Factors determining the degree of lift include the amount of retained water, depth of insulating snow cover, severity of low temperatures, and duration of exposure. In the course of a typical northern winter, accompanied by normal snowfall (which acts as an insulating blanket); only a very small percentage of surface areas actually freeze. When this snow blanket is absent, plowed away (roads), or prevented from accumulating (under decks or unheated outbuildings), ground freeze and subsequent frost heave can be severe.
Decks and porches, attached on one side to a stable house, are annually subjected to structural contortions and stress. Outlying exposed supports are lifted and released to varying degrees while the house side remains stable.
Free standing structures, such as unheated out buildings subjected to frost heave, cannot maintain door and window frames in “square”. Windows may shatter and doors often become inoperable “jamb”. In severe cases, damage can be done to asphalt roofing
materials (cracking)
Eliminating Frost Heave
Unfortunately, there are few shortcuts to providing a stable foundation for unheated structures in cold climates.
Various “spikes” and on-ground concrete deck blocks are available but simply ignore the frost heave problem and will not provide for a stable structure.
In-ground (construction tube formed) poured concrete column (pier) foundations are considered the best defense against frost heave.
This however, is true only if these columns are equipped with footings.
Simply drilling an 8” - 10” - 12” hole, inserting a construction tube and filling it with concrete is (in any area with a relatively severe winter) a concession to a quick, neat job and usually a recipe for future foundation related problems.
A popular misconception is that as long as the bottom of the column is below the frost line (maximum depth to which freezing occurs) the column will not heave (lift). In fact, most lift, is generated along the columns’ sides and occurs long before freezing could ever reach to below the column.
As ground freeze and expansion occurs, the column is first squeezed, then lifted upwards along with the freezing soils, frost penetration as little as 1 1/2 ft. will lift columns without footings embedded 4 ft deep.
Aside from the immediate stress developed in structures supported with lifting (heaving) drilled columns, these columns appear to “grow” annually. They do not simply slide back into place with spring thaw, returning the structure to level.
As surface thaw occurs, heat from the exposed portion of the column is transmitted throughout the column. Surface aggregates, suspended in melt water, run down the thawed column sides and are deposited in the void left under the lifted column. The column cannot return to its original depth, the column is “frost heaved”. Meltwater aggregate, running down the column, often accumulates on just one side of the void, causing the settling column to tilt.
In cases of severe frost heave distortion and structural stress, the only remedy is to remove and replace the heaving columns with columns on footings. This is at best a very difficult and expensive task (if even possible). Columns with footings resist frost heave because the
footing acts as a plug, holding the column down and forcing upward moving freezing soils to slip up the column instead of lifting it.
Added benefits of footings include greater lateral stability (sizemically active areas), much greater load bearing capacities (fewer columns) and resistance to washout in flood prone areas.
