With the combined concerns of load and mucky underground conditions, the support structures would need to be especially deep, with the smaller requiring a 10-ft-deep concrete foundation and the other needing a 13-ft-thick base.
Supporting the 7.5-MW section are 40 35-ft-long steel shoring piles, 88 70-ft-long steel piles and an estimated 250 tons of reinforcement steel, ranging in size from No. 4 bar to No. 11. For the larger test rig, 54 35-ft-long steel piles and 115 75-ft-long concrete piles form the base, along with 650 tons of reinforcement steel.
To enable flexible positioning of the equipment under test, AEC included an anchor sleeve system that allows for a range of load locations for the various drivetrains, Lorentz adds.
In each case, however, the piles' depth—and location within an existing structure—required some creativity, says Chris Palmer, project manager for Choate Construction Co., the general contractor.
"We had to modify the pile-driving rig to drive the piles 20 ft at a time," Palmer said via e-mail. "We would then have to full pin moment weld the next 20-ft section, which took about three hours per weld." In all, he estimates it required more than 1,200 worker-hours just to weld piles back together.
Altogether, the two test rig beds utilize 900 tons of rebar for their respective 10-ft- and 13-ft-deep concrete foundations. Most of the test-rig foundation rebar was No. 11 bar, measuring 60 ft in length, Palmer says. At that scale, placement would require as many as eight workers hoisting each bar.
For the concrete placement, only self-consolidating concrete would work in this situation, he adds.
"With the amount of rebar we had in each foundation, it would prove impossible to get traditional concrete and vibrators down 13 ft and know you have a solid foundation," says Palmer. Even so, designers had to be convinced it would work.
Key to that sales pitch was the recent experience of local concrete contractor Cooper River Contracting, which had used the method on another area project.
"They were really the driver of that [choice]," says Palmer. "That was a huge asset to this project as far as time and constructibility of the foundations. I don't think we would've gotten [the foundations] built" otherwise. The 7.5-MW section required 880 cu yd of concrete, while the larger one consumed more than 3,600 cu yd.
Precision Planning
Clemson's Tuten says he's most impressed with the accuracy with which these support structures were constructed, as the tolerance of most components was 30/1,000th of an inch, he says.
