...leveled for laydown and to fit offices, warehouses, mechanic shops, general storage, two RCC plants, a conventional concrete batch plant, parking for 600 cars and a 750,000-gallon reservoir for production water on the mountain outside the bowl.
All the aggregate for the RCC as well as the conventional concrete was produced by crushing the rocks from the old dam. Because it was an earth-and-rockfill dam, some of the material had a parasitic coating of loose dirt. “It’s not uncommon for dirtier material to be used in RCC,” says Lee Schermerhorn, Ozark construction manager representing ASI Constructors in the joint venture. The new dam required 3 million cu yd of crushed rock, and by the end of the project they were scraping the bottom of the barrel and telling people to shake out their boots and pockets before going home, he jokes. “We used every bit of it,” Rizzo agrees. “We had very little waste piled at the end.”
Fred Weber Inc. began setting up a crushing plant in March 2007, and production crushing began in September. Ozark Constructors began placing roller-compacted concrete under a $283-million cost-plus-fixed-fee contract with AmerenUE in late October.
The new dam uses 2.8 million cu yd of RCC and 300,000 cu yd of conventional facing concrete. “It’s the largest RCC dam in North America,” says Carl Rizzo. It is 1.26 miles long, 120 ft tall and 150 ft wide at the base. The reservoir’s storage volume is the same as the original’s. RCC has been used in much larger dams elsewhere. China’s Long Tan Dam is the world’s largest RCC dam, according to Malcolm Dunstan & Associates, a U.K.-based engineer that specializes in design of RCC structures. Long Tan stands 710 ft tall and contains 6.474 million cu yd of RCC. Completion was scheduled in 2009.
The biggest challenge at Taum Sauk was not the dam’s size; it was sequencing the work because of the dam’s configuration, says Rizzo. For a conventional dam, the RCC system is set up to deliver the concrete to a fixed point, with conveyor extensions added or removed as the leading edge moves. But the Taum Sauk Dam consists of nine monoliths with no abutments. “The excavation drove the placement of RCC,” says Rizzo. The work was done in all directions around the RCC plants, which were set up inside the bowl. The frequent change of delivery points meant “you couldn’t do it continuously,” he says. Instead of stationary conveyors, the contractor used mobile telescoping conveyors to deliver concrete.
The foundations also presented difficulties. Clay seams run through the site with geologic anomalies, says Rizzo. “When this dam was built in 1964, it was dumped rockfill,” he says. The builder shaved off the mountaintop and placed the excavated material in a kidney-shaped configuration to form the reservoir. “They didn’t consider the foundations at all.”
Redesigning the reservoir after its catastrophic failure, Rizzo Associates took extra precautions. “When we did the subsurface investigation we realized that we had some clay seams throughout the foundation,” including a clay seam “we called a geologic anomaly—a shear zone that ran directly across the reservoir from east to west.” The RCC could not be placed on the clay because it couldn’t support the structure so it had to be removed. “We had to take it down some 40-50 ft below the designed foundation elevation.”
The discovery threw a serious kink in the schedule. “When we found the geologic anomalies, it was like hitting a wall at 90 mph,” says Roger Gagliano, Ozark construction manager representing Fred Weber Inc. in the joint venture. “We pushed the labor to beat the schedule.” Labor availability also was a problem early in the project. Holcim (U.S.) Inc. broke ground for a 4-million-tonne-per-year cement plant in nearby Ste. Genevieve County, Mo., in 2006, and work was still in full swing in summer 2008. “It was a struggle to get enough people because the Holcim plant was in construction,” says Mark Denton, Ozark project manager.
Some excavations went as deep as 60 ft, while others were superficial, but all had to be brought back up with “leveling” concrete before RCC could be placed. The extra excavation cost the project 70 to 80 days, Rizzo says. Even with this delay, the project’s cost and schedule have not slipped much. Weather has been the main cause of delay. “It’s very unusual to place RCC in the wintertime,” he adds.
“RCC placements are very temperature-sensitive,” agrees Ozark’s Schermerhorn. “If it was too hot, we couldn’t place as much because we had certain parameters we had to stick within, and it was the same with rain or cold.” Ozark operated two 10-hour shifts six days per week in spring and fall, cutting back to two 8-hour shifts in summer. It did little RCC placement at all from early December to early April. When the last RCC was placed in November 2009, placement had occurred in only 18 of the project’s 24 months.
Roller-compacted concrete is a stiff, zero-slump concrete mixture with the consistency of damp gravel, according to the Portland Cement Association, Skokie, Ill. PCA says the durable paving material was developed in Canada to carry heavy loads, such as loaded logging trucks in remote areas. The U.S. Army Corps of Engineers first used RCC for a gravity dam in 1982 at Willow Creek Dam in Oregon. Since then it has been increasingly used for dam construction.
“[Conventional] concrete is really not a good thing to build a dam out of,” says Schermerhorn. “[RCC] is a low-heat mix design. It won’t crack as much. If you built the whole dam out of conventional concrete, it would crack because the heat buildup would be too much.”
Dam builders using conventional concrete have had to work around the drawbacks of cement’s chemistry. Schermerhorn points out, “You build up high heat in the core of mass concrete, so it expands. As it cures, the...