When the main runway at Alaska's Yakutat Airport (YAK) experienced damage from harsh hydrogeological conditions, the state's Department of Transportation and Public Facilities tread carefully when crafting a solution. As owner and operator of the airport, the department knows that any project in the lowlands area around the Gulf of Alaska includes extra hurdles-specifically weather and logistics.
Located 225 miles northwest of Juneau, the remote fishing village of Yakutat (population 613) has no road access. People, provisions and supplies must arrive and depart by air or sea. Barges deliver goods monthly during the winter and more frequently in summer. Alaska Airlines, the airport's only commercial passenger service, provides jet service from YAK twice daily; and the region is also served by air taxis and floatplanes.
During calendar year 2014, YAK logged 19,000 flight operations.
Project: Runway Repaving
Location: Yakutat (AK) Airport
Owner/Operator: State of Alaska
Length: 7,745 ft.
Width: 150 ft.
Repaving Cost: $12.5 million
Project Designer: AK Dept. of Transportation
General Contractor: Knik Construction
Infrared Joint Heater: Heat Design Equipment
Intelligent Compaction System: Hamm AG
Microblisters & Birdbaths
In the roughly 10 years since its last repaving, YAK's main runway, 11-29, had experienced frost issues due to high ground water, explains Robert Trousil, P.E., senior materials engineer with the state's transportation and facilities department. Hydrologic conditions, coupled with freezing weather, compromised the integrity of the runway pavement, causing small blisters to emerge on the asphalt runway's surface.
"They would be pretty small, no more than 6 inches in diameter-somewhat indiscernible, especially to a landing aircraft," notes Trousil.
The "microblisters" as he calls them, formed during winter due to frost heave and were scraped off when maintenance crews plowed the runway. When the weather warmed in the spring/summer, small pooling areas appeared on the surface.
Trousil says there were noticeable trends based on, to some degree, the jointing of the WWII-era concrete panels underneath the asphalt and the joints of the pavement itself. "We could see some trends, but there were literally a thousand of these blisters that would manifest themselves as little birdbaths in the summer and would be somewhat raised-maybe a tenth of an inch-in the winter."
Each subsequent year, however, the birdbaths grew a little deeper.
In 2010, the Alaska Department of Transportation (DOT) and Public Facilities developed a significant hydrologic monitoring and evaluation report. Because the runway issues were directly related to the groundwater, the department installed eight ground water monitoring wells and two surface water monitoring locations to assess the hydrologic/geohydrologic conditions in and around the airport.
Data was needed to determine the elevation of the ground water table at the airport, explains Trousil. Transducers in each of the wells and the surface water sites logged water surface elevations every 15 minutes. Water temperature and observations about the runway surface were also recorded. In total, the department collected data for about 18 months. "We had a good idea of how the water table reacted to precipitation and the seasonal fluctuations of the ground water table and how that played out," Trousil notes.
The department used the data to determine that shallow ground water conditions caused ice to form, predominantly along runway joints. A lack of subsurface drainage and capillary breaks beneath the runway structure caused differential heave of the runway subgrade and finished surface. According to DOT research at that time, the installation of dewatering mechanisms such as French drains and dewatering wells would not significantly reduce ground water elevations, and ditch improvements would have limited impact reducing the water table beneath the runway.
Based on the research, the department made fairly specific recommendations about how to address the situation, Trousil says. "It drove a lot of the thoughts and ideas of how we approached the project," he recalls.
The state's project design manager, Chuck Tripp, P.E., collaborated with the department's Southcoast Region Materials and Airport Planning staff as well as the FAA to formalize a plan to address the challenging asphalt conditions.
Ultimately, the strategy was to install a capillary break (to allow water movement) above the water table, remove frost susceptible soils and add non-frost susceptible fill materials on the 29 end of Runway 11-29 to provide separation between the ground water and pavement surface.
Additionally, the project removed a mild but broad hump where the crosswind runway, 2-20, joined the main runway.
Once the job was awarded, one of the first major tasks was establishing a mix design for the asphalt. Knik Construction, the general contractor, submitted an aggregate source, and the DOT developed a mix design that dictated how much asphalt needed to be mixed with the aggregate.
Getting the mix correct, and within FAA specifications, was one of the biggest challenges of the project, Trousil recalls. Typical highway projects often require two iterations of mix designs to nail down ideal proportions. For this project, the team went through six iterations before finding the right gradation to meet air voids. "It was challenging," he relates. "FAA specs are a little more challenging than highway specs to get the right design."
