The Lake Murray Backup Dam

$275 million dam should be complete by year's end

By Debra Wood

As it has for nearly 75 years, the Lake Murray Dam will continue to serve the people of South Carolina, diverting water from the Saluda River for energy production and creating a beautiful recreational lake - even after completion of a new $275 million backup dam.

"The existing dam will remain as it is and be the primary dam," said Sam Stockman, project manager with South Carolina Electric & Gas of Columbia, S.C. "The new dam will act strictly as a backup."

At one time, Lake Murray Dam was the largest earthen dam in the world built for power production. Federal Energy Regulatory Commission earthquake safety criteria forced SCE&G to construct a backup dam with the ability to withstand a major earthquake, after learning the current dam was vulnerable to widescale embankment liquefaction.

"It was determined the existing dam had problems if we had a seismic event, like a repeat of the Charleston earthquake of 1886," Stockman said. "This is the reason we are retrofitting on the downstream side and building another dam."

That seismic event caused extensive property damage and killed about 60 people. Although this earthquake occurred nearly 50 years before Charles F. Richter developed his earthquake scale, the U.S. Geological Survey estimates the Charleston quake had a magnitude of 7.3. It remains the most damaging earthquake to hit the Southeast and represents one of the East Coast's largest shocks.

Paul C. Rizzo Associates of Pittsburgh, Pa. and Columbia, S.C., developed the remedial design and is serving as construction manager on the backup dam. The structure has rock-fill berms at the base of the existing dam and a 2,300-lin.-ft. center section built out of 1.3 million cu. yds. of roller-compacted concrete.

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Rizzo's plan called for constructing a portion of the new structure along the back of the old dam, between that dam and the utility company's hydroelectric and coal-fired steam generating plants.

Both plants needed to stay operational during construction. The steam plant had experienced two short outages in the spring associated with the project, and the hydro plant had stayed online except during planned down times to re-route water-supply lines.

"We went to a lot of effort to put together detailed drawings," said Jeff Bair, project manager for design with Rizzo Associates. "The level of detail has really saved time, and it gave us a good idea of what we would find when we dug down."

While preparing to bid on the project, Barnard Construction Co. of Bozeman, Mont., built a scale backup dam as a visual tool and to figure out sequencing. The company came up with a plan that shaved 10 months off the schedule.

"Instead of starting in the center, which had a lot of constraints with the two power plants, we worked the north and south side at the same time and got that work done early before the more difficult sections," said Neil VanAmburg, Barnard's project manager. "We looked at it as a couple of different projects."

Barnard began constructing the 1.5-mi.-long dam in August 2002. Stockman said the project is on time, with a December finish for the dam and a spring 2005 completion of all associated work. About 250 people are working onsite, during two 10-hour shifts, six to seven days per week.

SCE&G awarded Griffin Dewatering Southeast of Houston the dewatering system contract prior to letting the construction work, enabling Griffin to lowering the water table within the dam for stability and to allow for the foundation excavations to proceed without water intrusion. Extensive dewatering, using 1,000 wells, was required to provide safety against slope instability during construction.

"It is the most challenging job we have ever had," said Harry Bagherzadeh, project manager for Griffin. "We had at the same time eight pumps running both shifts."

Griffin pumped the water to a discharge area that leads to the Saluda River. During dewatering and excavation, Rizzo monitored the existing dam so it could observe any movement and make sure it was reacting as expected.

Before starting the project, SCE&G lowered Lake Murray from 50,000 acres to 40,000 acres. An independent panel of experts recommended lowering the lake to increase the margin of safety during construction. Stockman expects to allow the lake to begin refilling later this year, as soon as the foundation work is completed.

"That lake has huge economic impacts for the central part of South Carolina, and we were mindful of that," said Brian Duncan, public affairs coordinator for SCE&G. "That is one reason we wanted to do the work, do it safely, but do it as quickly as possible."

The abutments have a soil core topped with sand and gravel filters. On each side of the abutments, crews placed 5.5 million cu. yds. of 30-in. or smaller shot rock from an onsite rock borrow area. The borrow area will become an ash landfill for the coal plant at the completion of the project.

The center section of the dam sits on a bedrock foundation. During excavation, crews encountered crevices that were deeper than anticipated that required remedial measures to fill the crevices or trenches prior to placing RCC concrete. Barnard used 30,000 cu. yds. of conventional concrete mix to fill the crevices. Excavation went as deep as 70 ft., 50 ft. below the nearby river.

"The bedrock was not as uniform as we anticipated," Stockman said. "We had to build retaining walls to address the deeper excavations."

Crews constructed cement-bentonite walls and soldier pile and lagging retaining walls to accommodate the additional depth of some of the foundations. During the process of wall design, Barnard moved its crews to the other end of the structure to continue work and maintain schedule. None of the difficulties resulted in work stoppages. Excavation was completed in the spring.

"Anytime you dig into an existing dam foundation, you never know what you're going to find until you get to the bottom," VanAmburg said. "As a group, we have changed the schedule and plans and methods at least a half-dozen times. You deal the hand you are dealt and move around and adapt. We've been able to do that successfully."

Barnard uses a roller-compacted concrete mix design that was developed during a preconstruction testing phase for the concrete section of the dam. The use of waste ash from the coal plant and aggregate from the onsite borrow area have reduced the total project cost.

The RCC is placed through specialty placement equipment on the lift surface and then spread with bulldozers and ultimately compacted with smooth-drum vibratory rollers. No steel reinforcement is used.

"We are depending on the mass to hold back the lake should it ever become an active dam," Stockman said.

Crews can place 500 to 600 cu. yds. an hour, or 8,000 cu. yds. in a day. The pours are completed in 1-ft. lifts. Workers place 15 in. of concrete at a time, and the rollers compact it to 12 in.

Barnard can complete up to nine 1-ft. lifts in one day. At the base, the concrete section of the backup dam is 160-ft. wide and at the crest, 20-ft. wide. The midsection will be a sheer, vertical, 150-ft. concrete wall on the lake side and terraced on the downstream side. Work continues on the center section.

"We have a long way to go, but we proved as a team that we can get through it," VanAmburg said.

Project Team:

Owner: South Carolina Electric & Gas, Columbia, S.C.
Construction Manager: Paul C. Rizzo Associates, Columbia, S.C.
General Contractor: Barnard Construction Co., Bozeman, Mont.
Dewatering Contractor: Griffin Dewatering Southeast, Houston