O’Hare International Airport in Chicago is one of the world’s busiest. On a typical day, nearly 2,500 aircraft take off and land from the location.
The demands on the airport are only going to increase, which has led to the development of new runways. Handling the sub-base work for the runways, which one day will be pounded by jets weighing more than 45 360 kg (100,000 lbs.), is Rock Solid Stabilization, Ringwood, Ill.
“We’re proud to be part of this high-profile project,” said Jonathan Pease, owner of Rock Solid Stabilization. The project also builds on the lessons the company has learned since entering the soil stabilization business.
That entry came about almost by accident in early 2000, when a supplier invited Pease to witness a fly ash modification/stabilization demo. “I saw a demo with fly ash, and it was really interesting,” Pease said. But that’s about all it was. “I thought it was a neat process, but I didn’t leave thinking, ‘We’re going to do this.’”
But a week later the family firm, Pease Construction, was bidding on a job. The engineering plans called for removing wet soils and replacing them with stone and clay. Pease instead bid on modifying the soil, and guaranteed the subgrade. The firm landed the bid and took on the project, including the modification work.
Pease Construction continued to offer modification/stabilization work as part of its overall business. As time went on, it continued to add more and more of the work until Pease bid—and was awarded—a strictly soil modification/stabilization job.
“Handling the entire process was a natural progression,” Pease said. He went all-in during 2007 with the formation of Rock Solid, and has been building his stabilization, modification and reclamation business ever since.
Rock Solid worked as a subcontractor on the O’Hare job. They were hired to handle 585,000 m2 (700,000 yd2) of lime stabilization. The work at the airport also included 36 280 metric tons (40,000 tons) of lime soil modification.
Handling the work was a Cat RM500 Rotary Mixer, but the job started with a spreader that Pease purchased from a manufacturer, who added custom modifications especially for the challenges faced on the varied jobsites.
"We have three styles of spreaders that can handle site conditions from hard, solid ground to wet, soft conditions," said Jim Hegemann, Vice President of Rock Solid Stabilization. The spreaders provide precision as well, delivering spread rates that were measured to one-tenth of 1 percent on the O’Hare job.
Lime kiln dust was the additive used on the job. It is not as readily available as it once was for a variety of reasons, including the fact that it is a byproduct of a slowed manufacturing industry. On the O’Hare job, the supply issues led Rock Solid to purchase the additive from three sources. “Each source has its own spread rate,” Pease said. “It adds to the complexity of the job.”
After spreading is completed, a motor grader makes a single ripping pass. Next comes a water truck, which adds minimal moisture. “Just enough to help activate the lime,” Pease said.
Next, a Cat RM500 Rotary Mixer makes a dry cut at a depth of 305 mm (12"). The machine works at a speed of 12-13.7 m (40-45') per minute on the O’Hare job, and covers strips 488 m (1,600') in length. It’s tough going, with an operator saying the rotary mixer typically can dry cut at a rate of about 20 m (65') per minute.
“The ground is so hard out there,” Pease said. “We usually move at a faster rate, but on this job the hard soil conditions dictated that speed.”
A second RM500 then makes a “wet cut.” Leading the train is a water truck, which is pushed by the Cat Rotary Mixer. Water from the truck is pumped and precisely sprayed into the mixing chamber, where it is mixed with the soil and lime. A second water truck replenishes the first truck so the mixing train can keep moving.
Running water directly into the drum is a key process for Rock Solid. “The amount of water is controlled with this project,” Pease said. “Water injection is the best way to control the moisture.”
Davin Heikkinen agrees. “The controlled method is better than mass flooding,” said Heikkinen, who handles quality control for Walsh Construction, one of the firms on the job. On a typical job, a water tanker “floods” the area and leaves it to the stabilization team to achieve the proper moisture content.
“The controlled flow saves money because you don’t waste water,” Pease said. “You also get the right amount of moisture the first time, meaning you don’t have to go back and make adjustments. That can add a lot of extra time.”
Heikkinen is a believer in the controlled system, too. “It’s the first time I’ve seen it,” he said. “It’s working excellent.” With the controlled system, the operator is able to look behind him and make a fairly informed judgment based on sight. With a flooded system, more passes—and speculation—are required.
“With the water injection, the operator can tell if the material is consistent,” Heikkinen said. It’s Heikkinen’s job to confirm that it is. “I follow him with a nuke gauge and test, and it’s been right on target,” he said. “We haven’t had to make any after-the-fact adjustments.”
Production is still good. “We can stabilize 20,000 yd2 (16,700 m2) per day or better with the two reclaimers—one wet and one dry,” Pease said.
Following the wet cut is a Cat CP-563C Soil Compactor. Its front blade helps even material, while the vibratory drum compacts the freshly mixed soil in 6-8 passes.
The motor grader—which earlier ripped—makes a shaping pass. Next a smooth drum roller makes 1-2 static passes to seal the surface. The general contractor applies an emulsion, and the surface then cures for five to seven days.
“It’s a big job, but it’s one that’s been made easier by the equipment, and the precision of the material spreading and the water control,” Pease said.