The following sections describe the more common methods for dealing with frost action:
- Typical state minimum pavement depths
- Those contained in the 1993 AASHTO Guide
- Those outlined by the Army Corps of Engineers
- The use of granular rock caps
- The use of geotextiles
Table 1 shows some specific practices of several northern states:
|State||Use of Frost Protection in Thickness Design|
|Alaska||Pavement structure > 50 percent of freeze depth|
|Maine||Pavement structure > 50 percent of freeze depth|
|Oregon||Pavement structure > 50 percent of freeze depth|
|Washington||Pavement structure > 50 percent of freeze depth|
|Ohio||Remove and replace frost susceptible materials|
|Utah||Remove and replace frost susceptible materials|
|Idaho||Capillary break / rock cap|
Minimum Pavement Depths for Frost Protection
Pavement structures are often designed to penetrate a significant distance into the average frost depth. In the simplest case, several State Highway Authorities (SHAs) use a rule-of-thumb that the pavement structure should equal at least one-half of the expected depth of freeze.
State Highway Agencies (SHAs) such as Alaska, Maine, Oregon, and Washington use knowledge about expected frost depths in the design process. Presumably, limiting the depth of frost into the subgrade soils limits the potential for frost heave and thaw weakening for most projects/locations. The above percentages (pavement structural section as a percentage of expected frost depth) are further reinforced by Japanese practice. Kono et al. reported in 1973 that on the island of Hokkaido the pavement structure is set at 70 percent of the expected frost penetration (the pavement materials are non-frost susceptible).
The 1993 AASHTO Guide
The 1993 AASHTO Guide contains a treatment based on reduced subgrade support. Its goal is to estimate the differential effects on the road profile and ultimately to estimate the residual effects on the Present Serviceability Index (PSI). Thus, the decrease in PSI with time due to frost effects is added to the loss of PSI due to ESALs.
Army Corps of Engineers
The Army Corps of Engineers design procedures for pavements subject to freezing and thawing in the underlying soils are based on two concepts (Lobacz et al., 1973):
- Control of surface deformation resulting from frost action
- Provision for adequate bearing capacity during the most critical climate period
Based on the above concepts, three separate design approaches can be used:
- Complete Protection Method. Sufficient thickness of pavement and non-frost susceptible base course is provided to prevent frost penetration into the subgrade.
- Limited Subgrade Frost Penetration Method. Sufficient thickness of pavement and non-frost susceptible base course is provided to limit subgrade frost penetration to amounts that restrict surface deformation to within acceptable limits.
- Reduced Subgrade Strength Method. The amount of frost heave is neglected and the design is based primarily on the anticipated reduced subgrade strength during the thaw period. Yoder and Witczak (1973) note that “…design of highway pavements should be based generally on this method, with additional thickness (based on local field data and experience) used where necessary to keep pavement heave and cracking within tolerable amounts.”
Using design charts in Lobacz et al. (1973) and traffic information in Yoder and Witczak (1973) and the National Stone Association (1985), Table 2 shows thicknesses developed from the Army Corps of Engineers design charts.
Table 2. Combined Thickness of Surface Course and Non-Frost Susceptible Base,
units of mm
- Assumes a 20-year design life.
- ESALs can be estimated using AASHTO LEFs.
- Higher Design Indices are available (up to DI-10), with a maximum combined thickness of 1.1 m (42 in.) for F3 and F4 Frost Groups.
Granular or Rock Cap
One fundamental way to reduce frost action in a pavement is to stop (or reduce) the available water from forming ice lenses and provide a positive drainage blanket to prevent saturating the upper layers of the pavement structure. Tabor commented on this in 1930:
“The troubles resulting from the formation of segregated ice under pavements can be entirely prevented if, in addition to the usual methods of draining, a thick layer of coarse material is introduced under the pavement extending down to the extreme depth of frost penetration.”
