Buried Bridges Elevate Fort Lauderdale Runway
Creating a mechanically stabilized mountain and building a pair of massive, buried bridge structures are all part of a day's work for the crew working on the elevated and sloping runway project at Fort Lauderdale-Hollywood International Airport (FLL).
The entire project, estimated at $750 million, is designed to extend the boundaries of the airport by raising one of its runways up and over a major highway and rail line.
Targeted for completion by September 2014, the work is testing the limits of contractors as they deal with void-ridden soils and considerable muck on a hemmed-in site.
The Fort Lauderdale runway elevation over active road and rail lines makes it the second of its kind in the U.S., following the fifth runway at Atlanta Hartsfield-Jackson International Airport, which opened in 2006.
One part of the FLL plan, about a $500-million project, is to expand an existing 5,300-ft-long landing strip into a new 8,000-ft-long south runway that partially sits on a 60-ft-high man-made embankment at the new eastern end.
West of that embankment, and somewhat perpendicular to the new runway, a joint venture of Tutor Perini and Baker Concrete Construction is working on its $180-million contract to erect two concrete bridge structures that will separately support the future runway and taxiway. The runway support structure measures 848 ft long and 584 ft wide, while the other taxiway structure is 723 ft wide and up to 486 ft long.
Both consist of prestressed, 6-ft-deep concrete beams and post-tensioned deck slabs; the runway segment will be supported by solid walls, while the taxiway portion will be built on piers. The structures will accommodate highway and rail lines underneath, functioning like an overpass or tunnel roof, says Ron Murtha, program director with Parsons Transportation Group, the construction program manager for the Broward County Aviation Dept.
While this work is going on, a joint venture of Odebrecht and Central Florida Equipment is progressing on its approximately $226-million contract to build an embankment totaling 6.5 million cu yd of aggregate, supported by 40-ft-tall mechanically stabilized earth walls totaling 323,500 sq ft.
In mid-April, Archer Western Contractors was the apparent low bidder for the runway paving contract, with a bid of $87.7 million.
Additionally, a separate, $250-million "noise mitigation" effort that upgrades nearby residences with new roofs, windows and other improvements pushes runway-related costs to roughly $1 billion. The runway expansion is occurring in tandem with an $800-million terminal renovation and expansion program.
Up and Away
The tight site presented a challenge to the crews. Boxed in on all four sides—by highways on the east, west and north, and by a residential area on the south—the airport had no available land on which to expand. That meant it would have to use the land previously utilized as a light aviation landing strip for the new runway. Even so, that stretch of property was still too short.
As a result, the project team, opted to jump the runway across the east side's highway/rail corridor and onto the embankment.
Starting from the west end, the new 150-ft-wide runway will first extend on a level course for roughly 30% of its ultimate 8,000-ft length, says Darin Larson, vice president and operations manager of aviation services with Tampa-based Atkins, the project's design criteria engineer for the bridge structures. Then it begins to slope upward, to a maximum of 1.31% before flattening out to 0.8% just before the bridge structures. Over that stretch, the runway gains roughly 55 ft in height, ending up about 60 ft above the east-side rail line—which was the controlling factor with its 24-ft clearance requirement.
The structure supporting the runway spans 583 ft along its centerline, while the taxiway structure span is longer, at 723 ft, due to the wider configuration of the highway/rail corridor at that location.
In all, the structures will use 2,661 24-in. concrete piles, 69,859 cu yd of cast-in-place concrete and 857 prestressed I-beams. The cast-in-place walls for the runway structure will be 3 ft thick and continuous. The runway and taxiway decks will be 15.5 in. and 14.5 in. deep, respectively.
"They'll take a new A380 or a fully loaded 747 hitting the bridge deck, [which is a] very unusual load for a bridge structure," says Michael Hernandez, design coordinator with Tutor Perini/Baker. "The pile loading is much greater than you would expect if this was a regular bridge."
As a result, compared to more traditional highway bridges, Fort Lauderdale’s runway structures feature decks that are 4 to 7 inches thicker than usual, and precast beams that are up to 4 inches thicker than standard, says Scott Dean, design manager with HNTB Corp., the engineer for the joint venture. Additionally, Dean says, “We have roughly 50% more pile on this project than would be necessary on a highway bridge of similar proportions.”
