The Importance Of The Frost Line And The Height Of The Water Table When Excavating A Foundation

When excavating for a foundation, a homeowner should ensure that the foundation will be below the frost line, but above the height of the water table. Here is a basic introduction to the frost line and the water table.

Frost-Protected Shallow Foundations

When a foundation is exposed to freezing temperatures, the concrete is more susceptible to cracking. This is because the temperature conditions cause water to vacillate between freezing and thawing.

To combat the risk of slab cracks resulting from the freeze-thaw cycle, the ground should be excavated so that the top of the foundation will be well below the frost line. Frost penetrates well below the surface because of moisture present in the underground soil; the "frost line" is the distance below ground at which this moisture no longer freezes.

In Canada, the frost line is generally two to three meters below the ground surface. The southern regions of Canada will have frost lines 2-2.5 meters below the surface, and the northern regions will have frost lines 2.5-3 meters below the surface. Thus, optimal foundation excavation depth depends on the location of the construction site.

Height of the Water Table

The "water table" is the underground area that is fully saturated with water. During foundation excavation, the height of the water table is the distance below the projected house's foundation at which this fully saturated area begins. 

When excavating for a foundation, the excavation hole should not extend below the height of the water table; if it does, the house is more susceptible to leaks and floods originating from the water table. If cracks or even tiny holes are present and the foundation is below the water table, then the water can seep into the home. Heavy rain, melting snow, and thawing frost and ice can increase the underground water amounts that will exploit these seepage opportunities.

The Balance Between the Frost Line and the Height of the Water Table

In a best-case scenario, the foundation can sit below the frost line but above the height of the water table. In situations where this optimal situation is not present, the excavators and the civil engineers involved will need to make arrangements so that the foundation will not be at risk for frost-related and flood-related damages.

The Importance of Proper Foundation Excavation 

In Canada, homeowners' insurance policies do not cover flooding if the flooding is the result of flawed foundations. Thus, the homeowner must pay out-of-pocket for flooding caused by flawed foundations that are built below the height of the water table. Foundation flaws include cracks caused as a result of excavating below the frost line. (go here for questions regarding haulage)


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