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Concrete footings 101
Bearing capacity of soil
Understanding soil type and bearing capacities
Footing size
How to determine the minimum size for soil conditions
Footing problems
Pouring in wet soil and more
Frost heave & foundation footings
Frost protected shallow footings
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In most parts of the north United States the ground freezes during the winter months to a depth of several feet. Such ground freezing can lead to heaving of buildings located above or adjacent to it. The forces involved can be very destructive to lightly-loaded structures and cause serious problems in major ones.

How Frost Heave Works

The volume increase that occurs when water changes to ice was at first thought to be the cause of frost heave, but it is now recognized that the phenomenon known as ice segregation is the basic mechanism.

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Water is drawn from unfrozen soil to the freezing zone where it attaches to form layers of ice, forcing soil particles apart and causing the soil surface to heave. Without physical restraint there is no apparent limit to the amount of heaving that may occur. (Movements in excess of 4 in. developing under basement floors in only three weeks have been recorded.)

Where restraint in the form of a building load is present, heaving pressures may or may not overcome the restraint, but they can be very high: 19 tons/sq ft has been measured, and a seven-story reinforced concrete frame building on a raft foundation was observed to heave more than 2 in.

A different form of frost action, called "adfreezing," occurs when soil freezes to the surface of a foundation. Heaving pressures developing at the base of the freezing zone are transmitted through the adfreezing bond to the foundation, producing uplift forces capable of appreciable vertical displacements. If constructed of concrete block a basement wall may fail under tension and part at a horizontal mortar joint near the depth of frost penetration.

Controlling Factors

For frost action to occur three basic conditions must be satisfied: the soil must be frost-susceptible; water must be available in sufficient quantities; and cooling conditions must cause soil and water to freeze. If one of these conditions can be eliminated, frost heaving will not occur.

Frost-susceptibility is related to size distribution of soil particles. In general, coarse-grained soils such as sands and gravels do not heave, whereas clays, silts and very fine sands will support the growth of ice lenses even when present in small proportions in coarse soils. If frost-susceptible soils located where they will affect foundations can be removed and replaced by coarser material, frost heaving will not occur.

Water must be available in the unfrozen soil for movement to the freezing plane where the growth of ice lenses occurs. A high groundwater table with respect to the location of the ice lenses will therefore favour frost action. Where proper drainage is prescribed water can be prevented from reaching the freezing zone in frost-susceptible soils.

Depth of freezing is largely determined by the rate of heat loss from the soil surface. Besides the thermal properties of the soil, this heat loss depends upon such climatic variables as solar radiation, snow cover, wind, and air temperature, which is the most significant. If loss of heat can be prevented or reduced, frost-susceptible soils may not experience freezing temperatures.

Freezing Index and Frost Depth

Air temperature records can be used to gauge the severity of ground freezing by using the degree-day concept. (If the daily mean air temperature is 31F this will be one degree-day.) The "Freezing Index" is simply the accumulated total of degree-days of freezing for a given winter.

Frost Action and Foundations

The conventional approach to the design of foundations to prevent frost damage is to place the foundation beyond the depth of expected maximum frost penetration so that the soil beneath the bearing surface will not freeze. This measure alone, however, does not necessarily prevent frost damage; if the excavation is backfilled with frost-susceptible soil it may lead to damage from adfreezing. Depths at which foundations should be placed are normally determined by local experience, as incorporated in building bylaws, but in the absence of such information the correlation shown in the preceding chart can be used.

By their very nature frost-susceptible soils do not drain well, and though inflow of groundwater may be prevented the quantity of water available in the unfrozen soil is often sufficient to produce significant heaving. Where possible it is good practice to remove frost-susceptible soil and replace it with coarse granular material that is easy to drain. Good drainage practice should also be followed, including the provision of drainage tile around the perimeter of the foundations.

Importance of Drainage

Good drainage is important with any foundation and FPSF is no exception. Insulation performs better in drier soil conditions.

Ensure that ground insulation is adequately protected from excessive moisture through sound drainage practices, such as sloping the grade away from the building. Insulation should always be placed above the level of the ground water table. A layer of gravel, sand, or similar material is recommended for improved drainage as well as to provide a smooth surface for placement of any horizontal wing insulation. A minimum 6-inch drain layer is required for unheated FPSF designs. Beyond the 12-inch minimum foundation depth required by building codes, the additional foundation depth required by an FPSF design may be made up of compacted, non-frost susceptible fill material such as gravel, sand, or crushed rock. Additionally, adding free-draining backfill helps to minimize or eliminate frost heave potential

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