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Aesthetics Play Key Role in Dam Rehabilitation
Gregg W. Hudock, P.E. Golder Associates Inc.
A rapidly growing metropolitan county depends on RCC to rehabilitate
an aging dam and restore the
focal point of a prominent park.
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| Aerial view of the Y-17 dam and downstream
reach prior to rehabilitation. |
The Yellow River Watershed Dam No. 17 (Y-17) is located approximately
30 miles (48 kilometers) northeast of Atlanta, Georgia. The dam
is part of a 92 acre (37.2 hectares) facility owned and operated
by the Gwinnett County Department of Parks and Recreation.
For more than a decade Gwinnett County has been one of the fastest
growing counties in metropolitan Atlanta. As a result of this rapid
development, the Y-17 structure which was originally constructed
to protect rural pasture and farmland now protects recently constructed
subdivisions, apartment complexes, office parks, retail businesses,
and a high school located downstream of the dam. This development
caused the classification of the dam to change from low hazard to
high hazard.
The Y-17 structure consists of an earthen embankment dam approximately
30 ft (9 m) high and 900 ft (274 m) long. Studies of the Y-17 structure
determined that the existing 200 feet (61 m) wide vegetated earthen
spillway was inadequate for the design flows. An options assessment
determined that the most cost effective solution was to armor the
entire downstream slope and abutments of the dam with roller-compacted
concrete (RCC).
The RCC forms a spillway approximately 550 ft (152 m) wide that
overtops the dam and allows the existing earthen spillway to be
abandoned. The spillway is asymmetric and designed to correspond
with the varying topography along the downstream abutments. The
crest has three stages to control flow over the spillway and to
provide flood protection during more frequent storm events, with
the lowest stage crest elevation corresponding to the crest of the
existing earthen auxiliary spillway. The crest was designed to maintain
peak stage during the 1/3 PMP design storm. To prevent dam overtopping
during the PMP design storm, the spillway sidewalls extend an additional
4 feet (1.2 m) above the original dam crest and an earthen berm
was constructed up to the PMP design storm peak stage in the existing
auxiliary spillway.
Construction began in October 2004 and was completed by early April
2005. The total project cost was $2.1 million. Approximately 65%
of the funding for the project was provided by the National Resource
Conservation Service (NRCS) Watershed Rehabilitation Program since
the structure was originally constructed by them. ASI RCC, Inc.
was awarded the job at a cost of $1.46 million with an in-place
RCC cost of $110 per yd3 ($143.87 per m3) for approximately 6,700
yd3 (5,123 m3) of RCC.
Early in construction, thick deposits of alluvial soils with high
liquid limits and very low shear strength were identified along the
downstream toe of the existing embankment. This discovery necessitated
the removal of more than 8,000 yd3 (6,116 m3) of additional soil.
A re-design of the RCC spillway stilling basin was carried out after
removal of the unsuitable material to correspond with the newly excavated
site topography. The re-design resulted in an additional 900 yd3 (688
m3) of RCC. RCC placement began in mid December 2004 and was completed
in early February 2005.
The RCC design compressive strength was 2,250 psi (15.5 MPa) at
28-day. The average compressive strengths of RCC cylinders molded
per ASTM C 1435 were 2,300 psi (15.9 MPa), 3,400 psi (23.4 MPa),
and 3,550 psi (24.5 MPa) at 28, 60, and 90 days, respectively. The
moisture content during placement was controlled by the Vebe consistency
time which was specified at 15 to 30 seconds.
The completed RCC spillway safely routes approximately 18,950 cfs
(537 m3/sec.) through the Y-17 watershed during the PMP design storm.
Since the structure is located in a prominent park, the downstream
slopes of the RCC spillway were covered with earthfill and vegetated
to hide the RCC. The crest and crest sidewalls are still exposed.
The spillway activates for storms at or less than a 25 year frequency.
As a result, earthfill covering the RCC may erode during these storms
requiring some future maintenance.
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Completion of the RCC stilling basin and construction
of the RCC spillway steps and sidewalls. |
Placement of earthfill over the
completed RCC spillway. |
Completed project with grass
covered downstream slope. |
Vital Statistics
Remediation Features
Maximum height of Embankment . . . . . . . . . 30 feet (9.1 m)
Overall length of dam . . . . . . . . . . . . . . . . . 900 feet
(273 m)
Width of RCC spillway crest . . . . . . . . . . . . . 524 feet (160
m)
Height of RCC spillway steps . . . . . . . . . . . . 12 inches (30.5
m)
Width of RCC steps . . . . . . . . . . . . . . . . . . . 9 feet
(2.74 m)
Slope of chute steps . . . . . . . . . . . . . . . . . . .3H:1V
Slope of training wall steps . . . . . . . . . . . . . .3H:1V
RCC Mix Design
Type I/II portland cement . . . . . . . . . . . . . . 250 lbs/yd3
(148 kg/m3)
Class F fly ash . . . . . . . . . . . . . . . . . . . . . . .80
lb/yd3 (47.5 kg/m3)
Graded road base aggregate
1-1/2”(37.5 mm) MSA. . . . . . . . . . . . . . . . . .3,488
lbs/yd3 (2,069 kg/m3)
Moisture content . . . . . . . . . . . . . . . . . . . . . 6.5%
Specified comp. strength @ 28 days. . . . . . . .2,250 psi (15.5
MPa)
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Credits
Owner/Construction Manager:
Gwinnett County, Georgia
Federal Sponsor:
Natural Resources Conservation Service
Designer:
U.S. Army Corps of Engineers, Savannah District
Resident Engineer/QA Testing:
Golder Associates Inc., Atlanta, Georgia
RCC mix design:
Schnabel Engineering Associates, Inc.,
Atlanta, Georgia
Contractor:
ASI RCC, Inc., Buena Vista, Colorado |