Tutor Perini/Baker has experienced challenges with the underground conditions during pile driving.
"There's a lot of variability in the subsurface conditions," says Hernandez.
The depths needed for individual piles have differed significantly, for instance, with final depths varying by as much as 40 ft for piles within 10 ft of each other.
"That kind of variability has been challenging," he says, adding that it's slowed production considerably. That work is approximately 86% complete now, Hernandez says, adding that some resequencing of scope and maintenance-of-traffic plans was necessary to keep the contractor on schedule for its original February completion target.
Mounting the Runway
Like Atlanta's elevated structure, this runway was designed by PBS&J, since acquired by Atkins. But unlike Atlanta's runway, this project could not use a conveyor belt to transport the 6.5 million cu yd of material to the site, due to the extensive foundation supports that would be necessary with the site's mucky conditions resulting from its proximity to the ocean, says Murtha.
Odebrecht-Central Florida's roughly 300 trucks are in near constant movement six days a week over two 10-hour shifts, delivering an estimated 2,000 truckloads of material daily, for a total of 40,000 tons.
Roughly 15,000 tons comes in daily via rail, on a spur to the airport property built by Florida East Coast Railway. Once it's arrived, Herzog Contracting, working for FEC, maneuvers its excavators atop the train, where they then offload the material car by car. The setup reduces the number of trucks that would otherwise be necessary, says Murtha.
The embankment consists of two sections: a segment east of U.S. 1 with about 4 million cu yd of material and another west of the highway that's about half that size. Atkins' Larson says the speedy construction of the massive embankment—especially the eastern section, which sits atop considerable pockets of muck—is key to the project's schedule.
"The fact that we're creating an embankment as high as we are in a fairly short period of time" remains a main focus of project officials, he says. Of particular attention is the settlement of that material.
"We're really concerned about making sure that the embankment that's on either side of those structures is sufficiently settled before we construct the runway, so we don't end up with a differential settlement between the structures and the runway," Larson explains.
So far, says Larson: "We're pleased with the results. It's flattening out at a faster rate than we [expected]."
Filling the voids
To the west of U.S. 1, throughout the runway area, layers of fractured limestone—called pinnacle rock—feature widespread existing voids that had to be addressed, since the new runway will handle much larger aircraft than the old one did, Murtha says.
"The old runway didn't handle big aircraft, so what was there was OK," he says. "But it's not sufficient for the beating this runway's going to get."
Odebrecht-Central Florida brought on the Japanese firm Fudo to propose using a method known as mammoth vibratory tamper, or MVT, instead of the traditional deep dynamic compaction that had been originally specified for the project.
Odebrecht became aware of Fudo and the MVT method from its work on Louisiana levee projects, says Mauricio Gonzalez, project executive.
Reducing noise and vibrations—a benefit of MVT over the traditional deep dynamic compaction method—was vital considering the proximity of residents, aircraft and a control tower, says Rodolfo Armenta, an Odebrecht project manager.
Crews vibrate the deeper layers of ground for 90 seconds, two times, using a 10 ft by 10 ft steel plate, says Murtha. "In between, there's a 7 ft by 7 ft plate that is vibrated for 60 seconds," he adds.
"The larger plate densifies soils at deeper elevations, and the smaller one gives us concentrated energy on the upper layers," says Armenta.
If the specified compaction isn't achieved, "we trench and then we tamp again," Gonzalez says.
Additionally, the team is preparing to address some 1,777,500 sq ft, out of a total 5.8 million sq ft, where compaction is needed for depths below 10 ft, due to more extensive pinnacle rock.
This spring crews will begin using a vibratory rod method, where a rod or H-beam attached to a motor is inserted approximately 12 ft deep at predetermined spacings, says Gonzalez.
The rod is then lifted up a few feet and the hole is partly filled with material. Then the rod goes back down, compacts that material, and the process repeats until the hole is fully backfilled, he says. All compaction work is slated for completion by the end of year. Paving crews could mobilize in July, laying the groundwork for the runway's opening next year.
