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Canadian Building Digest, 1962-02
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Ground freezing and frost heaving
Penner, E.
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Canadian Building Digest
Division of Building Research, National Research Council Canada
CBD 26
Ground Freezing and Frost Heaving
Originally published February 1962
E. Penner
Please note
This publication is a part of a discontinued series and is archived here as an historical
reference. Readers should consult design and regulatory experts for guidance on the
applicability of the information to current construction practice.
Frost damage to building foundations, retaining walls, driveways, walks and similar structures
is common throughout Canada, and although it is not equally serious in all areas the resultant
cost each year is high. This Digest contains a brief description of the physical processes
involved in ground freezing and frost heaving and some suggestions on ways to prevent or
diminish frost damage to various structures.
The results of frost heaving have been observed from earliest times. Swedish literature dating
back to the 17th century indicates that the uplifting of boulders in the field and the breaking of
plant roots in the winter were associated with frost heaving. At first, frost heaving of the soil
was thought to result from the expansion of water on freezing. The present concept is that
growing ice crystals draw water from the surrounding soil and develop into ice lenses.
Ground Freezing and Frost Penetration
When wet soil freezes, the main process is the physical change of soil water from liquid to solid
that turns the soil into a hard mass resembling concrete. Its relatively high strength can be
attributed in part to the binding together of soil particles with ice. In a porous body like soil,
water exists in a network of inter-connecting pores; when it freezes, this network becomes
rigid and encloses the soil particles in a solid block. If the soil is dry it cannot "freeze" in the
accepted sense although its temperature may be well below 32°
It has been found that all the water in soil does not freeze at the same temperature. In studies
with a saturated silty clay half the water remained unfrozen at 28°F; 1/6 was still unfrozen at -
4°F. Because all soils have a similar freezing pattern, it is not surprising that the strength of
frozen soil increases as the temperature is lowered and more water freezes. It bas been shown
recently that the strength of heavy-textured soils increases 3 or 4 times as the temperature is
lowered from 18 to 0°F.
The rate at which soil freezes is dependent upon its thermal properties, moisture content, and
the ambient air temperature. Of these, probably the most important is the amount of water to
be frozen, since it requires 144 heat units (Btu) to freeze each pound of water and by
comparison only about 0.20 heat units to change the temperature of a pound of dry soil by 1°F.
The density, conductivity of the soil particles and water content all influence the over-all
thermal conductivity of soil. Because clay particles have a higher insulation value than silt or
sand particles and since clay soils normally hold more moisture than silts and sands, the depth
of frost penetration is usually greater in silt and sandy soils (light-textured soils) than in clays
and silty clays (heavy-textured soils).
There are other factors that influence the depth of freezing. The insulating effect of snow
deserves special mention. It bas been shown that each foot of undisturbed snow reduces the
depth of soil freezing by approximately the same amount. Among meteorological factors such
as air temperature, sunshine, precipitation, and wind velocity, air temperature is probably the
most significant.
The use of "degree-days of freezing" as a guide in calculating frost depth for a given area
illustrates the strong influence air temperature has on soil temperature. A degree-day of
freezing results when the mean outside air temperature for one day is 1F deg. below 32°F. For
example, if the average air temperature for a given day it 31°F this is one degree-day of
freezing. The "freezing index" is simply the total number of degree-days of freezing for a given
winter.
The use of the freezing index to predict the depth of frost penetration must be used with
caution since it is based only on air temperature and does not take into consideration other
factors such as soil type, snow cover and local climatic differences. In areas where no actual
frost penetration information is available, the freezing index is a useful guide. Figure 1 shows
the freezing index plotted against depth of frost penetration as determined from an analysis of
many records of frost penetration in the northern United States. This design curve was
developed by the U.S. Corps of Engineers and is used as a guide to the depth of frost
penetration in the design of airport pavements. A "freezing indices" map of Canada has been
prepared by the Department of Transport and may be obtained from the Division of Building
Research in a paper describing its use (NRC 3573).
Figure 1. "Design Curve" of observed frost penetration in excavations.
