Engineers struggle to keep houses, buildings and roads from sinking
Alaska Business Monthly; February 2007
Jack Hebert is an optimist. He expects the ground under the Cold Climate Housing Research Center's new, 15,000-square-foot Research and Testing Facility to sink. And he can't wait to see what happens to the building when it does.
"One of the reasons we chose this site is that it was on degrading permafrost and we wanted to do a demonstration project that addressed building on that kind of soil," said Hebert, who is president and CEO of the CCHRC in Fairbanks.
People have been building on Alaska's permafrost since the first roads and houses were constructed here. However, just the process of building a road, for example, raises the surface temperature about 3 to 5 degrees Celsius. "You're stripping native vegetation away and you're replacing it with a nice black solar-collector," said Doug Goering, professor of mechanical engineering at the University of Alaska Fairbanks. In areas near the thawing point, such as around Fairbanks, those few degrees are sufficient to start melting the frozen ground beneath.
Over the years, various strategies have been developed to deal with building on permafrost. The Research and Testing Facility rests on a adjustable foundation that compensates for settling soils, while not a quarter of a mile away, Thompson Drive, at the entrance to the UAF campus, incorporates techniques to keep the underlying permafrost frozen.
Location, Location, Location
The best approach to building on permafrost is to avoid it. With permafrost underlying 80 percent of Alaska's land area, finding a permafrost-free site is not always easy, especially for projects on the North Slope, in Western Alaska, and in the Interior. However, some areas are more likely to cause problems than others.
"If it's what we call thaw-stable permafrost, then if the permafrost thaws, it still remains structurally strong with little or no settlement," said Billy Connor, director of the Alaska University Transportation Center (AUTC). "Most gravels are thaw-stable." In contrast, silty soils form thaw-unstable permafrost because the ice provides the structural stability. When it melts, the area turns to mud and settles. "The thaw-stable permafrost we don't have to do a lot for. We construct over it, let it thaw, and life is generally pretty good," Connor said. "It's the thaw-unstable permafrost we have to get worrying about."
Roads and Railroads
While avoiding permafrost, especially the thaw-unstable variety, is the best strategy, it is not always feasible. As a result, the Alaska Department of Transportation and Public Facilities spends about $12 million each year repairing damage due to melting permafrost and frost heave. To avoid this ongoing cost, the DOTPF has experimented with ways to keep the permafrost cool.
Putting a layer of insulation between the road and the ground has been tried with varied success. "Insulation works north of the Yukon Bridge reasonably well, and that is because the mean annual temperature is low enough that we get complete freeze back (each winter)," said Connor, who headed DOT's research program for 10 years before moving to the Alaska University Transportation Center. "But in the Fairbanks area or anything south of the Yukon River, it doesn't work very well. It slows down the thawing process but does not eliminate it."
Ironically, insulation is sometimes used in warmer parts of the state to prevent frost heave, another cold climate process that wreaks havoc on roads. While permafrost is ground that remains frozen for two or more years, frost heave results from the annual freezing and thawing of the ground. Insulation can help prevent the annual freezing.
In extreme situations, removing permafrost can be cost-effective. Such was the case on Farmer's Loop Road on the UAF campus. "At times maintenance was in there twice a day," Connor said. "We were getting three-quarters of an inch of settlement per day in that location."
The Alaska DOT finally solved the problem by digging out 20 feet of permafrost from underneath the road, and replacing it with fill. Some permafrost remains underneath the new fill, but now repairs are needed every few years rather than daily.
Rather than removing permafrost, Thompson Drive was designed to stay cold. An air-cooled embankment, or ACE, consists of an open rock matrix that allows cold wintertime air to flow through and cool the ground. In the summertime, natural convection patterns stop the flow of air, keeping cool air trapped within the matrix.
A Big Price Tag
ACEs are also well suited for railroad beds. Miles of ACE underlie the Qinghai-Tibet rail line that opened in 2005 to connect China with Tibet. Airstrips could also benefit from ACEs, but the problem is often finding or transporting the large rock.
"A lot of Alaska is in the Yukon-Kuskokwim Delta and other places where you have hundreds of miles of flats with nothing available but silt, most of which is frozen and full of ice," Goering said. "Gravel even is at a premium."
The second piece of keeping the soils under Thompson Drive frozen incorporates active cooling devices called thermosyphons. These hairpin-shaped tubes contain a refrigerant that transfers heat from the soils underneath up to the surface of the road. Building the road in winter also helped keep the permafrost intact.
