Building reserves
Fort Smith triples size of water reservoir, constructs new earthen dam

By Mark Friedman

Pete Temple is dealing with wet conditions above and below ground during a $180 million water supply expansion project in Arkansas.

Temple, the project manager for Granite Construction Co. Inc. of Watsonville, Calif., said his crew is about halfway finished with its portion of the project, which requires the construction of a new earthen dam and water intake tower at Mountainburg, just north of Fort Smith. The new water supply is projected to supply the Fort Smith region, which serves about 145,000 customers, until 2050.

Fort Smith currently uses two reservoirs for its water supply - Lake Shepherd Springs and Lake Fort Smith. Water from Lake Shepherd Springs is released into Lake Fort Smith to maintain Lake Fort Smith's water level.

"And when they don't need water (at Lake Fort Smith), they basically shut the water off," said Timothy Pitman, the project representative for project engineer Burns & McDonnell of Kansas City, Mo.

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When the $98.9 million job is completed in mid-2006, the two lakes, which are about 6,000 ft. apart, will merge into one 84,000-acre reservoir. The contract also requires relocating a state park, county roads and utility lines.

To merge the lakes, Granite must first complete the new dam at Lake Fort Smith, then degrade the dam at Lake Shepherd by removing its crust.

Pittman said a slit will also be cut through the center of the Lake Shepherd dam to allow the water to flow down the valley and stop at the new earthen dam.

Granite is using about 690,000 cu. yds. of clay to create the dam, all of which comes from the hills surrounding the lake. Steve Parke, director of utilities for Fort Smith, said that has helped lower the cost.

"This is an unusually large project for a locally funded municipality," Parke added. Water rate fees, some city sales taxes and bonds sold by Fort Smith will pay for the work.

One of the biggest challenges has been constructing the new dam and intake tower while working along an active reservoir, Pitman said.

"We use a lot of silk curtains and hay bails, and we structure our activities so that we have protection from any runoff that's going to flow into the reservoir," he added. Silk curtains are plastic sheets with micro fibers that allow water to pass while filtering out the silt.

Granite also had to excavate the side of a hill to make room for a new 225-ft.-tall, 65-ft. wide concrete intake tower. The tower will control the outflow of water to Fort Smith.

"We excavated a 60-ft.-deep shaft for the tower and supported the hillside with 20-ft.-long bolts," Pitman said. Granite used explosives and a drill to carve out the shaft, then placed steel rebar and 4,000-psi concrete.

Working underground resulted in water seepage problems.

"There was a lot of ground water leaking in because it's actually below the lake elevation," he said. "And we were doing it in winter, so there was a lot of frozen water on the side of the shaft, which also made it difficult."

To solve the problem, Granite workers channeled the water to sumps through gutters and pumped the water out.

With the underground portion completed, Granite began slip-forming the concrete tower.

"Normally, with conventional forms you set a form up and you put the rebar in and place the concrete," Pitman said. "The slip form continually moves and it slips up the side of the tower, and you just keep placing rebar and concrete in the form."

After completion of the intake tower, water was diverted through it so the contractor could work on the dam 500 ft. upstream.

To build the new 196-ft.-tall, 30-ft.-long dam, which is 101 ft. higher than the current dam, Granite had to excavate the dam footprint down to sandstone and shell, Temple said.

"The excavation was not a problem," he added. "The problems came when we hit the rock. Some of the rock is good and some of it is unsound."

Granite had to either remove the unsound rock or inject grout into the rock for reinforcement.

The new dam is made of a clay core, followed by fine and course filters and sandstone. Since the intake tower is nearly finished, Granite is re-using the same equipment on the dam, Temple said.

"We're able to increase production by increasing the amount of equipment at the dam site," he added. "We had one truck spread going at first and now we have as many as three, so we're able to go a lot faster."

Temple said the dam is about 40 percent complete.