- FROST HEAVES: Cause and Prevention
- Real Estate
- Preventing Frost Heave In Your Garden
- What is Frost Heave?
- Protecting Your Plants from Frost Heave
- What is frost heave and how does it affect ground-mount solar projects?
- CBD-26. Ground Freezing and Frost Heaving
- Frost Heave
- How It Works?
- Understanding and Preventing Frost Heave
- In every issue you’ll find…
FROST HEAVES: Cause and Prevention
Frequent temperature cycles above and below freezing cause water near the soil surface to freeze, expand, and pull up more water from underground. This causes desiccation underground and, due to pressure, compaction. The ice layers near the ground surface create pressure that displaces soil, rocks, hardscape material and plants (perennials, trees and shrubs). This can expose plant roots to cold air and cause their desiccation, which can sometimes lead to the death of the plants.
What to do:
Frost heave usually starts in natural dips in soil. Keep an eye on those locations and rake them out and add compost wherever you see cracks where roots could be exposed. If the ground allows, you can try tamping dislodged plants back into place. You can also add a layer of mulch, like pine needles or straw, on top of the soil after frost has occurred to insulate it from further freeze-thaw cycles.
When planting a new plant, make sure to plant it correctly by ensuring that the soil level of the garden matches the soil level of the pot that the plant came in.
Author: Veronica Lanz, MG, Vancouver Chapter
Freezing temperatures can cause a number of heartaches for homeowners, ranging from winter heating costs to burst pipes. They can also lead to a problem you may not notice until the spring: damage caused by frost heaves.
Frost heaves occur when water in the soil freezes, causing it to expand and put pressure on parts of your home. Debra Judge Silber, writing for Fine Homebuilding, says that not only will ice crystals expand, but capillary action and vapor diffusion will draw more water up into the frigid zone. This moisture freezes into ice lenses, which put upward pressure on the soil and anything built into it.
This movement of soil can affect your home in a number of ways. Polli Construction, a company in South Burlington, Vt., says frost heaves may put enough pressure on your home to cause cracks to form in the walls. You may notice that your doors and windows are sticking. Frost heaves can also damage your foundation, cause bulges or cracks to form in your driveway, or push up on pilings enough to warp a deck.
The spring thaw can exacerbate the problem. Silber says that once the ice lenses melt, structural components will sink back into the soft soil and may be misaligned.
Some types of soil are more prone to frost heaves than others. Polli Construction says clay soils are most vulnerable to freezing due to their higher moisture content. Erie Insurance, a company in Erie, Pa., says loamy and silty soils also tend to freeze during the winter. Sandy soils are less likely to develop frost heaves, but can still freeze if the water table rises high enough.
Structures built in areas with cold winters usually extend deep enough into the soil to avoid frost damage. Mark Wallace, writing for the concrete industry resource Concrete Construction, says heated buildings are also less likely to be affected by frost heaves since some heat is lost to the soil around them.
If you are experiencing problems caused by frost heaves, you might need to strengthen your foundation. Erie Insurance says helical piers and wall anchors may be necessary to reinforce the foundation and bear the weight of the home.
Polli Construction says a good foundation will have a sturdy footing at least twice as wide as the walls it is supporting, along with rebar to keep the wall intact in case of cracking. It should also extend farther below the surface than freezing temperatures are likely to penetrate.
One effective way to keep frost heaves from forming is to ensure that the soil around your home is not retaining too much water. There should be a sufficient slope away from the foundation to let water run off, and downspouts should also discharge water away from the home. Some residences may benefit from a drainage system around the perimeter of the foundation.
Driveways, patios, and deck pilings are more likely than foundations to suffer frost heave damage, since they are unlikely to have nearby heat sources that can keep moisture from freezing near them. Reuben Saltzman, a Minnesota home inspector writing for the real estate community ActiveRain, says pilings tend to use concrete footings to prevent this problem. However, these footings can still be dislodged if freezing water and soil are able to adhere to it.
Bell-shaped footings are more capable of resisting pressure from frost heaves, but can also break if they are put under too much strain. Sleeves or insulation can help keep the pilings from being gripped by frost heaves. Silber says you can also use gravel to backfill around a footing to improve drainage.
To minimize the chances of frost heave damage to driveways and patios, you can consider a capillary break. This mechanism will prevent more water from being absorbed into the freezing area of the soil, thus reducing the severity of the frost heaves.
