Permafrost and Foundation Types
Almost all northern housing is built in areas where conventional foundation systems are not appropriate. Frost heave and differential displacement are major problems that can cause racking and subsequent damage to the foundation, building frame, window and door openings, and finishes of a building. The loss of airtightness and the constant need for repairs mean costly maintenance if an appropriate foundation system is not properly installed.
The following briefly describes ground conditions in the North and their relevance to foundation design, and presents several foundation types.
Permafrost is soil that remains frozen throughout the year. It can occur as large continuous areas of frozen soil, several hundred metres thick, or it can occur in scattered patches surrounded by soil that experiences normal freeze-thaw cycles. The latter situation is termed "discontinuous permafrost". In all cases, permafrost is overlain by an "active layer" of soil that experiences normal freezing and thawing during the seasons. This active layer can vary in thickness from a few centimetres to several metres.
Frost heave is created by the freezing and thawing of water in the soil. Ground surface movements of 5-15 cm are common and movements of as much as 30 cm have been observed during a freeze-thaw cycle. Differential frost heave can cause severe distortion of structures which may not return to their original position at the end of the thaw season because the underlying voids have been filled with soil or trapped water.
Permafrost soils must be kept frozen for the soil not to lose its bearing capacity. When soil has a relatively high water content in the active layer, measures must be taken to freeze the soil and keep it frozen. When soil contains massive ice deposits, significant creep deformations will occur with time; extra care must be taken in the design of the foundation if this is the case.
Buildings transmit heat to the underlying soil through convection and radiation. This can thaw frozen soil, leading to differential displacement of the foundation.
There are two methods of building on frozen ground: the passive method and the active method. Passive methods include ventilation and insulation techniques. By placing a thick compacted granular pad on top of the frozen ground and raising the structure above the surface of the pad to provide a cold air space, ventilation allows heat to dissipate from the structure above while the soil remains frozen. The floor of the structure should be well insulated to prevent excessive heat transmission to the ground. The other passive technique is to insulate the structure so that heat does not escape to the underlying soil.
The active method means designing or locating the building so that it won't be affected if the soil does thaw. This approach can be used if the foundations can bear on a stable layer at a shallow depth. Stable layers include bedrock or a dense, thaw-stable soil layer. The active method can also be used when total or differential settlement of the foundation soils do not exceed tolerable limits and the thawing soil will maintain adequate shear strength.The choice of method must include consideration of both current and future uses of the site. Care must be taken to ensure that the foundation design of one structure will not have any detrimental thermal effects on the foundations of adjacent structures. For the construction of larger structures, it is a good practice to install temperature, movement and other instrumentation during construction to monitor the performance of foundations. Information collected will allow the validity of design assumptions to be checked and warn of any development requiring attention.
Poured Concrete Foundation Walls
Poured concrete foundation walls can be used in non-permafrost areas or where bedrock is near the surface to provide a stable bearing layer. This type of foundation allows for the installation of a basement or crawlspace. Its use is limited to areas which have access to sand and proper aggregates and is recommended only where soils are well drained. It is difficult to provide adequate insulation to poured concrete foundations in cold areas.
Preserved Wood Foundations
Preserved wood foundations can be used in areas where it is difficult to prepare concrete. Like poured concrete foundations, they allow for a crawlspace or basement. They can only be used in non-permafrost regions with well-drained soils. Advantages are: they can be built during cold weather; they are suitable for areas where concrete is not available or is expensive; ordering and shipping of materials is easy; they can be built by a standard framing crew using standard methods; and they are easy to insulate to high levels. They do, however, require careful levelling of excavation, bracing to resist earth pressures, and enough supervision to ensure that pressure-treated wood and proper fasteners are used.
Pads and Wedges
Pads and wedges are a type of foundation in which timbers are laid in alternating layers on a prepared gravel base or exposed rock outcrop (see Figure 1). They are used on soils that have low bearing capacities, and have been widely used in northern permafrost areas for small buildings which are not likely to suffer damage from a reasonable degree of movement. Preserved wood should be used in the construction of the pads and wedges.
Wood Cribs and Drum Footings
Wood cribs and drum footings are set into excavated holes in prepared gravel pads and filled with coarse gravel or rocks (see Figure 2). They are a more permanent solution than single pad and wedge systems. This system is appropriate for underpinning and stabilizing existing pad and wedge systems that have been undermined. Pressure-treated wood is recommended for cribs.
Pilings are a type of foundation in which a pile or post is driven or drilled into the ground to a solid bearing level or bedrock, or set into permafrost to provide structural support (see Figure 3). They are made of wood, concrete or steel, and are used at sites with soil of low bearing capacities and in permafrost applications. Some advantages of pile foundations include: they have a small cross-sectional area, which minimizes heat transfer from the building to the soil; they can be drilled or driven into frozen permafrost, which ensures stable support even if the soil in the active layer has low bearing capacity; there is minimal differential movement; and they can provide anchorage against wind uplift. Lifting forces may occur due to annual freeze and thaw cycles but this can be overcome by a combination of proper anchorage in the permafrost and the incorporation of a "slip sleeve" arrangement at the active layer. Disadvantages with this system include difficulties in drilling during the thawing of the active layer, the expense of skilled labour, and the expense of transporting and maintaining heavy drilling or pile driving equipment in remote areas.
Grade beams are strip footings of concrete or laminated wood laid on a prepared gravel base. Strip footings provide a greater bearing area than pads or wedges; therefore they are less susceptible to racking. Independent rows of strip footings allow for more movement of the soil than a continuous perimeter grade beam. Grade beams can be susceptible to uneven settlement if the gravel base is not extremely stable. The relatively high thermal conductivity of the foundation can increase the risk of thawing permafrost. No heavy equipment is required, but the system is relatively expensive because of its reliance on concrete or laminated wood and specialized labour.
Footings or Pier Foundations
There are several types of footings or pier foundations: individual footings carrying one column or post, combined footings carrying more than one column or post, and continuous or strip footings carrying a wall. Individual footings are preferred to continuous footings in permafrost areas since there is less structural damage if there is movement, and the footings can usually be individually adjusted to correct for the movement. Piers are square or cylindrical units with large bases to distribute the loads for an acceptable bearing pressure. These foundations are very stable provided they are formed within the permafrost, or above the permafrost on thaw-stable soil. If formed within the permafrost, provision must be made to ensure that the ground below the footing does not thaw. Protection must be provided against frost heave on piers and columns, and consideration must be given to the long-term bearing capacity and creep settlement of the frozen foundation soil.
Multi-Point Spaceframe Foundations
Multi-point spaceframe foundations are designed to withstand major shifting and settling of the soil while maintaining a level plane for the building. The spaceframe consists of metal tubes, called chords, and a connector, called a hub. Mechanical joints are made by inserting the chords into slots in the hubs. The spaceframe is a grid of squares in the top layer, separated from a lower grid by chords placed at diagonal angles. It acts like a floating slab. For further information on this type of foundation, refer to A Three-Point Spaceframe Foundation for Houses in the North.
Monocoque StructureIn areas where foundation movement cannot be avoided, a monocoque structure can be used. This method of construction uses plywood sheathing and rigid connections between walls, roof and floor. Four pad footings minimize the effects of differential movement of supporting soils and reduce the area exposed to permafrost. For further information on the Monocoque Structure refer to Monocoque Structure for an Arctic House.
Numerous types of foundations can be used in the North, but their design requires knowledge and due consideration of the existing ground conditions, an awareness of surrounding structures, and good knowledge of the advantages and disadvantages of each type.
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