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RCC Dam Saves City’s Sole Source of Water Supply
Stephen L. Whiteside, P.E., CDM, and Fares Y. Abdo, P.E., PCA
Franklin Dam is located in a rural area along the West Fork Drakes
Creek in Franklin, Kentucky. The dam impounds water from a watershed
encompassing approximately 108 square miles (280 km2). Its primary
purpose is to serve as the City's sole municipal water supply.
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| Aerial view of original dam prior to replacement. |
The original 16-foot (4.2-m) high dam was built in 1975 to impound the
immediate creek area. In 1987 the dam was raised 6 feet (1.8 m) to
increase storage capacity. The modified dam consisted of an 80-foot
(24.4-m) long concrete gravity spillway section adjacent to the left
abutment and a 500-foot (152.4-m) long embankment extending from the
spillway to the right abutment. The embankment consisted of a tiered
gabion wall along the downstream section and dredged material placed
on a 3H:1V slope on the upstream section. A filter fabric separated
the fill from the gabion wall. A membrane on the upstream slope served
as a seepage barrier. The top portion of the upstream slope, the dam
crest, and the area within about 18 feet (5.5 m) from the downstream
toe of the dam were protected against normal overtopping by a 9-inch
(230-mm) thick gabion mat with a thin concrete overlay. Even after
the raising of the dam, it was still overtopped several times a year.
In 1996, a large flood event overtopped the dam and caused severe
damage and voids in the dam. The dam was repaired by installing
sand bags within the embankment portion. However, in spite of the
repairs extensive seepage took place through and beneath the dam.
Sinkholes developed at several locations and required additional
repairs.
Replacement Dam
Due to the progressive deterioration of the dam, the City engaged
the team of CDM of Louisville, Kentucky, and Raleigh, North Carolina,
and Barge Waggoner Sumner and Cannon (BWSC) of Nashville, Tennessee,
to evaluate the feasibility of either repairing the existing dam
or replacing it with a new dam. Rehabilitation of the existing dam
was ruled out because of its poor condition, and it was decided
to replace the existing dam with a new dam immediately downstream.
The design team conducted a geotechnical investigation to evaluate
the subsurface conditions at potential locations for a new dam.
Based on the results of the investigation, they concluded that the
new dam should be located about 100 feet (30.5 m) downstream of
the existing dam, and that the normal pool should be kept at the
same elevation. By maintaining the normal pool at the same elevation
as the existing dam, upstream environmental impacts would be minimal.
It would also reduce the need for land acquisitions and make permitting
efforts less cumbersome. In addition, it would avoid karst features
located further downstream of the dam.
The team evaluated a range of alternatives—conventional concrete
dam, embankment dam with overtopping protection, and roller-compacted
concrete (RCC) dam. Based on the results of the geotechnical investigation
and hydrologic/hydraulic analyses, an RCC dam with a conventional
concrete spillway was selected as the preferred alternative.
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| A general view during construction. |
The RCC dam was designed to withstand overtopping flows during the
Probable Maximum Flood, with flood depths up to 8 feet (2.4 m) above
the top of the dam. The new dam is 450-feet (137.2-m) long and has
a maximum height of 20 feet (6.1 m). It has a vertical upstream face.
The upper portion of the downstream face is sloped at 0.8H:1V. Below
the sloped portion, the dam is shaped to form a basin sloped from
the right abutment to the spillway. See photo of completed dam. This
basin routes water during overtopping events toward the spillway and
the creek main channel.
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| Spreading mortar to control seepage
and bond RCC lifts downstream from facing concrete. |
The design included conventional concrete facing on the upstream
and downstream faces. A reinforced-concrete armor cap along the
crest of the dam and on the downstream portion was also included
to reduce the potential for damage from large trees and other debris
during overtopping events. The dam is founded on bedrock about 10
to 12 feet (3.0 to 3.7 m) below existing ground surface. An RCC
apron and 6-foot (1.8-m) deep cutoff wall are located immediately
downstream of the dam to reduce the potential for erosion during
overtopping events. Seepage control was provided by the use of the
conventional concrete facing systems, crack inducers with water
stops spaced at approximately 15 feet (4.5 m) on center, and bedding
mortar between RCC lifts for an approximate width of 10 feet (3.0
m) immediately behind the upstream concrete facing.
The spillway is an 80-foot (24.4-m) long ogee spillway constructed
with conventional concrete. It is keyed 5 feet (1.5 m) into bedrock
for stability and seepage cutoff. The spillway has a 40-foot (12.2-m)
long stilling basin.
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| Completed dam. |
ASI-RCC, Inc. of Buena Vista, Colorado, was the low bidder for
the project. Construction started in late January 2005 and was completed
in February 2006. The project required approximately 8,160 yd3
(6,240 m3) of RCC and 5,560 yd3 (4,250 m3)
of conventional concrete. The in-place cost of RCC (including cost
of aggregate, cement, fly ash, mixing, transporting, placing, and
curing) was $135/yd3 ($176.56/m3). The total
construction cost was about $4,776,000, approximately $200,000 below
the total bid amount.
RCC Mix Design |
| Type II portland cement |
205 lb/yd3 (122 kg/m3) |
| Class F fly ash |
105 lb/yd3 (62 kg/m3) |
Aggregate
(1-1/2 in. (37.5 mm) MSA) |
3,628 lb/yd3 (2,152 kg/m3) |
| Water |
202 lb/yd3 (120 kg/m3) |
| Specified 28-day compressive strength |
3,000 psi (20.7 MPa) |
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Credits
Owner: City of Franklin, Ky.
Designer: CDM and Barge Waggoner Sumner and Cannon
RCC Mix Design and QA Testing: CDM
Contractor: ASI-RCC, Inc.
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