Another option to combat frost heaves is to modify or even replace the soil. Erie Insurance says polymers can be injected into the soil to stabilize it and keep it from getting too saturated. You can also replace the soil with a type that is less likely to develop frost heaves.
Preventing Frost Heave In Your Garden
If you garden in a cold area or even one that experiences several hard frosts each winter, then you may need to consider protecting your plants from frost heave. Frost heave often occurs in early spring or late fall, when cooler temperatures and soil moisture are common. Heaves can happen in any type of soil; however, soils such as silt, loam and clay are more prone to heaving due to their ability to retain more moisture.
What is Frost Heave?
What is frost heave? Frost heave occurs after the soil has been exposed to freezing temperatures and plenty of moisture. The pressure that is created from alternating freezing and thawing conditions lifts the soil and plants up and out of the ground. As cold air sinks into the ground, it freezes water in the soil, turning it into small ice particles. These particles eventually come together to form a layer of ice.
When additional moisture from deeper soil layers is also drawn upward and freezes, the ice is
then expanded, creating excessive pressure both downward and upward. The downward pressure causes damage to the soil by compacting it. Compacted soil does not allow adequate airflow or drainage. The upward pressure not only damages the soil structure but also creates the frost heave, which is often characterized by deep cracks throughout the soil.
These cracks expose the roots of plants to the cold air above. In severe cases, the plants may actually be lifted, or heaved, out of the surrounding soil, where they dry out and die from exposure.
Protecting Your Plants from Frost Heave
How do you protect your plants against frost heave? One of the most effective ways to prevent frost heave from occurring in the garden is by insulating the soil with mulch such as pine bark or wood chips, or by placing evergreen boughs over the garden. This helps to moderate temperature fluctuations and reduce frost penetration.
Another way to help prevent frost heave is by raking out any low spots that may be present. A good time to do this is in the spring and again during fall as you are both preparing for and cleaning up the garden. You should also amend the soil with compost to further improve the soil’s drainage, which lessens the chance of heaving. Well-drained soils will also warm faster in spring.
Plants should also be chosen for their suitability to cold temperatures such as deciduous trees and shrubs, bulbs, or perennials that are cold hardy. Unprotected wet, frozen ground is one of the most common causes of death to garden plants in winter due to the havoc created from frost heave.
Don’t allow your plants to fall victim to frost heaves clutches. Take the extra time to insulate your garden beforehand; it only takes one good frost heave to destroy the garden and all the hard work you put into it.
What is frost heave and how does it affect ground-mount solar projects?
The following is an excerpt from a whitepaper produced by TerraSmart. .
As the demand for renewable energy grows across North America, an increasing number of utility-scale photovoltaic (PV) projects are being deployed in regions that experience severe winter conditions, including deep ground freezes. With that freezing comes frost heave and subsequent frost jacking of foundations, which can wreak havoc on installations not properly designed to compensate for those stresses.
To contend with the potentially catastrophic consequences of this geotechnical phenomenon, solar EPCs, developers, and asset owners need to understand the forces at play so that they can protect themselves from risks of overextended schedules, budget overruns, equipment damage, and system downtime.
Frost heave defined
Frost heave is the upward ground movement that occurs as soil freezes. Originally thought to occur as soil volume increased when water became ice, frost heave now is understood to occur because of ice lenses, which form parallel to the surface from water diffused within the soil. As frost penetrates the ground, water is drawn up into the freezing zone, forming layers that force soil particles apart and cause the soil surface to heave, as depicted in Figure 1.
Three factors need to be present for frost heave to occur:
1. Frost susceptible soil with pore sizes that promote capillary flow of water
2. Freezing temperatures that penetrate the ground
3. Presence of groundwater
Depending on the degree of these three factors, soil can heave at rates ranging from 1/64th of an inch to three-quarters of an inch each day. For PV plants with driven piles, the foundation also can be subject to adfreeze, in which the frozen soil adheres to the steel surface of the piles. This adfreeze, combined with frost heaving of the soil adjacent to the piles, results in an uplift force known as frost jacking, which lifts the foundation.
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Figure 1 shows how permanent vertical deformation of the foundation can occur if it has not been designed to resist the frost jacking force through sufficient embedment below the frost zone. Later, when the ice melts and water dissipates back into the soil, the foundation and structure drop into the resulting void.
Figure 1: Effect of Frost on Soil
Over time, repeated heaves and thaws can warp racking systems, cause connection failures, break PV module glass, disrupt electrical terminations, and severely shorten a solar plant’s lifespan.
