upstream Abutments. The possibility exists of relatively pervious and

continuous strata within the abutments which could carry significant seepage

around the dam. To provide for this possibility, however remote, it is recom-

mended that an impervious blanket of Zone I be extended to the upstream limits

of the dam on both abutments. This extension is shown in plan view on Fig. 3,

The seepage path through each abutment would thus be lengthened substantially,

reducing both seepage quantities and gradients. If unexpected seepage conditions

should develop during reservoir operation, the blanket could easily be extended

upstream of the dam. A secondary benefit of this modification is that it pro-

vides a transition between the abutment and the oversize rock zone in the upstream

shell. Should lenses of fine sand or silt be exposed during foundation stripp-

ing, the Zone I blanket will prevent migration of abutment materials into the

rock zone as a result of drawdown and wave action.

Downstream Abutments. Extension of Zone 2 on the downstream side of

both abutments, using a well-graded sandy material, is also recommended. This

modification will provide similar abutment protection where the oversize rock

zone could otherwise come in direct contact with fine-grained abutment materials.

As shown on Fig, 3, a normal thickness of 5 feet is considered adequate, pro-

vided the select material is carefully placed to avoid segregation.

The existing steep stream bank downstream of the left abutment is subject

to sloughing if foundation seepage emerges at this point. T6 avoid future

maintenance, it is recommended that any steep slopes which could eventually

undercut an extension of the 4:1 downstream dam slope be flattened to 3:1.