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RCC Hydro - Part 2

Limestone

Currently, two major milestone U.S. roller compacted concrete (RCC) flood control dam construction projects are nearing completion. In last month's Concrete Products, we focused on the Portugués Dam being constructed by the U.S. Army Corps of Engineers (Antilles) District in Jacksonville, Fla.

In the second of a two-part report, we describe a RCC hydro project by the San Diego County Water Authority (SDCWA), with work nearing completion in Lakeside, Calif., on the San Vicente Dam Raise, which is also a fortification of the dam for seismic soundness. These projects mark the largest RCC hydro projects undertaken in the United States since the Taum Sauk Reservoir in southeastern Missouri was completed in 2009.

IN THE WEST
The SDCWA imports up to 80 percent of the region’s water supply from more than 400 miles away, and its pipelines conveying this imported water to the county cross several major fault lines. Therefore, the Authority is increasing local storage to reduce the region’s reliance on imported water supplies and providing a more flexible conveyance system for use during emergencies. SDCWA’s project to raise San Vicente Dam is part of its water diversification strategy.

Preparation for the dam raise began in 2009, and by the following year, construction began to raise San Vicente Dam to increase water storage. This concrete gravity dam, owned and operated by the City of San Diego, was built in 1943 near Lakeside, an unincorporated community in San Diego County. (See Concrete Products, November 2002 on the Olivenhain Dam and Reservoir project.)

WORLD'S TALLEST RCC DAM RAISE
The San Vicente Dam Raise Project is the tallest dam raise in the United States and tallest RCC dam raise in the world. This project elevates the existing 220-ft.-high dam by 117 ft., and more than doubles reservoir storage capacity—the largest single increase in water storage in the region’s history. When completed later this year, the additional storage will bring the reservoir’s total capacity to 242,000-acre-ft. (An acre-foot is 325,900 gallons—enough to meet the needs of two average families of four for a year.)

The former marina serves both as a quarry and as a temporary RCC production site. Aggregates for the RCC are being mined from the existing marina’s hillsides to limit truck traffic through the community, reduce the carbon footprint of the construction activities, and ensure the aggregate is available when needed for the 24-7 operation. A ¾-mile-long conveyor system transports the RCC to the dam base, eliminating several-thousand truck trips to transport the RCC—a volume equivalent to that of the existing dam.

The San Vicente Dam Raise Project is one of many major SDCWA projects in this region of southern California that improve quality of life. After the dam raise is completed in 2013, refilling the reservoir will take two to five years, depending on imported water availability. Built to remain operational during a major earthquake, the dam will provide San Diego County residents with a secure water source, support the local economy, and offer enhanced recreation opportunities on the expanded reservoir.

SDCWA awarded the San Vicente Dam Raise RCC production and placement portion of the contracts to a joint venture of Shimmick Construction and Obayashi Constructors. Known as SOJV, the construction team developed a concrete production and cooling scheme as approved by the SDCWA owner’s representative firm Parsons Engineering.

METHODOLOGY
The San Vicente Dam Raise project calls for shorter duration of high-output RCC. The advanced output and fast track construction methodology required SOJV to look for a high-performance concrete production and cooling scheme. SOJV partnered with San Antonio-based industrial design-build contractor Plant Architects + Plant Outfitters in developing a retrofit plan for existing Shimmick batching equipment it wanted to utilize for the project. A standard, low-output dry batch plant was customized to meet SDCWA specifications for individual weigh batching and dual twin-shaft mixers. The modified plant would need to exceed output of 400 cu. yd./hour on a sustained basis.

A single plant with two, 7-cu.-yd. specially designed bombay door-type mixers were purchased from OMG-SICOMA by SOJV to allow for the high production from a single plant. Plant Architects designers developed a 10-in. structural steel tubing skid frame to hold the modified plant. The design group then removed the original cumulative batcher and remade it into three individual weigh batchers. The overhead storage bin was tripled in size to accommodate the required production, and finally a 48-in. x 45-degree-deep trough high-speed batch transfer belt was fabricated to transport the batchers through a “tipple chute” into two aggregate holding hoppers above each mixer. Below each mixer is a discharge hopper with a take-away metering belt to feed either an overland conveyor or a transfer conveyor to load dump truck chassis.

Two 1,000-ton bulk powder storage bolt-together silos were supplied by Plant Outfitters with a unique design whereby each silo is fit with a dual outlet and dual high-angle screw augers—each silo feeding an individual set of powder weigh hoppers above each mixer.

COOLING CONCRETE
As the aggregates are processed onsite, the two coarse sizes are washed and each is feeding dual chilled-water wet belts. The wet belts are approximately 150 ft. in length. The sand is air-cooled utilizing a modified rotary kiln and is delivered to the overhead bins by a conveyor system.

Batch water is chilled by centrifuge water-to-water chiller pulling condenser water from the reservoir and recirculating it back to the source. The chilled water storage is held in a 44,000-gal., insulated in-ground water pit and is used to effect the heat exchange on the sand cooler and to provide 37° to 40°F water in addition to supplying the cold water recirculation (through digesters to remove silt buildup) for the wet belts. The coarse aggregate is brought to within 1° of the water temperature prior to being deposited in the overhead plant bins. The sand is brought into the mid fifty-degree range, and combined with the chilled batch water, chilled coarse aggregate and the stabilized large volume bulk powders—thereby providing low enough constituent material temperatures to produce concrete below the required production temperature of 70°F.

Placement of the dam raise RCC is provided by an overland conveyor system from RCC contractor ASI, as leased to SOJV. A telescoping conveyor provides placement, with chute and pipe work at the face, with crane and bucket placement as required for dental and accessory work.

 

Additional nformation on the San Vicente Dam Raise Project can be found at www.sdcwa.org For information on SOJV, visit www.shimmick.com. For further information on the design-build offerings of Plant Architects + Plant Outfitters, or CTC concrete cooling and dry flake ice plants, visit www.robertober.com