This concept has been applied by several agencies in the form of a thick, large maximum aggregate sized open-graded granular layer commonly called a “rock cap”. The following information comes from Mathis (1991), Cook and Kyte (1995) and Uhlmeyer et al. (2002):
- Rock cap material characteristics:
- Open-graded, with typically 100 percent passing the 63 mm (2.5 inch) sieve and 0 to 15 percent passing the 12.5 mm (0.5 inch) sieve. Some agencies specify limits on intermediate sizes to ensure a more consistent rock cap gradation from job to job. Larger maximum aggregate sizes may make grading (adjustment to the plan elevation) and compaction more difficult.
- Backcalculated layer elasitic moduli for the rock cap layers range from 172 to 414 MPa (25,000 to 60,000 psi). These moduli remain relatively constant throughout the year.
- Rock cap layer permeability is estimated at 15,000 to 76,000 m/day (50,000 to 250,000 ft/day).
- Rock cap material typically provides more structural support than a typical crushed aggregate. Therefore, for structural design purposes, it is usually substituted on a 1.2:1 basis for untreated base material.
- The rock cap material should be 100 percent crushed material for constructability purposes.
- The basic structural design involves three layers:
- Moderate HMA surface course on the order of 100 to 150 mm (4 to 6 inches) thick. Surface thickness can change depending upon anticipated traffic.
- Moderate HMA or aggregate base course on the order of 100 to 150 mm (4 to 6 inches) thick. This layer is used to confine the rock cap layer in order to maintain its long-term stability. Approximately 25 to 30 percent of this layer will be lost into the surface of the rock cap (aggregate will fill the large voids in the rock cap) on placement. This settlement process may continue after paving.
- Thick rock cap section to provide a capillary break and free draining layer. Typically, structural design frost action thickness requirements are largely accommodated by the rock cap layer thickness.
- When placed on fine-grained subgrade soils, a geotextile is used as a separator (a filter layer could be used in lieu of a geotextile) to prevent erosion of the subgrade by the potentially high velocity water draining from the rock cap layer.
- Other observations:
- Rock cap thicknesses have ranged from about 30 mm (1.0 ft) to 814 mm (2.67 ft).
- Thick layers of such material are inherently unstable, requiring special construction techniques.
- The cost of the rock cap material is about three times less expensive than aggregate base but will depend upon the availability and location of a suitable quarry.
Geotextiles have frequently been used as separators and occasionally as filters. Work performed by Bell et al. (1983) at Oregon State University suggests that geotextiles have potential as capillary breaks.
The preceding methods are just several of the more common approaches to frost action prevention. They are not intended to be a comprehensive list. Other considerations not discussed include but are not limited to:
- Past pavement performance in the same area.
- The gradation of all materials used in the pavement section, which relates to frost susceptibility (recall “high” percent fines passing the 0.075 mm sieve can make a material (even crushed stone) frost susceptible).
- The anticipated seasonal changes in stiffness and/or strength of unstabilized materials.
- The need for positive subsurface drainage.
- The depth to saturated layers or the water table.
- The anticipated depth of freeze must be considered.
- Corps of Engineers Design of Highway Pavements in Areas of Seasonal Frost. Proceedings, Symposium on Frost Action on Roads, Norwegian Road Research Laboratory. Oslo, Norway.↵
- Principles of Pavement Design, Second Edition. John Wiley and Sons. New York, NY.↵
- Flexible Pavement Design Guide for Roads and Streets, Fourth Edition. National Stone Association. Washington, D.C.↵
- Rock Cap: A True Free Draining Base. Proceedings, Road Builders, Washington State University/University of Idaho.↵
- Open-Graded Rock Base for Asphalt and Portland Cement Concrete Pavements. Proceedings, 46th Annual Road Builders’ Clinic, Spokane, WA March 7-9, 1995.↵
- Uhlmeyer, J.S.; Pierce, L.M.; Lovejoy, J.S.; Gribner, M.R.; Mahoney, J.P. and Olsen, G.D. (2002). Design and Construction of Rock Cap Roadways – A Case Study in Northeast Washington. Paper prepared for presentation at the 2003 TRB annual meeting.↵