Resource: How to Build A Road

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Polar regions are cold year-round because sunlight strikes the Earth at a very low angle in latitudes above 65°. They are also dry. Winds generated in polar regions blow toward the Equator, keeping moister air from entering.

Influenced by cold, dry polar air masses, land that lies between 55° and 65° north latitude has a subarctic climate characterized by severe winters and brief summers. Annual temperature can range from -50°C to 12°C (-58°F to 54°F) and create conditions responsible for seasonal frosts, or frosts that form in the top layers of the soil and persist only during colder months. Underlying this so-called active layer is a layer of persistently frozen soil called permafrost.

Permafrost, which covers about 20 percent of Earth's land area, is soil that has been frozen for two or more years. It consists of large cavities of pure ice and ground ice, or ice that forms in pore spaces and bonds sediments in the soil. In the highest latitudes, the soil remains permanently frozen. In subarctic climates, however, frozen soils hover within a degree or two of freezing. Locations where warmer temperatures cause frozen soils to thaw to greater depths are part of what is called the discontinuous permafrost zone.

Working in the discontinuous permafrost zone can be problematic for road-building and maintenance crews. When ice-rich soils thaw, they can weaken the surrounding land so that it may shift or subside, together with any overlying structures. In addition, because frozen soil beneath the active layer creates a drainage barrier, melt water tends to pool and create boggy terrain that is difficult to traverse on foot or by vehicle.

Although climate largely determines the thickness of permafrost, other environmental factors such as vegetation density, depth of snow cover, and soil composition can influence ground temperature. So can humans. Removing natural insulation like vegetation for land development, paving roads and lots with asphalt, and heating buildings all warm the ground.

Whereas earthquakes and landslides are sudden ground system collapses, the shifting earth associated with permafrost goes almost unnoticed. However, permafrost shifts are just as capable of ruining roads, houses, and other man-made structures. To preserve permafrost in its more stable frozen state and allow for more sound construction in the discontinuous zone, engineers have been developing ways to prevent the subsurface from warming. For example, some newer buildings have piling systems that use convection to siphon heat away from the ground.

To learn more about frozen soils, called gelisols, including how widely they are distributed over Earth's surface, check out Soils Around the World.

To learn more about the plants and animals that live in the frozen tundra biome, check out Biomes.

Print Questions for Discussion

• Describe the "discontinuous permafrost zone" and why it is so difficult to build on.
• Muskeg produced some of the most time-consuming and difficult problems in building the Alaska Highway. Explain what it is and how the Army Corps of Engineers solved the building problems over this difficult terrain.
• Another problem along the highway was permafrost. In some parts of Alaska, towns are built atop permafrost. Explain why houses built on permafrost would need to have their basements insulated as well as their walls and roofs.
• What happens to the volume of water when it freezes? What is the impact on highway surfaces? How is water kept from collecting under road surfaces where you live?