Section 5.22 Frost Heave
The design foundation lengths required to resist uplift against ground heave and structural uplift can be calculated from the soil side resistance below the frost zone. The depth of frost penetration and adfreeze bond strength is used to determine the ground heave portion of the load to be resisted. Published adfreeze values generally range from 10 psi to 20 psi, with some silts ranging as high as 45 psi.
Many foundations have been installed according to the Tsytovich Rule, which is to embed the pile two to three times deeper than the frost depth. In some soil conditions and colder climates this rule can lead to excessive incorrect embedment depths.
Frost heave is a service limit state, consideration should be given to the appropriate use of load factors, phi factors, or safety factors to account for its transient nature.
Prevention of Frost Heave
Surcharge
Heaving will be prevented if the structure has ample dead load in the micropile to resist the heaving forces. Dead load could be added to offset heaving forces in micropiles that are otherwise lightly loaded. Adding dead load can be achieved by increasing the pile cap dimensions and / or reducing the number of micropiles to increase the load per micropile.
Isolation
Micropiles have been isolated from frost heave by installing a casing through the frost zone and installing the micropile inside the casing. The annular space between the micropile and casing can be left open, if protected from water infiltration. It is preferable to fill the annular space with a frost-stable, low shear strength, material such as an oil-wax mixture or grease. The annular void space will reduce the lateral capacity and increase lateral deflections of the micropile.
Insulation
Another option to help mitigate frost heave is to incorporate an insulated layer below and extending past the pile cap. The thickness of the insulation and the extension of the insulation past the edge of the pile cap is dependent on air freezing index. The insulation is sized to keep the soil temperatures around and beneath the pile cap above freezing temperatures.
Non-Frost Susceptible Soils
Removing and replacing any frost-susceptible soils present below grade around the micropiles to the frost depth will help limit the potential for heaving, but not eliminate it. The backfill should be non-frost susceptible such as SP, SP-SM. This will require a site that is large enough that active water management can be installed. The bottom of the excavation should be sloped toward one or more collection points so that any water entering the backfill can be collected and removed. A series of perforated drainpipes should be installed to collect and dispose of infiltrating water and/or groundwater that could accumulate within the backfill. The piping should be connected to a sump to remove any accumulated water, or “day lighted” if grades permit. If the water is not removed this option will not be effective in controlling heave.
References
- Canadian Geotechnical Society (2006), Canadian Foundation Engineering Manual, 4th Edition, 488p.
- NAHB Research Center (1994), Design Guide for Frost-Protected Shallow Foundations, U.S. Department of Housing and Urban Development Office of Policy Development and Research, 56p.
- Goldberg, L. F., (1999) Frost Protected Shallow Foundation Design Specifications, Minnesota Housing Finance Agency, 11p.
- Andersland, O. B., Ladanyi, B (2003) Frozen Ground Engineering, 2nd Edition, Wiley, 384p.