The thermosyphons and ACE in Thompson Drive come with a big price tag. "Everything in that one-mile stretch costs an additional $3 million over and above normal construction practice," Connor said. For now, the cheapest solution for most of Alaska's 2,000 affected miles of roads is to keep repairing the damage, but the AUTC is working to reduce costs. For example, Connor said they have found that incorporating the ACE structure in only the top 3 feet of fill, rather than the entire 20foot depth of a road, is effective. The AUTC is also experimenting with plastic thermosyphons instead of metal.
"We might be able to get a $3 million cost to under $1 million. Now those long-term costs start looking more attractive, especially in urban areas," Connor said.
Unlike roads, small buildings are often raised off the ground on pilings, separating the ground from the heated building, and allowing air circulation to cool the ground. For larger or heavier structures, a more active solution may be required.
"If the loads are high, a lot of times we'll use a self-refrigerated pile, an Arctic Foundations pile," said Rohn Abbott, senior vice president and a geotechnical engineer with Shannon & Wilson Inc., in Fairbanks. The piles are charged with a refrigerant like carbon dioxide. "When the ground temperature is warmer than the air temperature, the gas boils and rises to the top of the steel pile where the cold air in contact with the above-ground portion of the pile causes the gas to condense into a liquid." As the carbon dioxide condenses it gives off it heat, and returns to the below-ground part of the pile to start the cycle again.
Greater cooling for even larger structures such as warehouses, hangars and fire stations can be provided by using thermosyphons that are shaped differently but operate under the same principle as those used under Thompson Drive. Insulation is also provided between the building and the ground.
Adjustable foundations are often used for small buildings such as houses or cabins, especially in the Bush. "We use a pad with an adjustable beam on top of it so as the soil settles or moves, you can adjust the structure," Hebert said. "The pad is just sitting on the soil and you can adjust the structure." Adjustability is the strategy the CCHRC chose for their new research facility.
"We took a small-scale thought about an adjustable foundation that has typically been used on unstable soils for small residences and we applied it to a commercial building," Hebert said. "Everything was done in a big way with a lot of concrete and a lot of steel, but essentially what we have is a spread-footer foundation, that is like a snowshoe, a big bearing point." A grade beam sits on top of, but not attached to, the spread-footer. A jacking column allows the spread-footer and grade-beam to be pushed apart as necessary to adjust for settling. The void created between the two foundations will be filled with structural foam, which provides insulation and bearing capacity, and will break loose when future adjustments are needed.
Soil preparations done before the Research and Testing Facility was built could help to minimize the amount of settlement. The dirt dug for a storm water retention pond was stacked on the site for a while. "It got the soil used to the weight of this kind of a building on it," Hebert said. "One and one-half times the weight of the building was surcharged on the site." The building is also thermally isolated from the ground, so the building itself should not heat the soil. Hebert said the soils engineers predict only about 5 inches of settling, but the building has been built to withstand up to 2 feet to 3 feet.
In some instances, removing the permafrost, either through excavation or pre-thawing, is an option, although it can be expensive and time consuming. Abbott said the availability of large earthmoving equipment at Fort Wainwright made it cost-effective to remove the thaw-unstable permafrost there and replace it with sand and gravel fill for a military-housing project. The excavation allowed them
to use a conventional foundation.
A Changing World
Thermosyphons, ACE, and insulation are designed primarily to reduce or eliminate the warming caused by the road or building itself. How well these strategies will work if the climate continues to warm is still unknown. Some engineers are optimistic.
"When we build a road, we raise the temperature 5 degrees instantly, and our climatologists are talking about 5 degrees over 100 years, so the more significant impact is of our own doing," Connor said. "We typically rehabilitate our roads on a 12 to 15 year lifecycle, so we can take care of any changes that occur at that point. We don't feel like we have to panic on our roadways and suddenly change the way we do business.
Buildings are a different story. "We try to design buildings for 50 to 100 years," Hebert said. Even slow melting of permafrost could have serious effects within a building's life expectancy.
Orson Smith, professor of Arctic engineering at UAA, is working on a project to estimate how climate change will affect the value of property. "Thirty to forty years out we need to recognize that these long-lived buildings and projects, bridges and things, could be vulnerable," he said. "It's not going to be like the Roman bridges in Europe. These courthouses and these schools aren't going to last as long as you might hope."
Long-term monitoring of permafrost conditions could help engineers anticipate changing conditions, but that funding is hard to come by. "Monitoring is always a problem. It lacks glamour," Smith said. "The means to avoid these problems are available, but it has to do with knowing site conditions."
by Barbara Maynard
Article (C) 2007, Alaska Business Publishing Company