Frost action can be quite detrimental to pavements and refers to two separate but related processes:
- Frost heave. An upward movement of the subgrade resulting from the expansion of accumulated soil moisture as it freezes.
- Thaw weakening. A weakened subgrade condition resulting from soil saturation as ice within the soil melts.
Frost heaving of soil is caused by crystallization of ice within the larger soil voids and usually a subsequent extension to form continuous ice lenses, layers, veins, or other ice masses. An ice lens grows through capillary rise and thickens in the direction of heat transfer until the water supply is depleted or until freezing conditions at the freezing interface no longer support further crystallization. As the ice lens grows, the overlying soil and pavement will “heave” up potentially resulting in a cracked, rough pavement (see Figure 1). This problem occurs primarily in soils containing fine particles (often termed “frost susceptible” soils), while clean sands and gravels (small amounts of fine particles) are non-frost susceptible (NFS). Thus, the degree of frost susceptibility is mainly a function of the percentage of fine particles within the soil. Many agencies classify materials as being frost susceptible if 10 percent or more passed a 0.075 mm (No. 200) sieve or 3 percent or more passed a 0.02 mm (No. 635) sieve. Figure 2 illustrates the formation of ice lenses in a frost susceptible soil.
The three elements necessary for ice lenses and thus frost heave are:
- Frost susceptible soil (significant amount of fines).
- Subfreezing temperatures (freezing temperatures must penetrate the soil and, in general, the thickness of an ice lens will be thicker with slower rates of freezing).
- Water (must be available from the groundwater table, infiltration, an aquifer, or held within the voids of fine-grained soil).
Remove any of the three conditions above and frost effects will be eliminated or at least minimized. If the three conditions occur uniformly, heaving will be uniform; otherwise, differential heaving will occur resulting in pavement cracking and roughness. Differential heave is more likely to occur at locations such as:
- Where subgrades change from clean not frost susceptible (NFS) sands to silty frost susceptible materials.
- Abrupt transitions from cut to fill with groundwater close to the surface.
- Where excavation exposes water-bearing strata.
- Drains, culverts, etc., frequently result in abrupt differential heaving due to different backfill material or compaction and the fact that open buried pipes change the thermal conditions (i.e., remove heat resulting in more frozen soil).
Additional factors which will affect the degree of frost susceptibility (or ability of a soil to heave):
- Rate of heat removal.
- Temperature gradient
- Mobility of water (e.g., permeability of soil)
- Depth of water table
- Soil type and condition (e.g., density, texture, structure, etc.)
The Casagrande Criterion
In 1932, Dr. Arthur Casagrande proposed the following widely known rule-of-thumb criterion for identifying potentially frost susceptible soils:
“Under natural freezing conditions and with sufficient water supply one should expect considerable ice segregation in non-uniform soils containing more than 3% of grains smaller than 0.02 mm, and in very uniform soils containing more than 10 percent smaller than 0.02 mm. No ice segregation was observed in soils containing less than 1 percent of grains smaller than 0.02 mm, even if the groundwater level is as high as the frost line.”
Application of the Casagrande criterion requires a hydrometer test of a soil suspension (in water) to determine the distribution of particles passing the 0.075 mm sieve and to compute the percentage of particles finer than 0.02 mm.
Thawing is essentially the melting of ice contained within the subgrade. As the ice melts and turns to liquid it cannot drain out of the soil fast enough and thus the subgrade becomes substantially weaker (less stiff) and tends to lose bearing capacity. Therefore, loading that would not normally damage a given pavement may be quite detrimental during thaw periods (e.g., spring thaw). Figure 3 is an example of typical pavement deflection changes throughout the year caused by winter freezing and spring thawing. Figure 4 shows pavement damage as a result of thaw weakening.
Thawing can proceed from the top downward, or from the bottom upward, or both. How this occurs depends mainly on the pavement surface temperature. During a sudden spring thaw, melting will proceed almost entirely from the surface downward. This type of thawing leads to extremely poor drainage conditions. The frozen soil beneath the thawed layer can trap the water released by the melting ice lenses so that lateral and surface drainage are the only paths the water can take.
Tabor (1930) also noted an added effect:
“The effects of refreezing after a thaw are also accentuated by the fact that the first freeze leaves the soil in a more or less loosened or expanded condition.”
This observation shows that (1) the reduced density of base or subgrade materials helps to explain the long recovery period for material stiffness or strength following thawing, and (2) refreezing following an initial thaw can create the potential for greater weakening when the “final” thaw does occur.
Sources of Water
The two basic forms of frost action (frost heave and thawing) both require water. Water sources can be separated into two broad categories:
- Surface water. Enters the pavement primarily by infiltration through surface cracks and joints, and through adjacent unpaved surfaces, during periods of rain and melting snow and ice. Many crack-free pavements are not entirely impermeable to moisture.
- Subsurface water. Can come from three primary sources:
- Groundwater table (or perched water table).
- Moisture held in soil voids or drawn upward from a water table by capillary forces.
- Moisture that moves laterally beneath a pavement from an external source (e.g., pervious water bearing strata, etc.).
