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Foundation damage due to frost heaves or ice lensing: here we explain the difference between the vertical lift of frost heaves and ice lensing (or frost lenses) and we include details of the construction of a frost-lens-proof structural pier, and we provide data on the amount of soil pressure and expansion exerted by water and freezing. This article series explains of the the causes of foundation cracks, buckling, or collapse in areas of freezing weather, clay soils, or wet soils. Also see FROST HEAVES, FOUNDATION, SLAB. Photo (above) shows a crew constructing a structural pier (in clay soils) that successfully resists ice lens lifting in Minnesota. Some of the information in this article is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
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There are two distinct problems associated with frozen ground and its effects on building foundations: frost heaving and pressures from frozen soils.
Details About Ice Lenses in Soils
Frost heaving is what makes roads buckle during the winter in many northern states and provinces. It is caused by small plates of ice called ice lenses that form a foot or more below the surface. Ice lenses draw water from surrounding soil by capillary action, typically growing up to 1/4 inch to 2 inches in thickness and up to several feet across.
Often frost lenses form in multiple layers, adding to the total soil movement in an upwards direction. Ice lenses form in porous silts and also in clays that can support capillary action. They need extreme cold, combined with a subsurface water source - that water might come from ground water, surface runoff, or even roof runoff around a building. Roads and sidewalks are good candidates for frost heaves because melted snow provides a good water source as well, and frost can penetrate deeply into the soil because roads and sidewalks are cleared of their insulating snow cover.
Where the direction of heat loss is upwards to grade, ice lenses form horizontally. In these cases, ice lenses push mostly upward, not laterally. Theoretically, a frost heave could lift a building foundation wall or a pier if the soil above the ice lens were frozen to and adhered to the foundation wall or pier surface.
Healy has seen concrete piers for highway sign posts lifted as much as an inch per year by frost heaving, but he has never seen frost do structural damage to foundations. Where the direction of heat loss is horizontal, as through a retaining wall or the wall of a frozen basement, ice lenses could form in the vertical plane and push laterally.
Indeed frost lenses or ice lenses can lift a building foundation by adhesion to the foundation sides, especially at a a building corner. We (DJF) observed this condition at a 1960's home in Poughkeepsie, NY, where a building corner was lifted each winter by freezing wet ground.
Even though the wall's footings were below the frost level, ice lensing against the top few feet of the buried portion of the foundation wall was causing an annual movement problem that cracked the masonry blocks in a stair-step pattern at the building corner, and cracked the drywall in the bedroom above. The owner dug at the problem corner, found a ground water source, a spring, and cured by installing drainage to remove the water problem.
Soil frozen against a foundation wall or a pier, or ice lenses where there is a lot of ground water, can also pinch and lift piers and posts.
When Lon Church, director of Summer Blue Arts Camp in Two Harbors, Minnesota, decided in 2001 to construct an outdoor stage platform that would be exposed to both clay soils and very cold weather, we wrapped the platform's piers in several layers of 6-mil polyethylene plastic to reduce the ice grip around each pier, thereby reducing the ability of ice lenses in the freezing soil to lift the piers supporting the stage.
In our deck construction photos (see the center post in both photos above) you can see that we brought the poly well above ground level during construction. Here is a closeup of the frost lensing protection installed during pier installation. In our "Summerblue Arts Camp" winter photo (above right) you can see the same pier, still protected by plastic. Eight years after construction (in 2009), and after a number of very cold Minnesota winters, we had seen no frost movement in the piers for this outdoor stage. -- DJF. (Also see Deck Board Gaps, Spacing Guide).
Our photo (below) shows excavation and a new foundation under a home that suffered a catastrophic foundation collapse. The foundation wall in the right of this photo collapsed inwards into the garage, due to pressure from water-soaked earth that was caused, in turn, by surface runoff from a hill that sloped directly towards the home's rear wall. -- DJF
The necessary ingredients are water-saturated (often clay-containing) soil and continuous freezing temperatures. Water-saturated soils, where clay is involved, can hold up to two-thirds water by volume, and water expands by 8 percent when it freezes. So the total volume of soil can expand by as much as 5 percent during freezing conditions.
Unlike ice lenses, this freezing soil expansion exerts force in all directions. The force exerted by freezing soil against a foundation wall, according to Heley, could be enough to crack the un reinforced walls in Duluth, and we confirm that this phenomenon can occur even further south, to the limits of areas of freezing soils.
Even if the foundation wall does not cave in, the expanding mass of freezing soil may make a mess of exterior insulation or waterproofing coatings, as was reported in Fargo. As the soil expands, it may drag the exterior insulation or waterproofing foundation membrane with it. For this reason, some builders drape a loose layer of polyethylene plastic over their waterproofing or exterior insulation to act as a slip joint, such as we described for the outdoor stage piers. Others, like Heley, put foundation insulation on the interior.
Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
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