Frost jacking is an issue that many engineers face for lightly loaded structures and has become an important design consideration in recent years since the rapid growth of ground-mount PV solar systems being used for renewable energy. In many parts of the world freezing air temperatures lead to frost penetration into the ground. The maximum depth of frost penetration in any particular year in any area depends largely on the number of freezing days and the snow cover, which acts as insulation. In northern latitudes, seasonal frost penetration can be as much a 6 ft. (1.8 m) below the ground surface. As the water in the soil freezes, it expands and exerts an upward force caused by adfreeze (tangential heave stress) against foundations located within the frost zone.
Adfreeze: When soil freezes, the water in its pores becomes ice. That ice can stick (bond) to nearby surfaces—like concrete, steel, or wood—creating a strong adhesion known as adfreeze. This bond allows upward movement from frost heave to be transferred directly into the structure.
Frost heave is the upward swelling or lifting of soil that occurs during freezing conditions due to the growth of ice within the ground.
Frost heave is especially important for lightly loaded structures such as overhead lights, signage, fencing and ground-mount solar systems that can be lifted as the foundation heaves. The combined weight of the structure and the resistance of the foundation below the frost zone is often not enough to counteract the adfreeze uplift forces. Some examples of frost heave of lightly loaded structures in North Dakota and Vermont shown in Fig. 1. Frost heave of these structures is often cumulative over several seasons. Ultimately, new foundations are needed when the frost heave reaches an excessive state. Traditional foundations consisting of drilled cast-in-place concrete piers may actually accentuate the adfreeze because of the rough surface of the shaft.
Figure 1. Frost heave of lightly loaded structures in North Dakota (left) and Vermont (right)
Traditionally, several approaches have been used to reduce foundation frost heave, including applying low-friction coating (bond-breaker) on the pile surface, using a sleeve around the pile shaft, drilling and removing frost susceptible soils and backfilling with gravel and using ground surface insulation to reduce frost penetration. But these are not cost-effective solutions, and in some cases, may even exacerbate the problem. Helical piles can provide a cost-effective solution to counteract frost heave and eliminate upward movement. They potentially have a number of advantages over other types of foundations, especially drilled and cast-in-place piles and driven piles. Helical piles provide resistance to frost heave by placing a portion of the shaft and the helical plate below the depth of frost penetration. This resistance to uplift is developed by the strength of unfrozen soil above the helix and along the shaft.
Helical piles provide resistance to frost heave in essentially the same way that they provide additional resistance in uplift over a straight shaft pile with the same shaft diameter and length. The helical plate develops this resistance provided that the plate is embedded below the zone of maximum frost penetration. For most applications, helical piles with be used to replace straight shaft driven piles or drilled shafts for projects where frost heave can be a critical design consideration.
It has generally been thought that frost heave of pile foundations can be counteracted by placing a length of pile penetration below the maximum depth of frost penetration that is at least the same length as the pile within the frost zone. However, this is only true if the unit side resistance acting along the pile shaft in the unfrozen soil below the frost zone is equal to or greater than the unit adfreeze stress acing on the pile within the frozen zone.
Fig. 2 shows a comparison of the frost heave measured over one winter season in Hadley, Ma. of a single-helix pile and a plain pipe pile of similar diameter, both embedded to a depth of 5 ft. The site consists of stiff clay with a groundwater table near the ground surface. It can be seen that the helical pile performed somewhat better than the plain pipe. The estimated frost depth during this season was about 3 ft.
Figure 2. Comparison of Measured Frost Heave for Plain Pipe Pile and Helical Pile Over one Winter Season.
A similar comparison from the same site is shown in Fig. 3 for a larger diameter helical pile and a drilled cast-in-place concrete pier. In this case, the drilled pier showed a heave of about 1.4 in. (36 mm) while the helical pile only heaved about 0.2 in. (5 mm) through just one winter season. The adfreeze stress is higher on drilled concrete piers because of the surface roughness along the shaft. In both examples, the piles and shaft were unloaded. In reality. A downward force from the mass of the supported structure adds some resistance to heave as well.
Figure 3. Comparison of Measured Frost Heave for Helical Pile and Drilled Castin-Place Concrete Pier Over one Winter Season
In order for the engineer to estimate the Factor-of-Safety against zero frost heave, the following are needed:
Geometry of the foundation
Estimate of maximum depth of frost penetration
Unit adfreeze stress
The geometry of the foundation can be chosen and then analyzed to evaluate Factor of Safety against frost heave. Traditional design methods to estimate pile tension capacity can then be used to calculate uplift resistance.
Evaluation of the frost heave resistance provided by helical anchors or piles, requires an estimate of the unit adfreeze stress and an estimate of the maximum expected depth of frost penetration over the life of the structure. Values of reported adfreeze stress measured on steel pipe piles in Alaska and Canada range from about 1000 psf to 8200 psf (48 kPa to 390 kPa). The Canadian Foundation Manual recommends a value of 2100 psf (100 kPa) for steel piles in contact with fine-grain and silty soils. The magnitude of adfreeze stress depends on soil characteristics, such as water content and unit weight at each location as well as ground temperature, and pile characteristics.