Soil types and frost heave
Most soils can heave if there is a sufficient freezing rate and water supply. But the rate at which soil can heave is dictated by its grain size structure and subsequent permeability and capillary flow. Generally speaking, soil can be classified into three groups: sand, silt, and clay. Sand and gravel, with their large grain sizes, are most permeable to water, which can help to move moisture away from the surface before it freezes. Clay soils have the smallest grain size, which means low permeability that impedes the rate at which water can feed a growing ice lens.
With grains that are smaller than sand but larger than clay, silt is the most susceptible to frost heave because its fine grain structure holds water within the frost depth zone. Silt also promotes capillaries that allow more moisture to feed into ice lenses, which can grow to be up to four inches thick.
Where frost heave occurs
Geographically, frost heave can occur anywhere that experiences freezing temperatures, but it becomes a more significant issue in northern climates where temperatures remain below freezing for prolonged periods of time. A shallow, brief freeze does not allow enough time for ice lenses to form and deepen. In general, the deeper the average frost depth, the greater the likelihood of frost heave and the more significant damage can be.
As previously discussed, frost heave requires freezing temperatures, fine-grained soils, and the presence of groundwater. Fine-grained soils are common in the Northeast and Midwest, where glacial till is widespread, as shown in Figure 2.
Figure 2: Map of Glacial Till Occurrence in the U.S.
Northern regions in the United States typically have soils based on glacial till, the fine-grained silt sediment deposited by receding glaciers. Figure 2 shows where glacial till is most commonly found. The Northeast also experiences the country’s longest, deepest freeze conditions. Nonetheless, the region’s PV market is one of the fastest growing in the country, making frost heave considerations all the more critical there.
The U.S. Army Corp of Engineers conducted extensive research to classify the frost susceptibility of soils based on percentage of fine grain particles, soil type, and results from laboratory freezing tests. Results of this classification are shown in Figure 3 and Table 1, where it can be seen that soils with higher percentages of fine grain material are classified as the most frost susceptible and which show the highest rates of frost heave.
Figure 3: Susceptibility of Soils Based on Soil Type. U.S. Army Corps of Engineers
Table 1: Frost Susceptibility Classification of Soils (NCHRP 1-37A)
The degree to which frost heave will impact a site is directly related to the depth the ground fully freezes.
This is typically referred to as depth of frost penetration or, more simply, frost depth. When designing a foundation, estimating frost depth becomes the critical design parameter to determine frost heave’s potential effects. Figure 4 shows the frost heave forces acting over the frost depth on a driven pile.
Figure 4: Frost Heave Forces on a Driven Pile
Frost depth is related directly to how long the ground surface is exposed to below-freezing temperatures. Annual temperature data collected from weather stations quantifies historical exposure. This data is then used to calculate an air freezing index, or AFI, a metric that quantifies the annual below-freezing temperatures. An example of this metric is shown in Figure 5, which is a generalization of the temperature fluctuation for the site of interest. The AFI is represented by the portion of the curve — units of degree-days — that is below freezing temperatures.
Figure 5: Air Freezing Index (AFI) Curve
AFI values can be collected over multiple years and statistically analyzed as a generalized extreme value probability distribution, which is useful for assessing any return period. AFI values can be used to estimate a frost penetration depth. Northern regions of the United States are more prone to longer periods of below-freezing temperatures, which result in relatively deeper frost penetration.
Figure 6: Tension Resistance and Frost Depth Using Ground Screw Foundation
After assessing a site’s frost penetration depth, TerraSmart specifies a length sufficient to embed the ground screw’s threaded portion below the frost depth line. As show above in Figure 6, the screw’s threads are responsible for most of its tensile capacity; keeping the threads below the region of active frost heave ensures the design’s tensile capacity will exceed the project’s estimated frost heave forces.
This was an excerpt from a whitepaper produced by TerraSmart. .
Tags: education, ground-mounts, Terrasmart, utility-scale
CBD-26. Ground Freezing and Frost Heaving
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Originally published February 1962.
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.
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.
The problem of frost heave is doubly detrimental in northern climates where the freeze-thaw cycle can occur multiple times as winter wanes. When the soil heaves, abrupt bumps form on the surface, and If beneath a roadway, will make driving conditions rough and cause the pavement to crack and ultimately fail. In addition, ice lenses can grow larger in some areas of the road than others, causing differential heaving that also contributes to surface cracking. As the frozen ground thaws, the affected area will further soften and degrade under repeated traffic loading.
