Holding Down Pavement

Oct. 1, 2006

Crosswinds are a major concern at many airports, especially on the remote islands of Alaska where winds average 30-40 mph. Small passenger planes often have to turn back because of inclement weather. Imagine for a moment, your plane has begun its descent to the St. Paul Island airport, located 300 miles off the mainland. After a rough landing and a long roll to a stop on a runway of frozen dirt and gravel, you are stunned as you exit the plane and encounter the freezing cold and a 40-mph wind.

Crosswinds are a major concern at many airports, especially on the remote islands of Alaska where winds average 30-40 mph. Small passenger planes often have to turn back because of inclement weather. Imagine for a moment, your plane has begun its descent to the St. Paul Island airport, located 300 miles off the mainland. After a rough landing and a long roll to a stop on a runway of frozen dirt and gravel, you are stunned as you exit the plane and encounter the freezing cold and a 40-mph wind.

Predominantly inhabited by the native Aleut people, St. Paul Island is one of the two Pribilof Islands that sit in the middle of the Bering Sea. St. Paul Island is the home of the largest crab-production facility in the world, processing a half-million lb of crab per day.

The state-owned, 6,500-ft x 150-ft gravel runway was constructed during WWII by the U.S. government. An agreement with the U.S. government to return the land to the Aleut people included paving the airport runway for the wave of tourists who arrive each summer to visit the fur seal rookeries and observe the 248 species of nesting seabirds. More than 2 million seabirds nest on St. Paul Island each year, and the Aleuts wanted to make certain that their environment was protected during the construction of the new airport runway.

The climate of St. Paul is arctic maritime. Its Bering Sea location produces treacherous weather every year, and the island receives an average of 25 in. of rain and 56 in. of snow. Heavy fog is common during the summer months.

Knik’s knack

The gravel runway and the extreme weather presented many issues that needed to be addressed before construction began. Knik Construction, a large heavy-construction company that specializes in remote-site projects, was contracted to rehabilitate the airport. The company, headquartered in Anchorage with additional locations in Bethel, Alaska, and Seattle, works year-round in the arctic elements. Knik carefully schedules the best season and equipment for each phase of a project. In many cases, materials are moved via waterways in summer and construction is scheduled during the winter.

“Due to the location and arctic elements of the island, we had to ship the equipment and materials needed for this project via barges on the Bering Sea,” Parry Rekers, Knik vice president, said of the St. Paul Island airport runway construction project.

Knik Construction has a fleet of ocean-going and shallow-draft tugs, barges and landing craft to offer effective material and equipment transport to almost any maritime destination. Their low-ground-pressure, off-road vehicles are engineered to carry heavy loads across tundra without damaging the environment. When workers and equipment have to be moved in the winter, ice roads are constructed on frozen lakes.

Knik Construction used a foamed-asphalt process, popular in European countries, to transform the gravel runway into a cement-asphalt mix strong enough to pave over. Before Knik ran the foamed-asphalt remixer, portland cement had to be spread over the old runway in order to mix it with the asphalt. Foamed asphalt, in combination with portland cement, is used to stabilize and improve existing road surfaces and produces high-quality base courses and cold mixes at a very low cost.

Foamed, or “expanded,” asphalt is created by carefully injecting a predetermined amount of cold water into hot penetration-grade asphalt in the mixing chamber of the pavement remixing unit. The old gravel runway already contained a fair amount of moisture. Moisture control is paramount.

“Some moisture is needed for the foamed-asphalt process,” Rekers said. “Careful calculations are made with the moisture contained in the existing material to obtain optimum moisture for the foamed-asphalt process.”

Knik used a Wirtgen remixing unit, which cuts an impressive 8-ft-wide x 15-in.-deep swath. At the same time the machine mills the treated runway surface of cement, soil and rock, it injects the water and hot asphalt into the mix.

“The oil was heated to an optimum temperature of 360° in storage tanks prior to transferring to the Wirtgen,” Rekers said. “At this temperature, the best results are achieved.”

As the material was placed by the Wirtgen remixing unit, a grader smoothed the surface and a compactor compressed the material to create a solid base. The application of foamed asphalt eliminates the cure, or “break,” period, allowing traffic to use the surface almost immediately.

