Frost Heave: Understanding, Avoiding, Mitigating
Your pole barn is attached to the ground with sturdy anchors and concrete piers. Short of an earthquake, what could affect those massive piers? A 36” pier surrounded by water that freezes into underground frost can be heaved up and down twelve inches or more, turning your solid foundation into a massive headache.
Everyone who installs decks, pole barns, and pier-foundation structures in climates that include winter freezes must understand frost heave and act. Without considering frost heave, you risk expensive warranty repair, or worst, a blown reputation from a heaved pier foundation.
The proximate cause of frost heave is freezing water, but the ultimate causes of frost heave are more complex.
What is Frost Heave?
Frost heave is caused when sub-surface water in the soil freezes. When frozen, this water may be drawn into regions that create structures called ice lenses. The entire pier may be surrounded by frozen water and soil mixture in saturated soil.
Water, when frozen, increases in volume by 9%. This increase in volume from among the ice lenses creates motion horizontally and vertically. The horizontal movement pushes on the subsurface portion of the concrete pier with tremendous force. The concrete pier resists the compressive forces, turning the ice expansion force vertically along the pier surface. The surface of the pier, being rough, will move with those forces.
Typically, because gravity has compressed the soil below the pier, the direction the pier must move is upward. This is frost heave.
As mentioned above, some of the forces can be downward, too, pulling against the upward frost heave. In these cases, the pier can fracture because of the vertical tension, heaving the top portion of the pier upward, no longer intact as one piece of concrete.
What Causes Frost Heave?
The proximate cause of frost heave is freezing water, but the ultimate causes of frost heave are more complex. For frost heave, you need freezing temperatures plus water moving into the frost zone by capillary action. When this occurs, “frost lenses” form, which expand, creating heave.
When heave affects piers, three or more of the following are in effect:
Climates cold enough to freeze subsurface water
Presence of subsurface water around pier due to capillary action – typically sourced from poor drainage, over-watering, or improperly routed sumps, drains, or gutters
Pier depth too shallow for the depth of frost
Pier geometry allows movement of the pier
Not enough internal pier strength to resist frost heave tension (cases where frost pushes both up and down, splitting the pier)
Pier sides provide an attachment surface for ice lenses and soil
Climates Cold Enough to Freeze Subsurface Water
The likelihood of sub-surface frost increases with temperatures below freezing over multiple days. The further below freezing and the number of consecutive days determine the expected frost depth in inches. Decks.com has a great frost depth map of the continental USA that can be used to estimate extreme frost depth. A more detailed map of current frost depth in real-time is available from the National Weather Service. However, you should always consult your local building code jurisdiction for exact requirements before digging.
Poorly draining, fine grain soil is susceptible to frost heave. We can’t always choose the soil in which we dig footings and piers. However, when it’s clear that subsurface water will drain slowly, it is essential to ensure that water is not pooling around piers.
Excessive water pooling around piers is caused by:
Poorly graded land (water should naturally flow away from any foundation structure)
Improperly routed rain gutters
Sump systems draining near piers
Broken sub-surface drain or gutter pipes near piers
Pier Depth, Geometry, and Strength in Tension
A pier with its base within the frost zone can most easily heave. Therefore, climate-based considerations for frost depth are essential to planning pier digs.
Fully cylindrical piers are most susceptible to frost heave because they have little structure to resist movement with the soil and rough sides that allow the frost to grab the concrete as the ice lenses expand. Therefore, updated code requirements for piers in many regions require a bell-shaped or “spread base” style pier. This geometry will resist frost heave by anchoring the base of the pier beneath the frozen heaving soil, with the base buried below the frost line.
Even in cases with bell-shaped or spread piers, the tension on the pier created by up and down heave on separate portions of the pier can break it, pushing the upper, broken piece upward. Pier reinforcement is an important consideration when in climates with frost heave potential.
Pier Surface Smoothness
A pier with a slick outer surface would not easily heave. The ice lenses and soil would not attach and would slide along the surface of the pier upward (or downward). Paperboard forms are slick compared to bare concrete, but they can still adhere to the ice lenses and soil, allowing frost heave.
Frost Heave Essentials
Understanding the causes of frost heave instructs essential prevention practices. The following are needed in most mid-western climate zones.
Before digging, check that grading and any local drainage will not allow water near piers. This includes sumps, gutters, leaky underground pipes, sprinklers, and property drainage grade. Although soil can wick nearby moisture using capillary action, the chances for frost heave diminish with no excessive water around the pier. Notify the property owner to avoid watering / over-watering near the piers, and to mitigate any drainage problems, including rerouting drainage and adding sub-surface drain tile, if needed.
Know the frost zone depth for your region. Make sure the pier depth is deeper than the frost zone. Use a frost depth tool for your area.
Always use bell-shaped piers. This shape anchors the bottom of the pier below the frost line and provides resistance to upward heave. Jess Tools’ belling augers make this a quick and easy standard practice.
Always use vertical rebar in piers to provide sheer strength in tension. This stops the bell from breaking off under strain if ice lenses and soil push up on the pier's upper portion.
Use cardboard cylinder pier forms for a minimum of 36” from finished grade (or to the bottom of the form).
As with any foundation, proper alignment and attachment to the piers should be practiced for all structures. Frost heave can amplify any misalignment problems significantly. The Concrete Pier System ensures this as standard practice while eliminating wasteful labor.
What To Do if Existing Piers Are Already Heaving
If frost heaving is already occurring, there are ways to mitigate the problem potentially:
Check for poor drainage: This includes sumps, gutters, leaky underground pipes, sprinklers, and everyday property drainage grade. Clear up any drainage problems, including rerouting drainage and adding sub-surface drain tile or French drains. Do what it takes to keep the soil around piers dry. (1)
After removing water from the equation, if heaving still occurs, consider isolating a few piers and removing and re-digging, and pouring them using the Essentials above.
Frost heave is one of the most pernicious problems in a simple pier foundation system. Building the essential and, if needed, freezing weather practices into your workflows will ensure that your pier system is the exception to frost heave. In addition, if you’re a builder, consistently using these practices should provide a competitive advantage in the form of better warranties and fewer callbacks to deal with pier foundation problems.