Eighty-four feet below the top of Monolith 1, the basin of the new Upper Taum Sauk Reservoir resembles an ant colony, with workers busy on several facets of construction at once.
Massive dump trucks haul crushed rock across the basin, where it is loaded onto conveyor belts and used to make special concrete. Trailer trucks rumble up Proffit Mountain and into the 55-acre basin, bringing supplies for the construction of the new reservoir that will allow the pumped storage facility to once again make electricity. A tanker drives round and round, spraying water to keep down the dust.
The 700 or so employees at the site are working to ensure the new reservoir will never repeat the Dec. 14, 2005, dam failure that sent more than 1 billion gallons of water crashing down Proffit Mountain. Ameren officials believe the design, type of materials being used and constant supervision are among the reasons they will meet that goal.
“We can 100 percent guarantee there will not be another breach,” Craig J. Giesmann, managing supervisor of hydro engineering, told reporters Monday at the site.
Members of the media were taking a tour of the reservoir construction for the first time since December 2007. Approximately three-quarters of the dam wall is completed and AmerenUE expects the $480 million project will be completed and ready to produce electricity by the end of May. The design meets current dam safety regulations and goes beyond, said Karen Foss, Ameren vice president of public relations.
“Engineers and scientists around the world have come to visit,” she said of the construction project. “This is considered a state of the art construction of a pumped storage tank. There is nothing else like it in the world.”
The new reservoir is built on the footprint of the previous dam, which failed at 5:15 a.m. Dec. 14, 2005. A 700-foot-long breach in the northwest corner of the reservoir released more than 1 billion gallons of water down the side of the mountain, through Johnson’s Shut-Ins State Park, and into the East Fork of the Black River during a 25-minute period.
The raging flood ripped up trees and soil from its path, dropped car-sized boulders into the campground and swept the ranger’s house from its foundation. The ranger, his wife, and their three children were carried across the road into a field, where they later were rescued.
The water pulverized the soil into silt, dirt so light that it remained suspended in the water of the lower reservoir instead of dropping to the bottom. Since then, Ameren has worked to restore the damaged state park and clean the silt from the water.
Construction on the new dam began in 2007 with a new design. The old, rock-fill dam had been constructed on soil and weathered rock and was made of rock that came from blasting on the mountain. The outside sloped up to a parapet — a wall that encircled the reservoir.
The new dam is built on bedrock and is made of roller compacted concrete (RCC). The special concrete, which contains fly ash, is drier than traditional concrete and is made on site in three plants. It is laid down in 18 inch layers, each of which is compacted with rollers until it is about 12 inches thick.
Preventing a breach
The dam is being built in sections, called monoliths. The inside of the kidney shaped reservoir wall is smooth, but the outside slope forms large steps. One section will be slightly lower than the rest and will serve as an overflow release structure. Should the water rise too high, the stairs on the release structure will diffuse the energy of any water that overflows. That overflow will be directed down the mountain on uninhabited Ameren land and into the Black River.
Giesmann is confident that will not be needed. Shut off valves with backups will keep the water from rising too close to the top.
“We are using high level and high, high level probes that are record setting in the industry,” he said. “We have continuous video camera and staff monitoring by people onsite and in St. Louis and the Osage plant.”
Plant manager Dave Fitzgerald said he would monitor the dam daily from the gallery, a walkway around the reservoir that runs inside the base of the dam. Inside, high-tech equipment will monitor water leaks, cracks or other occurrences that could become a problem. Data from that equipment, as well as from drain lines that come from the top of the reservoir to the gallery and from the gallery into the ground, will help Chief Dam Engineer Tom Hollenkamp determine whether it will be necessary to shut down production to fix a problem.
Mark Birk, vice president of power operations, said the plant will have a production capacity of 440 megawatts through two pump turbines, which is about one-third of the electricity generated by the Callaway Plant. Startup will take about 10 minutes compared to several hours for coal-generated plants, he added.
Once the reservoir is completed, water will be pumped up the mountain from the Lower Reservoir to the Upper Reservoir during off-peak times for electricity usage, usually from 11 p.m. to 7 a.m. To generate electricity during afternoon peak times, the water will be sent back down the mountain to the lower reservoir.
Paula Barr is a reporter for the Daily Journal and can be reached at 573-431-2010, ext. 172 or at email@example.com