WSP11325

• File Number: 8443.400
• Description: Narrows Unit - Reports
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 3/1/1978
• Author: Woodward-Clyde Consu
• Title: Review Of Data And Studies Developed By The Bureau Of Reclamation For The Feasibility Stage Design Of Narrows Dam Colorado
• Water Supply Pro - Doc Type: Report/Study

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Woodward.Clyde Consultants

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For 54 tests on composited materials having permeability

rates of less than 10 feet/year, the average permeability value
w~s abuut 0.6 feet/year (6 x 10-7 cm/sec--very low

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permeability). For the only 3 tests on composited materials
having permeability rates of greater than 10 feet/year, the

permeability values were about 10 to 60 feet/year (very low to

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low permeability). From our review of the tests and test pit

logs it is apparent that the proposed Zone 2 borrow areas

contain extremely variable soils with respect to location and

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depth, and will have very low permeability in the embankment.

The friction coefficient, Tan 0, the cohesive strength at

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optimum moisture, Co, and the saturated cohesive strength, Cs,

which we assigned to the Zone 2 materials are given in Table 2.

4. Filter-Drain Zone

Although not specifically described in Table 1, we were

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later informed that the pervious filter-drain material, now

proposed, is to be processed material secured from Borrow Area

A, apparently with materials from Borrow Areas B, C, Land M

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also being available. The processing is to remove the fine

fraction so that less than 5% will pass the 200 mesh screen.

The filter materials were classified by the Bureau (Table 1) as

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silty sand (SM) with 31% gravel and 69% sand, without an amount

of fines indicated. After processing the classification will

be well graded sand-gravel and poorly graded sands (GW-SW-SP)

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with an approximate dry density of 127 pcf at 70% relative

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