Frost Heaving
In many cases where ground freezes no outward change is visible, although as indicated earlier
the strength of the soil will be increased. In other cases, however, the ground heaves and the
resultant displacement of the soil may cause considerable damage. The actual vertical
displacement is far in excess of the expansion that occurs when water freezes. Heaving occurs
when the right combination of fine grain soil, soil moisture and soil temperature exists.
As the mean air temperature drops in the fall of the year, the surface of the ground will freeze.
With the lower air temperatures of approaching winter, the freezing plane slowly penetrates the
soil. In a fine-grained moist soil a peculiar phenomenon occurs. At the freezing plane, the water
in the soil turns to ice. This is, in effect, a drying action and water in the unfrozen soil beneath
moves toward the freezing plane in the same way that water will move from moist soil to dry
soil. This water, on reaching the freezing plane, is able to flow through and around the soil
particles there and to join the ice crystals above, thus adding to the growth of a lens or layer of
pure ice. Pressure is developed so that the ice and soil above it are lifted.
When there is an adequate supply of water to the freezing plane in soil of the proper type the
ice lens can grow almost indefinitely. At the same time the freezing plane is prevented from
penetrating further into the unfrozen soil because of the heat made available from the water as
it freezes.
In practice, the freezing plane seldom remains stationary for any prolonged period; the supply
of water may decrease or the rate of heat loss may increase due to a change in conditions. The
balance between the heat from freezing of the water and the heat loss to the surface is then
disturbed, and the freezing plane advances until the conditions for growth of a new ice lens are
restored. This results in the formation of a series of ice lenses separated by layers of frozen
soil, and is the most common situation in nature.
Ice lenses frequently develop in the soil under road surfaces and cause them to heave. As
thawing proceeds downward from the surface in the spring, these ice lenses thaw and
contribute water to the soil. In some cases the water that has accumulated as a result of the
ice lens formation and subsequent melting is sufficient to cause the soil to lose strength, and
the action of traffic may cause the paved road surface to break, through loss of support.
The expansion of soil from the formation of ice lenses varies over a wide range, but vertical
movements of 4 to 8 in. are not unusual and as much as 24 in. has been reported.
Heaving pressures also vary over quite wide limits and depend mainly on the type of soil and
its moisture content. A saturated soil will develop the maximum heaving pressure; as the
moisture content drops, heaving pressure drops also and is reduced to zero in a soil with low
moisture content. The type of soil has an influence, with clay soils developing higher pressures
than silts. Pressures in excess of 14 psi have been measured, and in a laboratory experiment a
pressure of 213 psi was developed in a clay soil. Pressures of this order are much in excess of
the pressures found under roadways or under the footings of most buildings, so that these
structures can be heaved quite readily when conditions are appropriate for ice lens formation.
No heaving can take place, however, unless the heaving pressure exceeds the load on the soil.
The three basic requirements for frost heaving are: 1) a freezing plane in the soil; 2) a fine
grain soil through which moisture can move; and 3) a supply of water. If any one of these
factors can be controlled, frost heaving can be prevented. Since it is seldom economically
possible to control soil temperature, frost heaving is usually prevented by replacing the fine
grain soil with a coarse granular material. Soil moisture can also be controlled by careful
attention to drainage, so that the extent of frost heaving is greatly reduced.
The Nature of Frost Heaving Soils
In a site investigation for a building project it is often necessary to determine whether ice
lenses will form in the soil. This may be very difficult to determine if the soil is at the borderline
between frost-heaving and non-frost-heaving material. The characteristics of a soil with
extensive frost heaving ability are well known, as are those of a non-frost-heaving soil. The
difficulty arises where there is a blending of both frost-heaving and non-frost-heaving soils.
The size of the particles in a soil has a marked influence on its properties, and this
characteristic is often used to assess the heaving potential. The determination of particle size is
relatively easy since most testing laboratories have facilities for making this analysis.
While a prediction of ice lensing based on the particle size of the soil is widely used, there are
many cases where frost heaving has occurred in soils considered safe after an examination of
particle size. Attempts have been made to use some other property, such as the height-of-
capillary-rise, that more adequately describes the frost-heaving ability of a soil. Although this
type of test is more difficult, the results provide a more realistic indication of frost heaving
characteristics, giving an indirect measure of the size and distribution of soil pores.
