Jun. 30--LESTERVILLE, Mo. -- Atop Proffit Mountain is a symphony of tractor-trailers and giant earth-moving machines, as work crews erect 100-foot-tall concrete walls that will form the sides of a mammoth pool used to feed AmerenUE's Taum Sauk hydroelectric plant.

The 440-megawatt plant, about two hours southwest of St. Louis, has been out of service since Dec. 14, 2005, when a 700-foot section of the old reservoir wall collapsed, releasing more than a billion gallons of water that scoured Johnson Shut-Ins State Park.

The jagged path left from water rushing down the side of the mountain remains a visible scar of that day. Now, St. Louis-based AmerenUE is hoping to turn one of its biggest failures into an engineering marvel.

The rebuilding project will cost $480 million and is expected to be complete by May, allowing AmerenUE to restart the Taum Sauk plant.

More than 600 workers are part of two eight-hour shifts working six days a week to complete the project on time. Crews have already poured more than 2.1 million cubic yards of concrete -- about 10 times what's been used so far in the rebuilding of Highway 40 (Interstate 64).

The new, kidney-shaped reservoir, almost a mile in circumference, is made mostly of roller-compacted concrete, a material that is less prone to cracking and is now the standard construction material used in dams and spillways, said Thomas Hollenkamp, AmerenUE's chief dam safety engineer.

The old reservoir was built mostly of uncompacted rock topped by a 10-foot-high concrete wall. For the new reservoir, contractors dug as much as 60 feet in search of solid bedrock.

Taum Sauk is an important piece of the utility's power generation portfolio. The power plant is unique because it can generate electricity within minutes compared with the hours it can take to start a coal-fired power station. Not having it available for the last three years has cost the utility millions of dollars in lost profit.

"This is the best way we have to store electricity in bulk quantities," said Mark Birk, AmerenUE's vice president of power operations.

A 2006 investigation by federal regulators showed the failure of the upper reservoir was triggered when water flowed over the top and eroded the rock-fill dam. Water-level sensors that were supposed to prevent the reservoir from being overfilled had been moved too high to be effective.

The new design incorporates redundant water-level sensors, continuous video monitoring and a spillway in case the reservoir is overfilled. "We never intend to use that, but it's there as a last resort," said Craig Geisman, managing supervisor of hydro engineering for AmerenUE.

In 2006, AmerenUE paid $115 million in penalties under a settlement with the Federal Energy Regulatory Commission. It also paid the state about $180 million to settle a lawsuit by then-Attorney General Jay Nixon. That money went to help clean up Johnson's Shut-Ins and to extend Katy Trail State Park.

Taum Sauk originally opened in 1963. It works like this: Water from the reservoir atop Proffit Mountain flows through a mile-long tunnel to spin turbines that generate electricity. At night, when electricity is cheaper, the turbines are reversed, and water is pumped up the mountain to refill the upper reservoir.

Almost all of the cost of rebuilding the upper reservoir will be covered by insurance, AmerenUE officials have said. The state Public Service Commission has barred the utility from recovering any costs from customers.

The Taum Sauk plant's 50-year operating license expires on July 1, 2010. The company has applied to renew the license and hopes federal officials will grant a 50-year extension, Birk said.

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