The estimated maximum depth of frost penetration is typically obtained from local or regional frost penetration maps, or local building codes. The engineer usually designs for the maximum depth expected over the life of the structure; this is the “worst case” scenario.Frost depth may also be estimated from available climatological data and actual frost penetration depths measured in a particular geographic region. Maps for the maximum freezing depth for the continental U.S. have been presented by DeGaetano & Wilks (2002) showing maximum frost penetration depth (in cm) for 10-year and 100-year return for snow-free bare soil which were based on a model developed and presented by DeGaetano et al. (1997). These maps appear to be some of the most reliable that are currently available.
Alternatively, some simple models have been suggested for estimating the maximum frost penetration depth for specific locations. For example, Soliman et al. (2008) suggested a model for estimating the maximum frost penetration depth in Manitoba, Canada. They attempted to develop a model based on air Freezing Index (°C-days) and cumulative rainfall precipitation during the previous summer season from March 1 to October 31, however there was considerable scatter in the model. As an alternative, they proposed a simple relationship between maximum frost penetration depth (DF in cm) and cumulative freezing degree days ((staring from freezing point (°C-days) as:
DF = 4.8 (F)0.5
where:
DF = frost penetration depth (cm)
F = cumulative freezing degree-days
Helical piles resist frost heave in essentially the same way that they act as an anchor to resist an upward tensile force applied at the surface. Resistance is provided by the pullout capacity above the helical plate(s) and shaft resistance along the section of shaft below the frost zone. In a plain pile, such as an H-Pile or pipe pile, frost heave resistance must be developed on the section of pile below the frost zone. Sufficient length is required to develop enough shaft resistance to counteract the uplift force from the adfreeze stress developed in the frost zone.
Helical piles develop frost resistance as a result of the large uplift capacity and anchoring developed predominantly by the helix. Both single-helix and multi-helix piles can be used, but in all cases, the key is to place the helices below the maximum depth of frost penetration plus a minimum of one helix diameter. Figure 4 shows the forces acting on the pile.
Figure 4. Schematic of Frost Heave Resistance Provided by Helical Piles.
Several field and model studies have shown that helical piles provide a substantial reduction in frost heave compared with straight shaft piles provided that the helix is embedded to a depth below the maximum frost penetration depth. A study by Wang et al. (2017) compared results of a field tests to measure the frost heave of helical piles at a site in China using different configurations of helical piles, Fig. 5. The influence of number of helices and helix spacing on uplift is shown in Fig. 6. These tests show that all configurations of helical piles showed a substantial reduction in frost heave compared to a plain pipe pile of the same length and diameter.
Figure 5. Configurations of Helical Piles Used to Evaluate Effectiveness Against Frost Heave. (from Wang et al. 2017)
Figure 6. Comparison Between Calculated and Measured Frost Heave for Screw Piles.
(from Wang et al. 2017)
Determine the Factor-of-Safety with respect to frost heave of a round-shaft single-helix pile with a shaft (DS) diameter of 2.875 in. and a helix diameter (DH) of 12 in. embedded to a depth of 14 ft. (LH) in soft clay in Ottawa, Canada. The depth of maximum expected frost penetration (LF) is 5 ft., the adfreeze stress (σF) is estimated as 2100 psf; and the undrained shear strength (su) of the clay is 600 psf. Assume the adhesion factor for this clay is α = 0.8. Also assume there is no load at the top of the pile. (Ignore the self-weight of the pile.) LS = length of shaft below the frost zone.
FHEAVE = (π)(DS)(LF)(σF) = (π)(2.875 in./12 in./ft.)(5 ft.)(2100 psf) = 7903 lbs.
Resisting Force = Capacity of shaft below frost depth + Capacity of helix
FRESISTING = (π)(DS)(LS)(su)(α) + (AH)(9)(su) = (π)(2.875 in./12 in./ft.)(14 ft. – 5 ft.)(600 psf)(0.8) + [(π)(6 in./12 in./ft.)2 - [(π)(1.437 in./12 in./ft.)2)](9)(600 psf) = 3251 lbs. + 3998 lbs. = 7250 lbs. (Note that the net helix area (helix – shaft) of the helix is used in uplift.)
The Factor of Safety against heave is FOS = 7250 lbs./7903 lbs. = 0.92
Since FOS < 1, and since several assumptions were used, the engineer now should try using a helical pile with a larger diameter helix or select a double-helix pile installed below the frost depth. Alternatively, the length of pile embedment below the frost depth could be increased.
Helical piles offer the most direct and cost-effective foundation system to resist frost heave for lightly loaded structures. The design procedure is straightforward and has proven to be reliable. The availability of a wide range of helical geometries allows a specific helical pile to be selected to suit each project.