A theory now held, based on the correlations between pore size and heaving pressures, is that
the smaller the pore size the, greater the pressure. The way in which pore size distribution
affects the heaving pressure is being investigated.
In general it can be said that coarse sands and clean gravels do not heave, while fine sand and
silts are very susceptible to heaving. Clays also are very susceptible although they normally
heave slowly but often with tremendous pressures. Silts show a high rate of heave but have
much lower heaving pressures. When silts, sands or gravels are contaminated with clay,
however, heaving ability is usually much enhanced and becomes less predictable.
At present the most reliable method of spotting a frost-heaving soil is to carry out a laboratory
freezing test, although soils that show frost heaving in the laboratory do not always do so in
the field. The test is therefore apparently on the safe side, but further research is required
before completely reliable predictions can be made.
Prevention of Frost Damage
Frost heaving is not usually a problem in heated structures since the heat loss from the building
keeps the temperature in the soil adjacent to the foundation above the freezing point.
Difficulties often arise, however, in unheated detached buildings or in unheated additions to
heated buildings. Damage also occurs to roads, sidewalks and shallow underground service
lines.
A detached unheated building located on frost heaving soil may show no signs of distress owing
to the fact that the foundation has been raised uniformly so that no stresses have been induced
in the structure. Because of the non-uniformity of soil and other factors such as variable snow
cover, it is more usual, however, to have differential heaving. This may also occur where the
building has supports carried on footings located inside the structure. Due to the protection
provided by the building, the penetration of frost under the interior column footings may be
less than that under the perimeter footings. Under these circumstances there is a possibility
that differential movement will occur.
If conventional foundation walls and footings are used for detached unheated buildings, the
footings should be located below the level of maximum frost penetration. In such cases the
backfill should be carefully selected and well drained. If this is not done, frost heaving in the
backfill may occur that will lift the foundation wall because of the adhesion of the soil to the
wall.
Where a detached building is located on a concrete slab on grade, protection will be provided
by placing the slab on a mat of coarse granular material, which will act as a buffer against any
movement of the soil under the mat. A mat 12 to 18 in. in thickness is usually adequate.
Unheated additions to buildings located on frost heaving soils are often damaged if their
foundations do not extend below the frost level. This is due to the fact that some or all of the
foundation of the addition is beyond the influence of the heated structure. In such cases frost
penetrating below the shallow foundations will cause heaving that will result in a racking of the
addition. Because of this danger, additions should have foundations extending below frost line
with suitable backfill to prevent lifting of the foundation walls.
Retaining walls can be protected from being forced out of line by backfilling behind the wall
with clean granular fill material and providing weep holes for drainage at the bottom of the
exposed wall.
While driveways can tolerate some differential movement, particularly when a flexible covering
such as asphaltic concrete is used, this movement should be kept to a minimum to avoid
cracking and subsequent entry of water into the subgrade. Normally it is desirable to have a
uniform subgrade to reduce differential heaving. This will often require a special mixing of the
soil at the site. An addition of 6 in. of clean granular fill will provide added support for the
covering during the thawing period if subgrade softening occurs in the spring from the melting
of the ice lenses.
Run-off water from buildings should be directed away from critical areas by proper landscaping
around the building. This will, at the same time, provide better subgrade drainage, which. is
particularly important for driveways when only a thin layer of granular subbase material is
used.
Frost heaving can be prevented if the soil temperature or the soil moisture content or the soil
type can be controlled. Where differential movement cannot be tolerated, it is usual practice to
replace the soil. Good drainage will reduce the extent of frost heaving, but it is usually not
possible to lower the soil moisture content by drainage alone to a point where heaving is
entirely eliminated.
While heated structures have little to fear from frost action, this does not mean that the depth
of their foundations should be decreased. A foundation located below the frost line will also, in
most parts of Canada be in a region of uniform soil moisture content throughout the year. This
can be as important a consideration in the design of a building as are the provisions to prevent
frost heaving.