PROJ00505

• Contract/PO #: C153419
• Contractor Name: San Miguel Water Conservancy District
• Water District: 0
• County: San Miguel
• Bill Number: XB 99-999
• Loan Projects - Doc Type: Feasibility Study

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Dakota. The sand is quartzitic and laced with carbonaceous stringers in places

and the shale is clayey and/or carbonaceous. Cretaceous-age Mancos Shale,

which overlies and intertongues with the Dakota, is similar to the Dakota

Formation shale. and the Mancos-Dakota contact was not distinguished during

geotechnical investigation. Slopes to the east and south of the reservoir

basin are formed by Mancos Shale, and portions of the reservoir basin may be

underlain by the basal part of the Mancos.

The only exposures of the bedrock in the reservoir area are on the right

abutment as mapped. The reservoir basin was covered with water during our

investigation which precluded mapping of the surficial deposits. Our drill

data indicates a clayey overburden on the bedrock beneath most of the dam. Our

Map 1124-3 depicts our estimate of the extent of the exposed bedrock based on

the drill data. Geologic maps by the USGS indicate the area is underlain by

Mancos Shale, bbt the exposed sandstone beds and recovered core indicate Dakota
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Sandstone. The dip of the beds indicate that at least part of the southern or

left side of the reservoir is underlain by Mancos Shale.

No measurements of joints or fractures were made. Visual observation of

the strata exposed in the downstream outlet channel indicate the bedding planes

dip a few degrees upstream and toward the left abutment. Some joints were seen

in exposed sandstone strata on the right abutment. The shale and clay beds

appear from the core to be unjointed. It is the bedding planes which control

the seepage and stability of the foundation. The distribution and orientation

of major joint and fracture sets and the bedding planes will be required for

final considerations primarily with respect to grouting and to a lesser extent

for other forms of seepage reduction.

Core recovery from four of the test borings averaged 86% and the RQD or

modified core recovery averaged 38%. In the top 20 feet of the bedrock core

recovery averaged 80% and RQD averaged 28%. Below 20 feet into the bedrock

core recovery averaged 92% and the RQD averaged 52%.

Clay from weathered clay shale either transported
bedrock and underlies the existing embankment except

or in-situ overlies
beneath the right

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