Residential foundations: behavior below zero
By Frédéric Gagnon Eng., M.Sc.,
In cold weather regions, such as Canada and the northern United States, structural deterioration associated with the effect of frost is a common occurrence each winter. A large part of this damage attacks residential foundations. This situation is worrisome, because it could affect not only the foundations, but could have a negative impact on the rest of the building. The principal effects to the foundations due to frost are direct heaving, lateral pressure and freezing. Understanding how these phenomena occur enables us to avoid them by using adapted design practices.
When water freezes, its volume increases by about 9%. Thus, when the water contained in soil freezes, it causes expansion and lifting of the soil level. Freezing of the interstitial water starts when temperature decreases below zero degree Celsius. Progressive ice formation inside the pores of the soil confines residual water in the smallest free spaces. Pressure differential between the solid and liquid phases of water attracts the unfrozen water toward the freezing soil and ultimately leads to ice lenses formation. Three conditions are essential in this process. The soil has to be susceptible to frost, there has to be a sufficient quantity of water in the soil and the temperature has to be below zero degree Celsius. Soil susceptibility to heaving is generally a function of particles' size distribution. Actually, fine soils like silt and clay are more sensitive than coarse grain soils.
Speed and amplitude of heaving depend on many factors such as soil type, load to lift, underground water condition and freezing speed. Heaving forces could, in certain cases, be very important. The Canadian Research Council (CRC) mentions that lifting pressures of 1,820 kPa (185,500 kg/m2) have been recorded and that a seven-story concrete building with a raft foundation has been lifted more than 50 mm due to soil freezing. Heaving can occur under footings or slabs on grade. Figure 1 shows an example of a garage slab on grade that has been lifted by frost. In certain cases, foundations are lifted by frost, but there is no visible sign of defect. Indeed, damage is usually associated with differential foundation movements. Actually, when the heaving is uniform over the foundation, it's possible that no damage appears.
In order to avoid damage from frost heaving, the conventional approach consists in placing the footings deeper than the frost penetration level. Under these conditions, the soil below the elevation of the foundations cannot freeze and thus, there is no heaving. Depth of frost penetration varies with regions, soil nature and type of expositions and can be estimated using analytical methods. Moreover, foundation type and usage can influence the frost penetration near the structure. For example, the fact that the basement is heated or not, as well as the level of insulation are factors to be considered.
One of the principal functions of a foundation wall is to support the earth pressure. Earth or soil placed against a wall pushes horizontally on it. The pressure is distributed triangularly and it depends on the type and height of the backfill soil. Obviously, the higher the backfill is, the higher the stress will be.
When the backfill material freezes, its volume increases and causes additional lateral pressure on the wall. Generally, frost appears first on the surface before going in depth, and so the swelling is usually more prominent near the soil surface. Inversely to the normal earth pressure, one caused by frost is more important at the level of the finished soil than near the bottom of the wall.
When the resistance of a foundation wall is not sufficient to support these additional pressures, nearly horizontal cracking can appear. Cracks are generally located slightly below the level of the finished soil. Figure 2 shows a typical example of a foundation wall damaged by backfill soil freezing.
Positioning the base of the foundations under the frost penetration elevation is not a guarantee that the freezing soil will not heave the foundation. Actually, one of the mechanisms often implied is adfreezing. This phenomenon appears when humid or saturated soil adheres to a foundation unit by freezing at the contact surface. The lifting pressure caused by the formation of ice lenses in the backfill material is transmitted to foundation by adherence. These forces can be sufficient to deteriorate residential foundations.
Ice lenses can only adhere to cold foundations. Generally, heat loss by the concrete basement foundation wall is sufficient to prevent adfreezing. However, some precautions have to be taken when the foundation wall encloses a non-heated space or when there is not enough heat loss at the level of the foundations. Placement of an exterior rigid insulation or impermeable membrane can be an effective protective layer to prevent adfreezing.
As explained, when the exterior temperature decreases below zero degree Celsius, transformation of water into ice can affect building foundations by different mechanisms. It is impossible for us to control the climatic conditions, thus a satisfactory design should take these mechanisms into account. Moreover, the National Building Code of Canada (NBC) has requirements regarding the frost resistance of foundations.
Finally, damage caused by frost to residential buildings is not limited to foundations. Sometimes, freezing deteriorates decks, fences, pools and pavements. When the soil and drainage conditions are favorable to frost heaving, the quality of workmanship and design of these structures is critical.