Forces Harmful to House Foundation & Footers
Gravity, soil swelling, soil collapse, frost heaving, hydrostatic pressure, etc. are forces of nature which can cause serious harm to footers and foundations. In the event that a footer or foundation has a structural failure, this failure usually transmits to the rest of the structure. Cracks or separations can begin to develop within your home or structure. These failures can be very serious and actually lead to the ultimate collapse of your home or structure.
Gravity works against your foundation walls. In the case of a typical full basement foundation wall, the foundation wall not only acts as a beam to support the weight of the structure, but also acts as a retaining wall to keep the earth around your house from entering your basement. This can be a tall order to fill, in the event your house is built into a hillside. Think of all the soil that your foundation wall is holding back! Gravity is pushing the dirt or earth against your foundation constantly. During periods of heavy rainfall, the situation intensifies. Many soils absorb vast quantities of water and this added water, combined with the soil, pushes against your foundation wall. This force is commonly referred to as hydrostatic pressure.
Certain soils exist which expand and contract depending on whether they absorb or lose water. This property can be extremely harmful to footings and foundations. If a foundation is constructed on two different soils - and this is a very common occurrence - one soil may expand when saturated while the other soil remains stable. This situation can cause tension to build up within the footing and foundation. This tension, if great enough, can and will crack your footing and foundation. You can combat expansive and contractive soils quite easily. The key is to keep them in a state of dynamic equilibrium. That's a fancy word for tricking the soil. Simply put, if you keep the soil around your footing and foundation constantly moist, the soil will generally not expand and contract. It will not expand or contract because the soil reacts to changes in moisture content.
Frost heaving is a condition which occurs when the moisture in soil freezes and expands. Due to the fact that it takes extended cold weather for soil to freeze, this phenomena rarely occurs in warmer climates. Frost heaving in certain soils can cause a volume of soil to increase by as much as 25 percent. A volume change of this magnitude beneath a footer or against a foundation wall could be catastrophic. I personally have witnessed concrete slabs lift three to four inches due to frost heaving. For this reason, when you construct footings and foundations, you should always be sure that the bottom of the footing is below the "frost line" in your area. You can determine the "frost line" in your area by consulting your local building department officials. The frost line can vary widely. For example, in the southern United States, the frost line is measured in inches below the surface. In the northern Midwestern states, the frostline can extend to several feet below the surface.
Soil collapse can occur in soils which have very large voids within the soil. Loose sandy soil is an example. Footings and foundations placed upon these soils can "sink," which subjects them to tension. As we have discussed, this tension can lead to cracking and failure.
Cutting and filling activities can also act as an indirect force which can harm footings and foundations. These activities occur anytime you move undisturbed soil from one location to another. Many new home subdivisions are subjected to cut and fill earthmoving activities. When you cut a soil, its natural compaction is disturbed. It becomes "fluffed." It has large voids and requires compaction to make it suitable for construction purposes. If you place dirt or soil on a site without properly compacting it, you are inviting disaster. This "fill" dirt will eventually compact itself through the actions of gravity, vibration and water infiltration. Footings and foundations placed on uncompacted fill dirt or soil will, in all likelihood, crack and fail.