Estimation of Freezing or Thawing Depths in Pavements
This section discusses freeze depth estimation techniques. Such an estimate is helpful in designing for frost conditions, but oversimplifies the complex conditions that accompany various pavement materials, depths of freeze, and water sources. Basic terminology is contained on a separate page. All units will be in U.S. customary due to the source material. Two formulas are presented on linked pages:
Mitigating of frost action and its detrimental effects generally involves structural design considerations as well as other techniques applied to the base and subgrade to limit the effects of frost action. The basic methods used can be broadly categorized into the following techniques:
- Limit the depth of frost into the subgrade soils. This is typically accomplished by specifying the depth of pavement to be some minimum percentage of the frost depth. By extending the pavement section well into the frost depth, the depth of frost-susceptible subgrade under the pavement (between the bottom of the pavement structure and frost depth) is reduced. The assumption is that a reduced depth of soil under frost action will cause correspondingly less damage.
- Removing and replacing frost-susceptible subgrade. Ideally the subgrade will be removed at least down to the typical frost depth. Removing frost-susceptible soils removes frost action.
- Design the pavement structure based on reduced subgrade support. This method simply increases the pavement thickness to account for the damage and loss of support caused by frost action.
- Providing a capillary break. By breaking the capillary flow path, frost action will be less severe because as Tabor (1930) noted, frost heaving requires substantially more water than is naturally available in the soil pores.
Freezing and Thawing Implications for Maintenance Operations
The calculated freezing index (FI) and thawing index (TI) can be used to estimate the depth of freeze at a specific site and the resulting thaw. Maintenance personnel can use the TI to assess the need for seasonal load limits (Figure 5). The following general guidelines relative to spring highway load restrictions were developed and evaluated by a study in Washington State (Rutherford et al., 1985; Mahoney et al., 1986):
- Where to apply load restrictions. If pavement surface deflections are available to an agency, spring thaw deflections greater than 45 to 50 percent of summer deflections suggest a need for load restriction. Further, considerations such as depth of freezing (generally areas with air Freezing Indices of 400 °F-days or more), pavement surface thickness, moisture condition, type of subgrade, and local experience should be considered. Subgrades with Unified Soil Classifications of ML, MH, CL, and CH will result in the largest pavement weakening.
- Amount of load reduction. The minimum load reduction level should be 20 percent. Load reductions greater than 60 percent generally are not warranted based on potential pavement damage. A load reduction range of 40 to 50 percent should accommodate a wide range of pavement conditions.
- When to apply load restrictions. Load restrictions “should” be applied after accumulating a Thawing Index (TI) of about 25 °F-days (based on an air temperature datum of 29 °F) and “must” be applied at a TI of about 50 °F-days (again based on an air temperature datum of 29 °F). Corresponding TI levels are less for thin pavements (e.g., two inches of HMA and six inches of aggregate base or less) in that the “should apply” TI level is 10 °F-days and the “must” TI level is 40 °F-days.
- When to remove load restrictions. Two approaches are recommended, both of which are based on air temperatures. The duration of the load restriction period can be directly estimated by the following relationship, which is a function of Freezing Index (FI):
Duration (days) = 25 + 0.01 (FI)
The duration can also be estimated by use of TI and the following rough relationship:
TI = 0.3 (FI)
Frost Action Summary
Frost action is a critical pavement structural design concern in those parts of the country that regularly experience ground freezing. Without proper precautions, severe frost action can destroy a new pavement in a matter of one or two years. In taking the proper precautions, there are two basic types of frost action with which to contend:
- Frost heave. Results from accumulation of moisture in the soil during the freezing period. These accumulations (ice lenses) expand perpendicular to the direction of heat flow and push the pavement up, often causing severe cracking.
- Thaw weakening. Once a subgrade is frozen it can be severely weakened when it thaws (usually in the spring time). Therefore, loading that would not normally damage a given pavement may be quite detrimental during thaw periods.
Frost action can be further characterized by the typical depth to which the subgrade freezes in a particular area. This depth can be estimated by several equations including the Stefan formula and the modified Berggren formula. Once this depth is known, it can be used as a pavement structural design input to mitigate the detrimental effects of frost action. Mitigation techniques can be classified into four broad categories:
- Limit the depth of frost-susceptible material under the pavement structure.
- Remove and replace the frost-susceptible subgrade.
- Design the pavement structure based on reduced subgrade support.
- Force a break in the groundwater’s capillary path.
If frost action cannot be adequately mitigated, severe pavement damage (in the case of frost heave) or a loss of bearing capacity (in the case of thaw weakening) can result. Maintenance options to correct these problems are limited to pavement repair or replacement (in the case of frost heave) or limiting pavement loading during spring thawing (in the case of thaw weakening).
- Freezing and Thawing of Soils as Factors in the Destruction of Road Pavements. Public Roads, vol. 11, no. 6. U.S. Department of Agriculture, Bureau of Public Roads. Washington, D.C.↵
- Guidelines for Spring Highway Use Restrictions. Research Report WA-RD 80.1 (WSDOT) and FHWA-RD-86-501 (FHWA). Olympia, WA.↵
- Research Summary Report — Guidelines for Spring Highway Use Restrictions. Research Report WA-RD 80.2, Washington State Department of Transportation. Olympia, WA.↵