“This process allowed us to use in-situ material as a base, which can make the work easier and more economical,” said Rekers. “Although foamed asphalt provides a strong surface once it is cured, it is only adequate for landing planes on a temporary basis. Following application of the foamed asphalt, Knik paved the runway with a conventional hot-asphalt mix.”

Dying winds

A major concern was keeping the portland cement on the ground in the high winds. Even on a calm day, spreader trucks have a difficult time controlling dust. With prevailing winds averaging 40 mph, Knik had to find a way to prevent the fine cement from blowing away before the Wirtgen had a chance to mix it with the foamed asphalt.

In addition, concerns about health effects of airborne dusts have led to regulations establishing exposure standards for particulates that can cause lung diseases.

Since the runway project was federally funded and St. Paul Island is a protected environment, the Aleuts were particularly alert to any detrimental impacts from the project. The damaging effects of uncontrolled cement dust on the island’s environment and wildlife habitat were of major concern to Knik. Furthermore, spreading operations can produce a cloud of emissions that obscure visibility and create a safety hazard for workers trying to operate in the construction zone.

Knik Construction needed a spreader truck that would dispense just the right amount of cement to stabilize the soil. Too little additive would not supply enough material to mix with the foamed asphalt, and too much would waste money and create excessive dust. Knik consulted Stoltzfus Manufacturing of Morgantown, Pa., for a solution. Stoltzfus has developed a line of soil-cement spreaders that can accurately place cement, quicklime, fly ash and other materials for soil stabilization. Their highly regarded machines are known for their ability to spread precise amounts of material uniformly, an ability that makes them the perfect choice for the contractor trying to keep the dust and operating costs down while maintaining safety standards.

Stoltzfus was one of the few suppliers that already had considerable experience with dust control. Dust emissions from spreading operations during highway projects had become a hot-button issue with several state DOTs concerned about the health and safety effects on workers as well as the general public. Residential and commercial property owners have become more vociferous in their complaints about dust. Moreover, accidents and injuries resulting from poor visibility in highway construction zones also have caused insurance underwriters to become more attuned to the problem of dust emissions.

As early as 2002, Stoltzfus had developed a method of controlling dust as part of a joint venture with NESCO. Located in Hackettstown, N.J., NESCO specializes in the design and development of wet-suppression systems to control dust from a wide range of material handling operations. Gary Lake, president of Stoltzfus Spreaders, worked with NESCO’s Dr. Mark Kestner, the country’s leading expert on dust control for mining and material-handling operations, to design a system that would fit the bill. NESCO’s spray system had to be mobile, had to knock down the dust without wetting the aggregate and use as little water as possible.

“We knew that just dumping water on the problem was not going to cut it,” said Kestner. “First of all, adding water to a site with a stabilization problem would be like pouring gas on a fire. Secondly, this is a mobile application that can’t tow water in a trailer or stop for frequent refills.”

NESCO developed the SpreadKleen Fog System, which is able to knock down dust emissions using a powerful high-pressure fog without adding any detectable moisture to the substrate.

“We can manage the dust with very little water on board,” said Kestner. “Our system can be adjusted over a wide range of water flows and pressures to achieve just the right balance.”

“The system developed and implemented definitely solved both the dust and the wind problems for us,” said Lake. “On St. Paul Island, we were able to adjust the spray system so that it not only kept the dust down, but also put a damp cap on the portland cement that held it in place, against 40-mph crosswinds, until the Wirtgen remixer arrived. We now offer it on all of our stabilization spreaders. The system is helping our customers avoid fines and is getting the NIMBYs off of contractors’ backs. Even more important, the health and safety of the workers, local public and the environment is being protected.”

Can’t beat the spread

The St. Paul Island airport project was successfully completed in four short months. Their ability to control cement dust emissions allowed Knik to use foamed-asphalt technology to stabilize and improve the runway without risk to the environment or the health and safety of their employees.

Rekers recognized the efforts of Stoltzfus and NESCO as “contributing factors to the successful completion of this project. I definitely would recommend these products to other contractors. The SpreadKleen Fog System suppressed dust without wetting the substrate, allowing us to apply the foamed-asphalt base while protecting the environment.”