Roller compacted concrete, also known as RCC, has been in the spotlight in Georgia's Cherokee County, where it was used to construct a massive RCC dam at the Hickory Log Creek Water Supply reservoir within the city limits of Canton, Ga.
Brown and Caldwell, based in Atlanta, and Schnabel Engineering, based in Alpharetta, Ga., handled overall design of the pump storage augmented project. Schnabel's Randy Bass served as project manager for the dam, assisted by on-site construction manager Chuck Kahler.
The contract for the first phase of the project — which included clearing, excavation, foundation exploration and preparation, and related work to ready the dam construction site for RCC placement — was awarded to Thalle Construction, based in Hillsborough, N.C. Thalle Construction is also the contractor on the current Phase 2, which includes construction of the actual RCC dam, with ASI Constructors serving as the RCC placement subcontractor. The overall project cost of more than $56 million is being shared between the Cobb County-Marietta Water Authority and the city of Canton. The project will help meet water needs for the city of Canton and the authority's service area through 2050.
The Hickory Log Creek dam has a design length of 980 feet and a height of 180 feet. Its width varies from 147.6 feet at the base to 18 feet at the top. The completed structure, which will be the highest non-federally regulated dam in the state of Georgia, will create a 411-acre lake that will provide as much as 44 million gallons of water per day. The lake's water will come from natural inflow from Hickory Log Creek, augmented by a pumped-storage system which will pull water from the nearby Etowah River when conditions permit it.
The dam is designed with a flat, vertical upstream face and a stepped downstream face. Its downstream steps were formed using wood forms, creating steps with a height of 3 feet. On the upstream side, the team used precast stay-in-place concrete form panels — typically measuring 16 feet by 6 feet 6 inches with a thickness of 5 inches — to define the face of the dam. The inside face of each of these panels is lined with an impervious geomembrane. Crews fabricated and cast these panels at an on-site precast yard.
To anchor these upstream forms during RCC placement and compaction, permanent anchors extending back into the RCC were attached to the inside face of each form panel. Additionally, temporary exterior steel stiffbacks were installed to provide additional support. Overall, the design utilizes close to 1,100 of the panels.
Not all formwork was on the outside of the dam. The Hickory Log Creek dam includes an inspection/drainage gallery which has been constructed deep within the dam itself. This gallery has a width of 7 feet and a height of 8 feet. The walls were formed during RCC placement using removable metal forms; gallery ceiling was constructed using precast reinforced concrete roof panels. Access stairways have also been incorporated into the mass structure. The presence of these openings and passages complicated construction somewhat but allows for easy instrumentation, drainage from the foundation drains and interior inspection of the structure.
Overall, the dam's design calls for the use of about 225,000 cubic yards of RCC. ASI Constructors produced the RCC on-site in a Johnson-Ross batch plant, utilizing aggregate from the nearby Lafarge quarry in Ballground, Ga., and cement from Signal Mountain Cement Co. Fly ash was also utilized in the mix design. The six-yard plant typically produced about 350 yards of RCC per hour.
The mix was delivered to the dam via a conveyor system manufactured, installed and maintained by Rotec Industries. An initial conveyor run of about 700 feet carried the mix from the plant up the hill to a transfer station; from there, a second conveyor transported the mix along the top of the dam to a "tripper." The tripper, which could be positioned along the second conveyor's run, diverted the RCC from the distribution conveyor and discharged it onto the working surface at the desired location.
As the RCC was discharged by the tripper, it was spread in 12-inch lifts by a dozer. Initially, during placement of the first lifts of RCC, a small Komatsu D21A dozer was used for maximum maneuverability. Then, as RCC placement continued and placement rates increased, the team switched to a larger Caterpillar D5MXL and then to an even larger Deere 850 dozer outfitted with wings on the blade. Toward the top of the dam, as the working area began to narrow, the Cat D5MXL was again called into service.
As the dam's elevation increased, the columns supporting the second stage of the conveyor system were raised using hydraulic jacks. Typically, the conveyor was raised about every two days. As the support columns were jacked up, they left behind openings in the RCC which provide drainage or ventilation to the gallery.
After being spread by the dozer, the RCC was compacted by a pair of Ingersoll Rand rollers — a DD125 and, for working close to the edge forms, a DD24. Additionally, vibratory plate compactors were used to compact the mix near the edge forms and around various penetrations.
On the downstream side of the dam, the RCC face will be visible. To enhance the appearance of those faces, the construction team utilized so-called grout-enriched RCC to yield a smoother face. This involved placing uncompacted RCC along the form edges, applying a neat cement grout onto the RCC surface, then vibrating the grout into the RCC to enhance a narrow zone of RCC and thus yield a smooth formed face. Komatsu and Deere loaders were used to transport the facing concrete from ready mix trucks to the locations where it was used.
Temperature is a concern during any RCC project, particularly one such as this where such a large volume of RCC is being placed. RCC produces heat as it hydrates and cures, and that heat can cause cracking if it is excessive.
To avoid overheating problems, RCC placement was scheduled to avoid the heat of day. Placement typically began at about 5 p.m. and continued through the night, wrapping up mid-morning before temperatures got too high.
Temperature management was further aided by the fact that most of the aggregate was stockpiled last winter, during cold weather. The core of the aggregate pile remained cool even as the weather warmed — an additional aid in controlling mix temperature. Should ambient temperatures have risen too much, the team had the ability to add liquid nitrogen to the mix to lower the temperature further.
Temperature was also a major factor in determining whether bedding mortar was used between each placement session. Although bedding mortar was used in a narrow zone on the upstream side and at each abutment contact at the beginning of each day's RCC placement, it was only necessary to bed the entire lift if the "degree-hours" (that is, the product of the temperature and time between lifts) exceeded 500 degree hours.
Because of the location and layout of the site, a key factor in maintaining production was ensuring that materials and equipment could be delivered to the dam construction crew where and when they were needed.
To that end, ASI utilized four large cranes on the project — a Kobelco 100-ton crane on one end of the dam, a Liebherr 120-ton crane on the other end, a Manitowoc 4100 on the downstream side, and a Manitowoc 888 on the upstream side — as well as smaller cranes and lifts elsewhere as needed. Having multiple cranes on-site allowed maximum lifting flexibility at any point on the dam.
Besides the RCC work, Phase 2 work at the Hickory Lock Creek project also includes significant conventional concrete work. Thalle Construction's crews handled this portion of the work, which includes construction of the dam's conventional concrete crest — which measures a foot thick and is 700 feet long — as well as construction of the overflow section of the spillway, the spillway training walls and the stilling basin at the bottom of the spillway.
Thalle's work also included additional site work in the abutment areas, handled after RCC placement was complete.
As work on the dam itself continues, subcontractor West Contracting, based in Marble, N.C., has been working to clear the actual reservoir area.
RCC placement was completed in early June. Additional work on the dam will continue over the next few months, with filling of the reservoir beginning this fall.