Permafrost in Construction
Permafrost is the soil or rock with or without water that continuously remains at a state of 00 C (320F) or below for two years or more. It is mostly found in subzero Polar Regions and high-altitude mountain ranges. Building construction on permafrost is quite different from normal construction methods that we adapted in other regions.
Detailed geotechnical and geological tests and studies should be carried out in permafrost for design of foundation and structures. It should be noted that change in atmospheric temperature and pressure highly influence the construction and materials used.
Unplanned construction on permafrost or careless disturbance of permafrost causes melting and that may lead to uneven settlement of foundation or structure. Permafrost is important for artic life cycle. The water accumulates on the surface of the soil does not infiltrate and during summer the plants and animals in Polar Regions rely on this accumulated water.
What are the design considerations for construction on permafrost?
The main factors that should primarily considered while designing a building in permafrost areas are:
- Temperature changes
- Type of soil
- Ground ice characteristics
The permafrost is highly sensitive to temperature changes. Permafrost is good for foundation if it is in a frozen state. The changes in the ground surfaces like melting of the top layer of soil may affect the rigidity of permafrost. Also a building can chance the temperature cycles in permafrost. Considering these risk factors elevated foundation or pile foundations are adopted to prevent the impact on ground from thawing.
Being always in frozen state the permafrost doesn’t permit the moisture drain into the soil. During summers (or small rain) the top layer ice gets melted and water remains on the surface ground and therefore a proper channel should be made to clear off water from ground surface to prevent water accumulation.
Type of soil
The ice content varies with the type of soil. Solid rocks, gravel soil, and sandy soil normally contain less amount of ice, so the thawing really doesn’t cause soil settlement. But in silty or clayey soils contain high amount of water or ice content and thawing may lead the slurry formation and reduces the bearing capacity of soil.
Large ice masses are connected to the sub terrain soil masses of silt or soft soil in permafrost. The silt soils have good water retention characteristics. If they melt down in seasonal changes, it may leave large holes or pits on the ground. This can create uneven ground condition unfits for structures.
How to maintain structures in permafrost?
Maintaining structural stability of a building on permafrost is important after construction. Pile foundations are mostly adopted in permafrost. The heat exchange from the foundation to permafrost causes thawing. This will adversely affects the structural stability of building. To solve this issue, following methods are adopted:
Maintaining structural stability by freezing the ground
The foundation is raised from the ground to maintain a gap between building base and ground. The air flow through the gap makes a ventilation affect and cools down the ground surface from thawing because of the heat transfer from the building.
Thawing of perennially frozen ground
In this method the foundation is put much deeper to the permafrost. Thawing of the ground is allowed for a pre-defined depth and the building is founded on the ground. Very close control of thawing is needed over upper ground layer.
Freezing the ground for construction on permafrost
The unfrozen ground or lose ground is frozen by artificial means. This method is highly expensive and time consuming. This method is used in hydro technical construction (such as Dam’s construction).
Freezing the ground during construction time and operations
Cooling systems are installed in the ground at a depth of 0.5m after the construction of foundation. The freezing of ground causes the increase in its volume and creates passive horizontal pressure, and that pressure holds up the foundation from settlement.
Types of Foundation adopted in Construction on Permafrost
As mentioned earlier wooden posts (piles) or piles are used as foundation in permafrost. These wooden posts or wooden piles are commonly used in small structures like wooden buildings or other light weight structures.
For heaver structures reinforced concrete piles are used for construction on permafrost. And used in single or pair of piles according to the design. If the pile diameter is less than the diameter of the bore hole, the excess gap is filled with mortar or mud and that freezes later. If depth increases, Thermal piles are used. Thermal piles stabilize the permafrost by its heat exchange action.
In some northern regions gravel pad of 4 to 6 foot length is used as insulation layer (also as foundation) in between the permafrost layer and the active layer. Sometimes the gravel is used to raise the construction ground or dig down the ground upto the permafrost layer and replaces the soil with gravel. The gravel pads act as an isolator and reduce the heat transfer from the buildings. This method is used in road construction and for light weight buildings.