What Does Permafrost Mean In Construction?
What Does Permafrost Mean In Construction?
Permafrost is a type of soil or rock that stays below 0°C for a minimum of two consecutive years. It is primarily found in the frigid Polar Regions and high-altitude mountain ranges. Building construction in these areas requires a different approach compared to traditional construction methods due to the unique conditions.
Before any construction work begins, extensive geotechnical and geological assessments must be conducted to ensure the proper design of the foundation and structures.
It’s crucial to keep in mind that changes in temperature and pressure can significantly impact the construction and materials used. Unplanned construction on permafrost or careless disturbance of it can result in melting, leading to uneven settling of the foundation or structure.
Permafrost is a crucial aspect of the Arctic ecosystem, as the water that accumulates on the soil’s surface provides a vital source of hydration for plants and animals during the summer months.
Principles Of Construction On Permafrost Bases And Foundation
The way in which foundations interact with the ground is a crucial aspect of construction, and this interaction is influenced by the type of soil present. In areas outside of permafrost regions, it is generally believed that the weight of a structure is transmitted through the base of the foundation to the bearing ground.
In contrast, in regions with deep seasonal freezing or permafrost, the weight is transmitted through all the ground surfaces that come into contact with the foundation. This is because the foundation and ground bond together through freezing, which results in the transmission of tangential and normal stresses from the ground to the foundation and vice versa.
The direction of these stresses can change over time and is dependent on the layer in which the foundation is situated. In the layer of seasonal freezing, tangential stresses transmitted from the ground to the foundation arise during the freezing and heaving phase, and are directed upwards.
In the layer of perennially frozen ground, the frozen ground carries some of the load from the foundation, not only through the base, but also through the sides of the foundation.
This increased foundation resistance to heaving during seasonal thawing is due to the freezing between the lower part of the foundation and the perennially frozen ground.
The stability of the foundation is determined by the combined effect of normal and tangential forces, heaving forces, and sometimes frictional forces. This can result in uniform or non-uniform movement of foundations.
What Are The Design Considerations For Construction On Permafrost?
When constructing in permafrost areas, there are several key considerations to keep in mind.
The first is the impact of temperature changes on the permafrost. As the permafrost is highly sensitive to changes in temperature, it is important to take into account how a building could alter the temperature cycles in the permafrost and how this may affect its stability. Elevated foundations or pile foundations are often used to mitigate the risk of thawing.
Another important consideration is drainage. As permafrost is always frozen, it does not allow for moisture to drain into the soil. During the summer or after rain, the top layer of ice may melt and water may accumulate on the surface, creating the need for proper drainage channels to prevent water accumulation.
The type of soil is another factor to consider when designing a building in permafrost areas. Soils with high ice content, such as silty or clayey soils, may lead to slurry formation and reduced bearing capacity upon thawing. In contrast, soils with low ice content, such as solid rocks, gravel or sandy soils, are less likely to be impacted by temperature changes.
Finally, it is important to be aware of ground ice in permafrost areas. Large ice masses in the permafrost can be connected to sub-terrain soil masses of silt or soft soil, which have good water retention characteristics. If these soils melt in seasonal changes, it may result in large holes or pits on the ground, creating an uneven surface that is unsuitable for structures.
Maintaining Structures In Permafrost
Ensuring the stability of buildings built on permafrost is crucial post-construction. Pile foundations are commonly used in these areas, but the heat transfer from the foundation to the permafrost can cause melting and impact the structural stability of the building. To address this issue, the following solutions are employed:
Raising the foundation from the ground to create a gap between the building’s base and the ground. This gap allows for ventilation which cools down the ground and prevents melting from the heat transfer from the building.
Placing the foundation deeper into the permafrost, allowing the ground to thaw to a pre-defined depth and constructing the building on the ground. Close monitoring of the upper ground layer is necessary to prevent excessive thawing.
Freezing the loose ground or unfrozen ground artificially, although this method is both expensive and time-consuming, it is used in hydro-technical constructions like dam building.
Installing cooling systems at a depth of 0.5m after the foundation is built. The freezing of the ground increases its volume and creates passive horizontal pressure, which supports the foundation and prevents settling.