FLOOD10481

• County: Douglas
• Basin: South Platte
• Title: Chatfield Reallocation Study - Roadway Analysis Study
• Date: 5/11/2004
• Prepared For: CWCB
• Prepared By: Sear Brown
• Floodplain - Doc Type: Floodplain Report/Masterplan

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most of them will be removed from under water. Time will need to be
construction schedule to dry the materials to a moisture content
compacting into the embankment.

wet because
provided in a
acceptable for

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BridQes, Three bridge sites were looked at. One of the sites is across Deer
Creek and the other two are across the South Platte River. One of the sites across the
South Platte River is the site of an existing bridge, We observed no geologic or
geotechnical reasons the construction or reconstructions of bridges at these sites are
not feasible,

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The ground under the abutments of the Deer Creek site is different. Our
observations suggest the foundation strata for the east or left (looking downstream)
abutment of the Deer Creek Bridge is expansive clay overlying expansive claystone.
Our observations suggest the foundation strata for the right abutment is several feet of
clays overlying sands, gravels and cobbles. Our experience in the area shows the
sands, gravels and cobbles are underlain by claystone at varying depths, We expect
ground water at about the elevation of Deer Creek will be found under the right
abutment and little or no ground water will be found in the clays or claystones under the
left abutment.

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We believe straight-shaft piers penetrating the claystone will be the better
foundation for the left bridge abutment due to the expansive clays and claystones,
Footings bearing on the sands, gravels and cobbles, straight-shaft piers penetrating the
claystone if it is not too deep or friction piling driven into the sands, gravels and cobbles
or end-bearing piling driven to "practical refusal" in the claystone are alternatives for the
right abutment. However, the larger cobbles and perhaps boulders in the sand, gravel
and cobble stratum may interfere with drilling pier holes or driving piling, Pier holes will
need to be cased due to the ground water,

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The existing bridge across the South Platte River appears to have performed
satisfactorily, from the foundation point-of-view, We do not know how the existing bridge
is founded, but suggest this be learned, Because the existing bridge foundation appears
to have performed acceptably, we believe it is reasonable to assume at this stage in the
project planning that the abutments of either of the proposed new bridges across the
South Platte River can be founded with the same type foundation used to found the
existing bridge. The alternatives discussed above for the right abutment of the Deer
Creek Bridge are also alternatives for the abutments of the new bridge across the South
Platte River.

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We believe that at a time not too distant in the future, if the Reallocation Study
progress to design, borings will need to be drilled along the new roadway alignment and
at the selected bridge locations, some laboratory tests will be needed and geotechnical
engineering will need to be done to confirm our observations, opinions and conclusions
discussed above. We would be pleased to provide these services to the project team at
that time,

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SEAR.BROWN
CHATFIELD RESERVOIR REALLOCATION STUDY
'ROADWA Y ANALYSIS STUDY.
CTL I T JOB NO. FC.2998

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