Large-scale solar projects are becoming more common in Minnesota, and as you might expect, installation of these projects requires a different approach than large-scale projects in those popular desert climates.
An example would be a new 5 MW AC Project south of the Twin Cities. On approximately 20 acres, two factors in particular require a different approach. The ground is mostly dense sand with little cobble, and a Minnesota winter’s frost can affect uplift. So, what’s the answer in such a situation? Longer piles with more embedment depth.
The project was contracted with Landwehr Construction to handle the pile driving. Landwehr, based in St. Cloud, Minn., had the experience with helical piers and bracing needed to find an answer to handling the extra pile length. Although the company has been around since 1895, solar work was not one of its areas of expertise until recent years.
“We began doing helical piers and bracing to support our precast division,” said Robert Schofield, Landwehr’s foundation estimator. “A few years ago we hired a senior estimator with over 25 years of experience building renewable sites. We paired his knowledge with our crews who typically perform highway heavy, crane and site work. The combination of experience and skill set has worked out very well for us.”
In addition to the right crews, Landwehr needed to acquire a new machine to handle this job. It turned to the Hercules STR-20 for a number of reasons, the biggest being its 29-ft boom. “On the west side of the array, it is very dense sand, so all the piles are 22-ft tall, requiring 15 ft of embedment,” Schofield said. “The Hercules machine is currently the only one on the market capable of driving a 22-ft pile without pre-drilling the holes.”
Schofield estimated that using standard equipment would have added a minimum of three crew members and at least one other piece of equipment to the job. The job also moved along more efficiently due to the STR-20’s ability to free-float the hammer, which can save time on each pile. “You can physically push the pile in the ground until you can’t push it in anymore, then switch to the hammer,” Schofield said. “So, on this site, we can push them in anywhere from 4 to 6 ft and then hammer them in the rest of the way.”
The STR-20 also sits on a turntable, allowing it to spin 360 degrees and do two rows in one pass. “It all comes down to saving a few seconds here, a few seconds there,” Schofield said. “It really adds up over a couple thousand piles.” Their success can been seen in the numbers. Landwehr began driving pile for this project on Sept. 1 and only experienced four refusals out of 1,800 piles.
— Solar Builder magazine