In fall 2006, Wirtgen’s largest milling machines played a big role in helping a prime contractor meet a strict runway project completion deadline, and avoid $225,000 per day liquidated damages for failure to finish on time.
At Hartsfield-Jackson Atlanta International Airport (ATL) — one of the world’s busiest in terms of passengers — twin W 2200s and a W 1200 F/L kept base removal on time and within budget as crews scrambled to completely remove and replace runway 8R/26L, a $91 million project scheduled for only 59 days.
The costly liquidated damages liability was based on the fact that ATL needed every square inch of runway it could use. Completion of ATL’s new $1.3 billion fifth runway in spring 2006 permitted the shutdown of 8R/26L, but with 8R/26L’s closure Sept. 8, 10 percent of all flights in and out of Atlanta were delayed, with half of those delays blamed on the runway’s closure. Taxiway E — a 75-ft. (23 m) wide taxiway parallel to 8R/26L, between the terminal and the runway — also was reconstructed.
The Wirtgen mills quickly and efficiently removed an average 8-in. (20 cm) of either granular aggregate base (GAB), or cement-treated base (CTB), once the existing concrete runway slabs had been removed. Whatever the base, the W 2200s with 7.7-ft. (2.4 m) drums cut through “like a hot knife through butter.”
Cold Mills Speed Base Removal
“They were ’walkin’ the dog,’ to put it in simple terms,” said Kevin Gamble, operations superintendent, The Miller Group Inc.
“We were doing the full-width runway, moving 8 inches of granular material out, minimum 500 feet per 12-hour shift. They cut either base without a problem.”
“In the first six days on the job, with only two W 2200s running, we pulled out some 30,000 cubic yards,” said Ryan Essex, vice president and general manager, U.S. operations, Miller Group. The total amount removed was 58,000 cu. yd . (44,344 cu m) in a total of 13 days of Phase I main runway and taxiway work.
The existing concrete runway was 150 ft. (46 m) wide by 10,000 ft. (3,048 m) long. Concrete slabs 16 in. (41 cm) deep comprised the old runway, and to start demolition, the slabs were saw-cut, then lifted out in large pieces by backhoes with grapples and placed in trucks for removal. This saw cutting took place at night while the runway stayed open during the day.
“They would shut the runway down on a night closure during the week and a contractor would cut a 25-foot by 10-foot grid,” Gamble said.
“Once the runway was closed beginning Sept 8, at 11 p.m., the claws would grab the 16-inch slabs and lay them in back of dump trucks for hauling off. Each truck would haul two slabs.”
Removal of the runway this way was much faster than demolition with guillotine breakers or similar equipment.
“The tailgates of the dump trucks were removed and the grapples would slide the slabs into the backs of the dumps,” Essex said.
And only 24 hours after slab removal commenced, Miller moved in with its Wirtgens to remove the GAB under the slabs.
“We removed an average of eight inches of GAB, above cement-treated base, or native soil,” Gamble said.
“Some areas of subbase below the GAB had to be milled to get to a proper grade. We may have had to remove an inch here, or half-inch there, it varied. Most of the runway was over GAB, but the taxiway was over CTB.”
Instead of replacement with a fresh GAB, the subbase was overlaid with a minimum 2 in. (5 cm) of .5 in. (1.25 cm) top size Superpave hot mix asphalt (HMA). Above the Superpave a concrete slab was placed. Due to the tight time frame, wherever Miller had completed base removal, paving would start within hours.
Where the concrete runway would abut a concrete taxiway, Miller cut what is called a “thickened edge,” usually an extra 6 in. (15 cm) of depth, some 25 to 50 ft. (7.6 to 15 m) wide at varying lengths.
“These areas would receive a low-slump concrete mix, with asphalt and concrete placed over all,” Gamble said.
Mills Put Two
Shifts’ Work Into One
The urgency of the project prompted Miller to start work with two 12-hour shifts, or 24 hours a day, but right away the productivity of the Wirtgens versus the amount of slabs being removed by the prime allowed Miller to cut back.
“We started doing 24/7,” Essex said, “but to keep up with the slab removal it was only necessary to run one 10-hour shift per day. Use of cold milling machines for excavation is becoming a very common practice, especially when it comes to projects constructed on-grade. It’s a matter of dollars and cents.
“You can throw conventional equipment like excavators and front-end loaders to excavate as fast as milling machines can, but cold mills are usually more cost effective because they can excavate and fine grade all in a single pass,” Essex said.
“And we still surprised the prime with our speed, because the contract was set up for two 12-hour shifts to keep up with the slab-pulling. But after a day we realized we could get accomplished in one shift what they expected us to do in two. The machines exceeded everyone’s expectations.”
Miller’s W 1200 also generated considerable savings for the prime contractor, GSC Atlanta Inc.
“Along the edge of the concrete to be removed, we used the W 1200 to cut four-foot widths from the edge of the concrete into the existing asphalt shoulder at six- to eight-inch depths,” Gamble said.
“This would accommodate new drainage and kept GSC from having to saw cut and remove the material with tracked excavators, with considerable savings. We were able to cut the material quickly with a nice vertical edge, and save the contractor a lot of money.”
After the 4 ft. (1.2 m) of shoulder was milled, concrete slabs were removed. Then, while the gravel base was milled out by the W 2200s, they also would cut deeper into the 4-ft. (1.2 m) width, down to the grade required. A trencher would follow to provide the trench needed for drainage.
“Our prime contractor, GSC Atlanta, was a first-class operation to work for,” Essex said. “They were very professional, on-time, and on-schedule. A milling job cannot be efficient unless you have trucks to put material in. There was a tremendous amount of material to be removed on this project in a short amount of time and none of it matters unless there’s a truck underneath the milling machines. There is no way we could have met these production goals if they didn’t provide the trucks we needed, when we needed them. It was truly a pleasure to not have to worry about the trucks.”
(This article appears courtesy of “Wirtgen Technology” magazine.)