While completely eliminating the supply of water and the resulting phenomena of frost heave is not possible, its impact can be minimized if the moisture intrusion into the subsoil is controlled. With the introduction of structural NPA (novel polymeric alloy) geocell reinforced granular layer(s) beneath the surface, accumulating moisture is continually drained and redirected while in a liquid state. When freezing occurs below the geocell layer, the structure of geocells distribute vertical pressure and therefore the force of frost heave horizontally across a wider area, eliminating abrupt frost heave conditions with its beam/slab reinforcement mechanism.
As a geotechnical company, Stratum Logics employs NPA geocell and other advanced geosynthetics in structural designs where danger of frost heave exists, to successfully minimize the effects of frost heave damage.
Pavement and Foundation Heave Protection Solutions
Ground Heave Solutions
Frost heave is an upward swelling of soil due to the formation of ice during freezing conditions. It usually occurs when the freezing temperature penetrates the soil and turns the present moisture into ice thereby generating an upward movement in the soil.
The size of ice mass increases because of the continuous supply of moisture through capillary action. Soil weight might restrain the influence of ice and generate ice lenses. Nonetheless, ice lenses can move the soil layer upward.
Frost heave inflicts considerable damage to roads, channels, foundations and subsequently, the superstructure. In order to prevent the detrimental effects of frost heave, it is necessary to understand how it works and identify the basic elements which lead to its occurrence. After that, proper measures can be set up to prevent it.
How It Works?
As freezing temperature penetrates into the soil, it converts the soil’s moisture into ice. When moisture at freezing area solidifies, water from other parts of soil would move toward freezing area through capillary action. This leads to the increase in the size of ice mass. Soil weight and other objects above would restrain ice size growth and consequently, ice lenses are formed.
Fig. 1: Ice lenses and Capillary Rise of Water
When freezing temperature further penetrates into the soil, it leaves ice lenses behind. These ice lenses continue to grow towards the area that loses temperature which is toward soil surface.
The ice lenses are capable of thrusting the soil layer upward. It creates cracks in the soil and causes damages to foundations and subsequently to the superstructure. It is reported that, when moisture converts to ice, its size increases by 9%.
Fig. 2: How Frost Heave Works
Fine grain frost-susceptible soil, moisture that continuously supplies water to ice lenses, and freezing temperature are the basic elements of frost heave action.
When temperature declines, the ice melts and the structure lows back to its location under its weight. When freezing and thawing process repeats, it would severely deteriorate and possibly collapse.
- Destruction of channels in freezing season.
- Decline in load carrying capacity of subgrade.
- Undulations and considerable damages to the pavement
- Damaged foundations and slabs.
Fig. 3: Effect of Frost Heave on Buildings Fig. 4: Frost Heave damaged Concrete Fig. 5: Pavements Cracks due to Frost Heave
Generally, frost heave can be prevented by eliminating one of its basic elements which include fine grains soil, frost temperature, and water. There are several measures which can be considered to avoid frost heave :
- Provision of frost heave prevention systems such as hydronic heating system.
- Extend foundation such as piers below frost line.
- Provide backfill materials such as gravel around foundation so as to encourage water drainage.
- Use sleeve to avoid ice from gripping the concrete.
- Construct footing that withstands upward movement.
- For road construction, replace fine grain frost susceptible soil with coarse granular soil.
- Use capillary breaker so as to prevent movement of water toward freezing front and consequently decline frost heave influence.
- Stabilize the soil.
Understanding and Preventing Frost Heave
Frost heave is a natural cycle that causes great damage to buildings, roads, and plants every year. There are three conditions that have to be present for frost heave to occur.
Climate is the first variable. The climate has to be able to support temperatures that are consistently cold enough to reach below the structure to the sub-base. There also has to be a supply of water in the area that will help cause the soil to expand. Water expands by 9% of its total volume when it turns from a liquid to a solid. Finally, the soil has to be susceptible to frost.
Frost heave is very hard to prevent, but it can be done. The two most effective ways to prevent frost heave are to either replace the soil that’s susceptible to frost or to prevent it from being frozen via heating or insulation.
These steps explain why frost heave is so common in roads. With so many roads it’s impossible to fully prevent frost heave. With houses, the foundations are usually built below the freeze line to help prevent any sort of frost heave.
In every issue you’ll find…
Why do I need to know about frost heave?
