Throughout Canada, our northern climate results in frost damage to buildings, roads, and equipment every winter. Smaller buildings and roads are particularly affected by ground movements due to frost heave. This article discusses the causes of frost heave, typical damage resulting from heaved ground and ways frost heaving can be avoided.
Everywhere in the country (except near Vancouver) mean air temperatures fall below freezing for at least several weeks each winter resulting in ground freezing. The depth of frozen soil ranges from very shallow to in excess of 2 metres in northern areas. In southern Ontario the depth of ground freezing varies from 1.0 metres near Windsor to 1.8 metres in the Ottawa area.
Ground movements of 20% to 35% of the thickness of the frozen layer (i.e. a maximum heave of 350 mm or 1 ft where the depth of freezing is 1 m) have been recorded, however these are extreme values. Movements of 50 to 100 mm (2-4 in) are more common and are large enough to cause substantial damage to foundations, slabs on grade and roads.
In the change from liquid to solid, water expands 9% in volume. In granular soils, such as sand, this volume change results in minimal, if any, frost heaving because even in a saturated soil the pore water makes up only a minor part of the total soil volume. In fine grained soils the expansion of water is also relatively unimportant when considering frost heave.
The large ground heaves are a result of ice segregation and ice lenses which form under the following conditions:
presence of frost-susceptible soil
availability of water
As the ground freezes, water is drawn from the lower unfrozen soil to the frost front where it freezes forming a series of ice lenses as the frost penetrates the ground. Slow freezing of the ground results in the largest movements since water is not frozen in place before it can move to the freezing front.
Heaving results by the ice displacing and forcing the soil upwards.
Fine grained soils, particularly silty soils are frost susceptible, as their pore structure allows the capillary movement of water but is fine enough to support ice lensing. Clays impede water flow and so the development of ice lenses is retarded.
The water source for ice lensing is usually a high ground water table. Poorly graded roads and backfills or cracked pavements can also supply the needed moisture.
To avoid frost problems, footings are usually placed below the frost level. This prevents lifting of the foundation. Exposed slabs and footings above the frost level should be placed on well drained granular soil which is not frost susceptible. Styrofoam insulation can be used to keep the soil below slabs and footings frost-free.
Adfreezing is another form of frost heave. It occurs when soil in contact with a foundation wall or pier adheres to the wall as it freezes. A strong bond is created at the interface. If frost susceptible backfill is present against the wall the heaving forces are applied to the building by the adfreeze bond.
Concrete block walls may fail in tension at about the level of frost penetration. Walls, which are not anchored to their foundation can be lifted. Distortion is caused by unequal lifting around the building. If soil falls into the cavity produced by the heaving the building cannot settle back into its original position.
Adfreezing is avoided by using a well drained granular backfill against walls.
Adfreezing also acts to push posts upwards. In a similar manner it forces boulders to the surface in fields.
Before any movement can occur the heaving forces must exceed loads on the building walls and foundations. Light weight buildings such as storage sheds or cottages are most susceptible as only relatively small pressures are needed to shift them. Very high pressures can develop, however, and one report details a seven storey concrete building which was heaved 2 inches when frost penetrated under the foundation.
Heaving often occurs in basement garages kept just above the freezing temperature. When temperatures dip as a result of a cold snap or because the door was left open, the underlying soil freezes. As a result, shallow footings and concrete slabs are heaved.
To conclude, frost heaving can be avoided by removing one of the 3 factors necessary for its foundation.
The use of:
footings below the depth of frost penetration or insulation for shallow slabs and footings
will prevent damage due to frost heaving.
The damage to roads and sidewalks due to frost heave and subsequent thawing will be discussed in an upcoming issue of Leading Edge.
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