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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GEOLOGIC REVIEW

Dr. Charles S. Robinson, consulting geologist, performed the geologic

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review for International Engineering Co. After an independent review of per-

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tinent data provided by Western Engineers, he inspected the Yamcolo Darn and

reservoir site on August 22 and 23, 1978. The second day included a brief

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conference at the site with Mr. Jay A. Olsen, consulting geologist, and with

representatives of both Western Engineers and International Engineering Co.

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His report was submitted in preliminary form on August 3D, 1978 and, with

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minor revisions, is incorporated in this section.

The geologic report is generally favorable with respect to the primary

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considerations of abutment and shoreline stability. Several conclusions can

be drawn from an engineering standpoint, based upon Dr. Robinson's geologic

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interpretation of the site:

1.

Due to glacial action, both abutments have been subjected to

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overburden loads comparable to those which the darn will impose.

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This preconsolidation increases shear strength and reduces both

permeability and compressibility.

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2.

The existence of a former lake at the site and seasonally high

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water table in both abutments and the valley floor reduces the

tendency for settlement when the reservoir is raised. The

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adjustment in 'soil structure sometimes associated with initial

saturation has thus taken place during the geologic history of the

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area.

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3.

The lakebed deposi~s evident in the valley floor immediately

upstream of the darn will have a sealing action against reservoir

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seepage losses, if left undisturbed.

4.

Both the debris flow material and the glacial till are re~atively

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well graded, dense, and unstratified; properties which make the abut-

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ments and foundation resistant to piping or internal erosion when

subjected to seepage gradients.