In any region where the ground freezes in wintertime, all structures that contact or penetrate the soil–foundations, basements, piers, retaining walls, patios, driveways, and more–are susceptible to damage if they aren’t built properly. Local codes will dictate the requirements, which is one of the primary parameters that will ensure that what you build will stand up to the seasonal cycles.
What exactly is frost heave?
Frost heave occurs when freezing temperatures penetrate the ground, causing subsurface water to form ice structures that displace the soil along with anything that rests on or in that soil. While it was once thought that frost heave happens because water expands as it freezes, the process is actually more complicated, involving not only expansion due to freezing, but also the accumulation of additional layers of ice as liquid water is drawn up from below the frost line.
Cold temperatures alone don’t cause frost heave
Frost-susceptible soil—finegrained, moist soil in certain climates—is the first prerequisite for frost heave. Engineers define this type of soil as either that in which more than 3% of the grains (by weight) are 0.02 mm in dia. or smaller, or that in which 10% of the grains are 0.075 mm or smaller.
Water is another requirement, as are subfreezing temperatures that penetrate beneath the surface. The depth to which freezing temperatures penetrate the ground is referred to as the freezing plane or frost front. The depth to which they can potentially extend in any given region is the frost line. Frost lines range from a few inches in Florida to more than 6 ft. in the northern United States.
If not controlled, frost heave can seriously damage buildings and other structures in cold climates. Mitigation typically involves removal of one of the three elements (frost-susceptible soil, freezing temperatures, or water) required for frost heave to occur. Here’s how it works.
When freezing temperatures penetrate the ground, water trapped in voids in the soil forms ice crystals along the frost front. As it solidifies, this water expands by about 9%. In addition, the freezing process desiccates the surrounding soil, drawing unfrozen water from below the frost front through capillary action and vapor diffusion. This water freezes to the ice crystals that have formed above, thickening it to create an ice lens.
An upward force
As temperatures change, the depth of the frost front changes, leaving behind a series of ice lenses with layers of frozen soil between. As they grow, these ice lenses may attach themselves to vertical surfaces below ground, an action known as adhesion freezing, or adfreezing. The ice lenses continue to grow in the direction of the heat loss—that is, toward the surface—lifting soil and structures along the way.
When the air warms, thawing occurs from the ground’s surface downward. As the ice lenses melt, water saturates the soil, weakening it. Structures raised by the frost heave slide back down, often resting askew from the combination of weakened soil and shifting load forces above. The cumulative effect of repeated heaving may aggravate the situation, causing a structure to collapse.
Controlling frost heave
Footings and Piers
Code mandates that support structures either extend below the local frost line or be protected by insulation so that the bearing soil is not subject to freezing and, thus, heaving. Frost heave also can be controlled by backfilling around piers with gravel to promote drainage, using a sleeve to prevent ice from gripping the concrete, or pouring footing bases that resist upward movement.
Driveways, Walkways, and Patios
The occurrence of frost heave can be minimized by replacing fine-grain, frost-susceptible soil with coarse granular material that is not subject to heaving. Drainage measures can reduce the presence of moisture, which also prevents heaving. Providing a capillary break is another option; interrupting the capillary action that draws water toward the ice lenses can make frost heave less severe.
Frost heave can seriously damage a basement if the ground surrounding that basement freezes to the foundation walls. When this happens, heaving soil around the house can carry the walls with it. This situation does not occur with heated basements, however. That’s because a heated basement (insulated or not) loses heat to the soil surrounding it. This outward heat loss pulls moisture away from the foundation walls. Because moisture is required for adfreezing, less moisture means the frozen soil has a less tenacious grip on the foundation.
Foundations can extend out instead of down
A foundation is one type of structure that allows for an alternative means of protection from forces exerted by the frozen ground. If you can keep the soil around a foundation from freezing, the foundation doesn’t technically need to be built to below the frost depth; instead you can use a detail called a frost-protected shallow foundation. This construction detail usually consists of a monolithic slab with wings of rigid-foam insulation extending out from the edges on all sides, buried just below the surrounding topsoil. Typically the insulation extends the same distance horizontally that the frost depth would require a footing to be down from the surface, but check with your local building officials to be sure.
This method of building a foundation uses less concrete and doesn’t require you to dig as deep of a hole. The insulation around the house might cause conflicts with your landscaping plans, but incorporating hardscaping, decks, or porches that extend past the insulation could counteract that issue.
Drawings: Christopher Mills. Inset drawing: courtesy of Acta Materialia.
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