JFK - under the microscope

On June 28, New York’s JFK International Airport reopened the Northeast’s longest and busiest landing strip, the ‘Bay Runway’ ( 13R-31L) It is the nation’s third-longest at 14,572 feet, and was resurfaced with an 18-inch layer of concrete to yield an expected maintenance savings of $500 million over an expected 40-year life span, compared with an anticipated eight-year asphalt life span. Because JFK is a key
component in the nation’s commercial air-traffic system, the first phase of the $376.3 million project was put on a fast-track schedule of 120 days, with overall completion due in late 2011. Here’s a look at how the project came together.

In addition to financing from the owner, the Port Authority of New York and New Jersey, the project received $73 million in Federal Aviation Administration and $15 million in American Recovery and Reinvestment Act funding.
Milling, grading, and paving subcontractor Intercounty Paving Associates, LLC of Hicksville, NY kept the first milling phase of the first phase on track by simultaneously utilizing a combination laser-Global Navigation Satellite System (GNSS) on multiple Roadtec RX-900 cold planers.

Preventing chronic delays

JFK has traditionally experienced problems with delays. According to U.S. DOT, it ranked 28th out of 31 major airports in on-time performance in 2009. The Bay runway is a key component of JFK’s infrastructure, handling about one-third of its annual operations and more than half of all departures, to the tune of about 440,000 flights and 48 million passengers in 2008.

To address the chronic delays, the general contractor, Sylmar, CA-based Tutor Perini, was awarded a $204 million contract to widen the runway from 150 to 200 feet to allow for new delay-reduction taxiways, among the improvements. The new shoulders along either side of the widened runway are 50 feet wide and sit adjacent to 30-foot-wide erosion pavement.

The runway will be the first at JFK to accommodate the massive new Airbus A-380 and also serve as a backup landing spot for the space shuttle. The taxiways have high-speed exits for landing aircraft, holding pads to enable planes to bypass those held on the tarmac, and a new drainage system; the sum total of the changes are designed to improve aircraft queuing and allow quicker departures and easier access between taxiways and terminal gates. According to the Port Authority, the improvements will reduce future delays by an estimated 10,500 hours.
Part of the Bay Runway had to remain open to traffic so that planes could taxi across the open portion between the terminals and the other runways. After the first phase, about 11,000 feet of the runway was to be reopened and the remaining 4,000 feet-plus were to be resurfaced in subsequent stages.

Procedural changes

Preparations began during night closures the month before the start date. Tutor Perini surveyors localized the jobsite using Intercounty’s GR-3 base station from Topcon Positioning Systems. The GR-3’s one-watt UHF radio left “dead zones” at both ends of the runway, typical in radio signal-dense airfield environments. So the surveyors set up Topcon’s 35-watt external radio, allowing them to position the base station along the runway, near the middle of the jobsite, providing full radio coverage without interference from voice traffic.

Also complicating matters were several unseasonably large snowfalls during February. Once control points were located, they had to be cleared before the shots could be taken and drifting snow routinely refilled the control point holes. These weather conditions would have made the conventional method of gridding and marking out the runway in time for early March milling difficult to impossible, making it clear that an alternative method of elevation control would be beneficial.

Even when construction scheduling is designed to minimize the effects of adverse weather, a few snags are inevitable. Despite the fact that Intercounty made adjustments on the Roadtecs to allow for unexpected issues with the lasers, the system turned out to be invaluable for milling efficiency.

The system made an initial impact in subgrade preparation and increasingly is being used for milling and paving. Just as Millimeter GPS+ has been used for fine grading, contractors are beginning to use it for “fine milling” — and achieving accuracies within a quarter-inch, in contrast to the tenth-of-a foot precision inherent in conventional machine control.

The role of GNSS

Conventional GNSS machine control uses satellite signals alone. Such a system uses a rugged antenna mounted to a shock-absorbing, vibration-damping pole along with a GNSS receiver box mounted in a secure location on the machine. Satellites send positioning data to another antenna/receiver combination at a stationary base station. The base station then sends a three-dimensional position and 3-D corrections via radio to the mobile or machine control receiver.

Positioning data is also sent to the machine. The stationary base and machine work together to provide real-time kinetic (RTK) position information, revealing the machine’s three-dimensional location on the site. Software compares the machine’s position to the design grade, which was determined using site plans, at a given location. The system also provides visual guidance for machine operators by displaying a site model on an in-cab color monitor, or it automatically adjusts the needed elevation and desired cross-slope of the blade as the operator guides the machine forward.

