Water works

Pump station fights seepage from Alabama lake

By Sam Barnes

Digging 50 ft. into the ground just a short distance from an Alabama lake has brought inevitable water problems, but nothing the contractor can't handle.

B. L. Harbert International LLC of Birmingham, Ala., is operating pumps around the clock to keep water out of the 75- by 50-ft. cofferdam created for the pump station. The station will transport the water 2 mi from Lay Lake to the new Shelby County South Water Treatment Plant near Wilsonville, Ala.

The $42 million treatment plant and pump station project broke ground in February 2006 and will be completed in February 2008.

"The pump station sits about 25 ft off the edge of the lake," says Harbert project manager Tim Anderton. "There were water concerns because the pump station is being built mostly below ground. There's only one level above ground … so we are well below the water line of the lake. This is a big hole near a lake so that's where the water wants to go."

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Harbert hired Morris-Shea Bridge Co. Inc. of Irondale, Ala., to create a cofferrdam consisting of drilled sheet piles around the perimeter of the pump station site, then excavated the hole and began the continuous dewatering of the site with pumps.

"It's just part of the process when you're dealing with water," says Harbert superintendent Charles Sammons.

The Harbert crew is drilling down about 10 ft. to anchor the pump station structure to existing subsurface rock, then is placing 4,000 psi concrete with a pump truck and crane and bucket to create a 3-ft-thick slab and 2- to 2-ft. 6-in. walls. Couch Ready Mix USA LLC of Dothan, Ala., is supplying the concrete.

"They're batching the concrete, then delivering it an average of 15 mi. to the site," Sammons says. Large-diameter reinforcing steel is going into the slabs and walls, some pieces as large as No. 10s.

"Most pump stations are real beefed up like that," Anderton adds.

Exiting the pump station will be 42-in-diameter ductile iron pipe buried an average depth of 4 ft 6 in, which will transport the water to the new treatment plant.

"The treatment plant will accommodate population growth in Shelby County," Anderton says. Municipal Consultants of Hoover, Ala., is providing engineering for the project.

The new treatment plant is being built on 52 acres of pastureland about 2 mi. from Wilsonville. Only minimal clearing and site preparation was necessary because the site is located on rolling pastureland.

"There was no new fill brought into the site," Sammons says. "Basically, we just redistributed the dirt that we had. There was one area where we had to build out the lagoons."

Once entering the plant, the lake water will go through a pre-aeration basin, two flocculators, a rapid mix area, setting basin, filter area, then finally to a distribution pump station. From there, the treated water will be transported to the county's main water lines through distribution lines.

The contract to install the distribution lines has not been awarded.

"We've got a lot of different types of foundations, but most of the structures rest on slabs and footings," Sammons says. The subsurface rock provides good foundation, in lieu of drilled shafts or piles.

The settling basin has the largest slab in the plant, measuring 2 ft thick on the outside perimeter of the slab, with 1-ft 4-in. walls.

Sammons says that the Harbert crew and subcontractors has already peaked at about 140, a number it will reach multiple times during the project.