Repairing the Runway
On May 1, YAK closed its main runway and Taxiway A to begin the repaving project. Diverting traffic to the crosswind runway allowed the airport to remain operational, while shutting down the main runway completely (vs. using a series of shorter nightly closures) allowed the project to proceed in a safe and timely manner, says Airport Director Robert Lekanof Jr.
On the main runway, crews milled off at least half of an inch to remove grooves, repair damage and prepare the surface for a new layer of asphalt. On the crosswind runway intersection, they milled off up to 6 inches to remove the hump and improve drainage. Local fill was used to raise the 29 end of the runway by 21/2 feet, and the entire surface received 6 inches of asphalt, paved in two 3-inch lifts.
With 62,000 tons of asphalt, YAK's runway work is the state's largest paving job in 2016, reports Amanda Gilliland, P.E., Knik's project manager. It is also the first runway project in Alaska where the company used an infrared joint heater, she adds. Typically, FAA specs require contractors to cut 4 to 6 inches off the edge of asphalt and then pave the area to eliminate the low-density portion. At YAK, Knik used a joint heater, from Heat Design Equipment, to reheat the old asphalt. This allowed crews to pave against it and achieve the necessary density to provide a seamless, watertight edge, explains Gilliland.
The joint heater, which Knik had used on other airport and roadway projects, provides multiple benefits. Notably, it saves money by reducing material waste. "If you're going to trim 6 inches of asphalt off 8,000 feet of runway, on each joint, it's just a lot of waste," she comments. "[The state] would be paying for asphalt that you just end up grinding up and throwing away."
The heater also makes jobs much less labor-intensive for crews, and ultimately produces better results for customers, adds Gilliland. "You're reheating the edge and getting more interlock with the aggregate in the asphalt, so I think it makes a better joint," she explains.
In addition, Knik used echelon paving, with pavers working side-by-side, but slightly offset to smooth the asphalt. The process reduces the number of joints, which are the weakest points of asphalt. At YAK, 14 joints were reduced to five.
A HAMM intelligent compaction system helped improve rolling accuracy during paving operations by leveraging global positioning system technology. The system's computer provides real-time data about the surface, displaying on screen precisely where workers have rolled. "It takes the guesswork out of it and guarantees they're going to have consistent coverage," Gilliland says.
The system also records data such as speed, amplitude and temperature, making it a valuable tool for troubleshooting if pavement issues emerge later. "You can go back and look at what settings the roller was on, how many passes crews were doing, what the temperatures were," she explains.
Weather & Logistics
From his seat as airport director, Lekanof feels that the runway project went smoothly and quickly, despite weather that sometimes halted progress. A wet August made painting a fourth-quarter struggle, with rain continuing for 10 days at a time.
Despite such challenges, the airport reopened its main runway on Aug. 24. For the nearly four months it was closed, YAK stayed operational by using its crosswind runway. Keeping on schedule was challenging for Alaska Airlines, because heavy rains on the ungrooved crosswind runway caused delays and cancellations.
Weather played a huge role throughout the project, Gilliland reflects, but her crew managed to complete the project within the limited construction season. (Yakutat receives some of the heaviest precipitation in the state, averaging 132 inches of rain and 219 inches of snow annually.)
When the weather cooperated, Knik's team produced up to 6,000 tons of asphalt per shift-impressive results when working with a mobile asphalt plant, notes Gilliland.
"They put down a record amount of pavement in a pretty short amount of time," adds Trousil. "That capability helped the project really take advantage of the weather they had, as well as using their resources to the best that they could."
Yakutat's remote location and complete lack of road access added considerable challenges for Knik. With air, ferry or once-monthly barge service, preparing for the project was a carefully orchestrated exercise.
The contractor not only brought in trucks and construction vehicles, it also imported a parts and maintenance shop to keep them running. In addition, crews manufactured all of their own hot rock and crushed aggregate with asphalt and other plants, which the company also transported from home.
To support the project, Knick delivered 3,500 tons of asphalt cement. In the "lower 48," a manufacturer would have simply trucked material to the plant each day.
Knick even transported a mobile camp to provide lodging for its workers during the project, because basic project necessities were simply not available locally. "It all comes down to planning," Gilliland reflects. "You really have to try and think of everything."