PROJ00143

• Contract/PO #: C153810
• Contractor Name: Wood Lake Mutual Water & Irrigation Company
• Contract Type: Loan
• Water District: 1
• County: Weld
• Bill Number: SB 97-008
• Loan Projects - Doc Type: Feasibility Study

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grade, The silty sands range from dirty sands with little silt and clay to sands that contain
up to 42 % fines, In general, the sands are very loose to loose with SPT blow counts
ranging from I bpf to 7 bpf. The clays are low plastic, wet, and soft with SPT blow
counts ranging from I bpf to 6 bpf. Unconfined compressive strengths of the clays ranged
from 225 pounds per square foot to 1082 pounds per square foot.

(2) Dam Foundation: The dam foundation immediately beneath the embankment fill consist
of severely weathered to weathered sandstone and was encountered in borings BH,2, BH,
3, and BHA, This material has the appearance of dense sand and shows little structure
after sampled with the split spoon. Blow counts ranged from 22 bpf to over 50 bpf.

Based on the Geologic Map of Colorado by Ogden Tweto, the sandstone is most likely
Fox Hills Sandstone which is a sedimentary deposit of the Upper Cretaceous Age,

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(3) Groundwater Data. Groundwater levels were measured in the borings at the time of
drilling and in the piezometer placed in BH,4 and found to be from 6,feet to 8,feet below
the dam crest in all the borings, The reservoir water level at the time of drilling was
approximately 4,5,feet below the dam crest.

Groundwater levels should be expected to fluctuate seasonally and yearly from the
groundwater readings noted on the boring logs. The time of year that the borings were
drilled and the history of precipitation prior to drilling should be known when using the
groundwater readings from the boring logs to extrapolate water levels at other points in
time.

B.I ENGINEERING INVESTIGATION

B.1.1. Dam Embankment

The dam embankment soils are generally low in density from what would be expected in a dam
embankment. The materials are predominantly sands with some silt and clay, The Standard Penetration
values range from I to 7 bpf which would indicate relative densities from 0,2 to 0.4. The permeability
of the sands would be expected to be medium to low with permeabilities ranging from 10"4 to 10.6 cm/sec
due to the amount of fines,

No obvious distress or structural problems with the embankment are visible at this time, The wide cross
section has o~viously enabled this dam to function successfully for many years even though the density of
the soils in the dam are very low.

The phreatic surface in the dam is high based on the levels found during the subsurface investigation. The
phreatic surface was only two to three feet lower than the elevation of the water in the reservoir. In spite
of the high water levels, low density, and sandy soils, little seepage can be observed through the dam
embankment.

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