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Arc-Shaped RCC Spillway Triumphs in Dam Rehab
Gregg W. Hudock, P.E. Golder Associates Inc.
 A
roller-compacted concrete (RCC) spillway proved the most cost-effective
method of bringing an aging flood-control dam into compliance with
modern safety standards.
Constructed in 1967, the Yellow River Watershed Dam No. 14 (Y-14)
resides approximately 30 miles (48 km) northeast of downtown Atlanta
near the city of Lawrenceville in Gwinnett County, Georgia. The
dam was built as part of the Soil Conservation Service Resource
Conservation and Development Program. Originally constructed to
protect pasture and farmland, the dam now protects subdivisions,
apartment complexes, office parks and retail businesses in a rapidly
growing Atlanta suburb. The watershed area is approximately 3.1
square miles (8.0 km2). The original height of the dam was 40 feet
(12.2 m). The structure was constructed with a two-stage principal
spillway riser and pipe outlet and a 50-foot-wide (15.2 m) earthen
auxiliary spillway located east of the dam site.
When officials found that the dam was not in compliance with current
Georgia rules for dam safety, they analyzed options for modernizing
the structure. The most cost-effective solution was construction
of an RCC spillway overtopping the existing dam and abandonment
of the existing earthen spillway. The RCC spillway crest was set
at an elevation of 1.5 feet (0.5 m) below the existing earthen spillway
crest. This elevation is approximately 1.2 in. (30 mm) above the
100-year 24-hour storm, so as to not impact the FEMA mapped floodplain
downstream of the structure.
Golder Associates Inc. designed an arc-shaped chute spillway with
an ogee weir. This design minimized convergence impacts and provided
favorable hydraulic performance with uniform streamlines, preventing
the formation of supercritical waves that could potentially overtop
the chute walls. The arcshaped spillway also minimized excavation,
encroachment on adjacent properties, and tree removal in the downstream
abutments.
The RCC spillway steps were constructed by forming the front edge
of the 1-foot-high (0.3 m), 10-foot-wide (3.0 m) steps. The chute
steps were constructed with a 3H:1V slope; the training wall steps
were constructed with a 2H:1V slope. The spillway’s arc shape
was formed using 10-foot-long (3.0 m) connected concrete forms.
Bedding mix was required on the first four lifts, last two lifts,
and on every cold joint having a joint maturity greater than 2000
°F-hours.
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Y-14 Structure during the early stages of
construction
(clearing and grubbing in process) |
Placement of the first lift of RCC in the
stilling basin area
at the toe of the existing dam |
Construction of spillway chute steps and stilling
basin
training walls |
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| Close-up view of RCC steps |
The contractor completed RCC placement in five weeks at a cost of
$1.33 million. The average price of constructed RCC (including cement
and pozzolan) was $142/yd3 ($186/m3). Approximately 4,650 cubic yards
of RCC were placed. The new RCC spillway safely routes
the full Probable Maximum Precipitation (PMP) storm through the
watershed, with a peak discharge capacity of approximately 16,000
ft3/sec (453 m3/sec) and maintains the peak storm stage below the
original dam crest elevation. Sixty-five percent of the total project
costs were funded by the Natural Resources Conservation Service
(NRCS) Watershed Rehabilitation Program.
Vital Statistics
Remediation Features
Maximum height of Embankment . . . . . . . . . 40 feet (12.2 m)
Overall length of dam . . . . . . . . . . . . . . . . . . 280 feet
(85.3 m)
Width of arched RCC spillway at crest . . . . . 190 feet (57.9 m)
Width of arched RCC spillway at downstream end . 82 feet (25 m)
Height of RCC spillway steps . . . . . . . . . . 12 inches (305
mm)
Width of RCC spillway steps . . . . . . . . . . . . . . . . 10 feet
(3 m)
Slope of chute steps . . . . . . . . . . . . . . . . . . . . . .
. . . . . 3H:1V
Slope of training wall steps . . . . . . . . . . . . . . . . . .
. . . . 2H:1V
RCC Mix Design
Type I portland cement . . . . . . . . . . . . . 250 lb/yd3 (148
kg/m3)
Class C fly ash . . . . . . . . . . . . . . . . . . . 250 lb/yd3
(148 kg/m3)
Graded road base aggregate (1-1/2”(37.5 mm) MSA) 3,273 lb/yd3
(1945 kg/m3)
Moisture content . . . . . . . . . . . . . . . . . . . . . . . .
. . 6.7 percent
Specified Compressive strength at 28 days 3000 psi (20.7 MPa)
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Credits
Owner/Construction Manager:
Gwinnett County, Georgia
Federal Sponsor:
Natural Resources Conservation Service
Designer/Engineer/QA Testing:
Golder Associates Inc., Atlanta, Georgia
RCC mix design:
Schnabel Engineering Associates, Inc.,
Atlanta, Georgia
Contractor:
Thalle Construction Company, Inc.
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