...temperatures go up, reach a peak point and then start coming back down.” The resulting thermal stress produces cracks.

In the 1980s, engineers tried reducing the cement and water in the mix to reduce the thermal stress. The resulting mix could be placed in a mass by dump trucks, spread by bulldozers, then compacted with smooth-drum vibratory rollers. Once they figured out the process, it proved to be an efficient way to build a gravity dam. “They found out that 100,000 cu yd of RCC” could substitute for “1 million cu yd of dirt,” says Schermerhorn. If there is “a good, solid foundation on rock, RCC is a lot more economical to build,” he adds. RCC was the only material considered for the several design alternatives for Taum Sauk, Rizzo says. “It was the most cost-effective and expedient way to rebuild the reservoir,” he adds.

Two RCC batch plants inside the bowl and one outside produced up to 14,000 cu yd of RCC per day, depending on weather and the number of shifts that the weather allowed, says Schermerhorn. Each plant had a pair of mixers. One was top-loaded with aggregate, cement and water, while the other dumped 7.7 cu yd of RCC onto a belt going to the placement site. The cycles alternated at 30-second intervals, producing a constant flow of RCC. Each plant inside the bowl supplied RCC to one of the dam’s nine monoliths. A rock-crushing plant also was located in the bowl. The third RCC plant, outside the bowl, fed another site, usually in the foundation, says Schermerhorn. Its production was delivered by dump truck, telebelt or mobile telescopic conveyor, rather than the belt system used inside the bowl.

Bump Up Production

A fourth roller-compacted-concrete batch plant was added in late summer 2009 to bump up production as pressure grew to complete RCC placement before winter. A structural concrete batch plant also was located outside the bowl to produce concrete for the upstream and downstream faces of the structure.

One tragedy marred the otherwise successful project. The project’s safety record was good for the first 10 months of the work, and in August 2008 the team celebrated the first million work-hours without a lost-time accident. But one accident broke the streak. Nine months later, in May 2009, Connie Munton, a laborer, was struck and killed by a dump truck working on a night time RCC placement.

The U.S. Occupational Safety and Health Agency investigated but closed the investigation in October without issuing a citation. “We found no basis for a citation being issued to Ozark Constructors,” says Bill McDonald, OSHA’s St. Louis area director. As the project comes to a close, more than 3 million work-hours have been logged.

The old reservoir’s failure manifestly had devastating consequences, and Rizzo Associates designed the new one to prevent the possibility of overtopping again. But another known threat is seismicity. The New Madrid Seismic Zone, which experienced a series of quakes up to 8.3 on the moment magnitude scale in 1812, is just 74 miles away. The new reservoir should be able to withstand a design-basis ground motion of 7.7 from that source, says Rizzo. As far as humanly possible, the new Taum Sauk Upper Reservoir is designed and built to justify AmerenUE President and CEO Thomas Voss’ 2007 statement, “After much analysis, we are now confident that this plant can be returned to service and operated safely to restore a critical source of reliable power to our customers.”