FLOOD10941

• County: Jefferson
• County: Douglas
• Stream Name: Chatfield Reservoir
• Basin: South Platte
• Title: Chatfield State Park Recreational Mitigation Study Roadway Feasibility Analysis Correspondence
• Date: 1/6/2004
• Floodplain - Doc Type: Correspondence

alluvial sands, gravels and cobbles (the segments of the roadway located on the flood
plain) or be built on existing embankment that bears on the alluvium (existing road
around west end of reservoir). We observed no geologic or geotechnical conditions that
preclude building the realigned road where proposed by the conceptual plan. We noted
the existing roadway has generally given satisfactory performance. The areas of
distress, we believe, are due to age and traffic loading that has exceeded that used for
the design.

The potential for damage from the expansive clays and claystones in the
subgrade for the stretch of the roadway that will approach the Deer Creek Bridge from
the east can be reduced. The commonly used methods by increased effectiveness are:
a) mix lime or fly ash with the upper approximately one foot of the subgrade; b)
subexcavated of the order of three feet of the subgrade and replace the subexcavated
materials as a compacted fill at a higher moisture content and stabilizing the top lift of
the subexcavated areas with fly ash, as needed; and c) subexcavate of the of the order
of three feet of the expansive subgrade and replace it with densely compacted,
impermeable, non-swelling granular soils. There are additional features that can be
added to the several methods, such as drains, that will increase their effectiveness.

The claystone bedrock can be unstable if cut at steeper slopes and the bedding
or fracturing or jointing is averse to the plane of the cut. We recommend the use of cuts
to build the road be avoided, where possible. The use of compacted fills would be better
and have the added advantage of raising the roadway surface, by design, above the
expansive clays and claystones in the subgrade.

Across the flood plain, we expect there will be stretches along the realigned road
where construction machinery will experience soft ground. Probably some method of
stabilization of the soft ground will be needed to build the new roadway. We recommend
including in budgeting cost estimates dollars for soft ground stabilization.

Raising Existing Embankment. One of the options for the stretch of the road
around the west end of the reservoir is simply using compacted roadway embankment
to raise the elevation of the roadway to above the new normal water level of the
reservoir. We observed no geologic or geotechnical reasons this is not feasible. The
base of the new embankment will be wider than the existing roadway and the slopes will
need to be flatter than usually used for a roadway embankment because they will be
inundated and also subject to “rapid draw down”. We expect the slope angle will need to
be of the order of 3:1 (horizontal:vertical) or 4:1. The slopes will also need to be rip
rapped or otherwise reinforced to resist erosion by wave action.

We understand there is some thought the materials to build the embankment
raise will be excavated from along the side of Chatfield Reservoir by dredging or some
other method. The near ground surface soils that will come from such an operation will
be clays and the deeper soils will be sands, gravels and cobbles. The types of materials
can be used to build the raised embankment. The materials will be

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SEAR-BROWN
CHATFIELD RESERVOIR REALLOCATION STUDY
“ROADWAY ANALYSIS STUDY”

CTL I T JOB NO. FC.2998