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Projects - McKinney Lake Dam
An RCC First in North Carolina
The first dam remediation project in North Carolina
to use roller compacted concrete (RCC) for overtopping protection.
 The
first dam remediation project in North Carolina to use roller compacted
concrete (RCC) for overtopping protection, the McKinney Lake Dam
is located on Hitchcock Creek, north of Rockingham, North Carolina.
Situated upstream of the McKinney Lake National Fish Hatchery, the
dam’s primary purpose is to supply water to the facility.
Originally constructed in the late 1930s as
a Works Progress Administration project, the 23-ft-high (7 m) dam
consists of an earthfill embankment with a concrete corewall. The
principal spillway is located in the middle third of the dam and
consists of a concrete gravity overflow section and a rectangular
concrete outlet channel. An unlined emergency spillway was cut into
the left third of the embankment.
When state officials determined that the dam
suffered from safety deficiencies, roller-compacted concrete (RCC)
was chosen as the material of choice for improvements. Plans called
for increasing spillway capacity and raising the top of the dam.
| Remediated Structural Features |
| Maximum height of embankment |
23 feet (7m) |
| Elevation of embankment crest |
284.9 feet (86.8 m) MSL |
| Overall length of dam |
718 feet (219 m) |
| Control crest width of principal spillway |
40 feet (12 m) |
| Control crest elevation of principal spillway |
277.5 feet (84.6 m) MSL |
| Upstream slope of embankment |
2H:1V |
| Downstream slope of embankment |
3H:1V |
| Crest width of emergency spillway |
240 feet (73 m) |
| Crest elevation of emergency spillway |
280.0 feet (85.3 m) MSL |
 The
project’s design concept addressed inadequate spillway capacity
by armoring a section of the embankment with RCC, and raising the
crest of the embankment to contain the 1/3 PMP flood level plus
freeboard. The armored section of the embankment serves as the emergency
spillway.
The RCC mix design was as follows:
| Type I/II portland cement |
450 lb/yd3 (267 kg/m3) |
| Modified NCDOT roadbase aggregate (1-1/2 MSA) |
3,428 lb/yd3 (2034 kg/m3) |
| Water |
262 lb/yd3 (156 kg/m3) |
 The
specified 28-day compressive strength was 3,000 psi (20.7 MPa).
The unusually high cement content was required to compensate for
poor grading of the sand fraction of the NCDOT base course aggregate.
In addition the silt fines were on the high side of the specified
range (3% to 8% by weight) for minus #200 (75 micron) sieve material.
The 1600 yd3 (1223 m3) of RCC armoring was
placed on the upper portion of the upstream slope, the crest, the
downstream slope, and an apron area beyond the downstream slope.
Sheet piling driven along the edges of the RCC serves as spillway
training walls to contain the design discharge. The training walls
converge to direct outflows up to the projected 100-year flood level
through an existing box culvert under the downstream access road.
The end result:< improvements with RCC increased the spillway discharge
capacity to more than 11,000 cfs (311 cms), or about 47 cfs per
foot (4.3 cms/m) of weir length—a substantial improvement
over the dam’s original capacity.
Project Team
URS Corporation, Denver, Colorado designed the
dam improvements and is the engineer of record.
Schnabel Engineering Associates, Greensboro, North
Carolina, was the project manager responsible for owner and dam
safety coordination, as well as construction oversight. Schnabel’s
Charlotte office performed field and laboratory testing.
Atlas Resource Management, Fayetteville, North
Carolina, was the general contractor.
Gears, Inc., Crested Butte, Colorado, was the subcontractor
for RCC mixing and placement.
S&ME, Louisville, Tennessee, performed the
RCC mix design.
Click here for
a printed version of this case study.
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