Millimeter GPS+ combines GNSS and laser. In addition to a GNSS base and rover, the system uses a PZL-1 Lazer Zone transmitter and a PZS-MC machine-control sensor or PZS-1 rover sensor that gets integrated with the contractor’s GNSS receiver. The PZL-1 transmitter sends out a wall of laser light 33 feet tall and up to 2,000 feet in diameter.

The contractor can link up to four transmitters for a total reach of 8,000 horizontal feet and 132 vertical feet. The PZL-1 transmitter can operate multiple machines equipped to accept its signals. The GNSS component of the system plots the location of the machine while the laser component guides the grader to position and elevate the blade precisely. The system “knows” the three-dimensional position of the laser transmitter and the three-dimensional position of the machine and is then able to calculate the vertical angle from the laser up to the sensor on the machine and provide a vertical correction.

The roughly 12,000-foot runway was divided into quadrants of some 6,000 feet in length and 200 feet wide from the centerline. The 200-foot widths were subdivided into two adjacent 100-foot-wide subsections to accommodate eight passes by the 12-½ foot-wide Roadtecs. With the entire job now accurately localized for GPS at its midpoint with a single control point file, Intercounty figured that mounting the PZL-1s on 15-foot-tall towers would provide the lasers with continuous coverage. But nonstop 40-mph-plus winds would not allow continuous rotation of the self-leveling Topcon PZL-1 lasers at that height.

So Intercounty’s Topcon dealer, Cleary Machinery Co., Inc. of South Bound Brook, NJ lowered the PZL-1s to their standard 2-meter height on tripods. Intercounty had five milling machine rovers and three survey rovers, all corrected by a single base station despite being located thousands of feet apart. A three-dimensional site model developed by Mesh Consulting, Eagleville, PA, was loaded into the Roadtecs’ machine-control systems.

Actual Milling

The original plan was to fine-mill the surface to within three-quarters of an inch of the specified elevation in a single pass using three machines deployed in a staggered formation. The machine making the “virgin cut” would have Millimeter GPS+ controlling both sides of the drum. Each trailing machine would “joint match” on one side using the RX-900’s hydromation system, while utilizing Millimeter GPS+ on the other side. Four PZL-1s were spaced apart by 750 feet, affording a pass length of 3,000 feet before the machines would have to “square up” and return to the original starting point.

Equipment blockages and weather caused the Roadtecs to cut to within 0.02 foot with laser reception and within 0.05 foot without it. These factors, coupled with severe machine vibration caused by the deep cut in hard material, made precisely milling to the specified elevation in one pass difficult. But the situation turned out to be a blessing in disguise, as it forced Intercounty’s milling department to devise an alternative process that proved to be even more efficient than the original plan.

The decision was made to disconnect the laser receivers on three of the Roadtecs and have them rough-mill the existing surface to an inch and a half above finished subgrade. It was reasoned that since the machines were going to cut the last lift with the Millimeter GPS+ anyway, the rough mill accuracy was not as critical. Three machines cutting without the lasers actually put Intercounty ahead of schedule, and two other Roadtecs fine-milled where possible at any given time.

The FC-120 field controller also mounted on the pole listed the elevation in feet and the cut in thousandths of an inch. The fine-milling machines were set at 18 inches (minus-1.50 feet) and the bulk-milling machines were set at 16¼ inches (minus-1.36 feet).

Intercounty reports that the hardness of the top of the existing asphalt layer made milling off an average of six inches in one pass difficult, too. After the rough start, Intercounty was hitting the daily production goal of 2,000 by 100 feet over several days in the second week of work.

Under conventional elevation-checking methods, reference stations were marked every 25 feet on a pass, typically along both edges of a pass. Elevation checks utilized tape, stringline, and reference marking on the milled surface. Millimeter GPS+ allowed for on-demand elevation checks — in varying locations within a pass — using GX-60 control boxes and the rover.

Continuous Accuracy

GPS may not be any more accurate than manual at station, but between stations, it is. It performs like a virtual stringline, calculating smooth transitions from station to station. And it eliminates the occasional blown grade.

The system is intended to take the human error out. The two-stage milling process proved to be almost the opposite of paving with asphalt, where one puts down a base and binder course and then comes back and levels the surface with a consistent final lift. The more consistent the lift depths are, the better the ride on the final surface is.

One of the biggest advantages of the system’s accuracy is prevention of overmilling. If overcut, it’s necessary to go back and pave and then mill again. That costs time and money.

The entire area was milled without a single mark on the ground, which is roughly 8,000-plus shots in a 25-by-12-foot grid over 11,000-plus feet. The mark-out costs associated with 16,000 shots is actually insignificant compared with the potential downtime resulting from waiting for the marks to be made, due to the large penalties enforced for not finishing the job on time.