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Causes of Foundation Failure by Matt Clark's Northern Basement Systems in Vermont and New Hampshire.

Different soil types are affected by moisture in different ways. Water passes through sandy soils rather than being absorbed, making them more predictable. Instead of expanding as they absorb moisture and contracting as they dry out, sandy soils maintain a fairly consistent volume and density. Because of their stability and good load-bearing qualities, sandy soils are less likely to shift and settle, so they rarely cause foundation problems. Clay Soils Soils rich in clay and silt have the greatest potential to damage a foundation. Clay absorbs water easily, expanding in volume as it becomes more saturated. So-called "expansive clays" can cause foundations to crack, heave and shift. When clay soils dry out, they shrink and crack, leaving gaps around a house where water from the next storm can penetrate easily and repeat the expansion cycle. Clay-rich soils can cause more foundation damage by expanding than by contracting. Sandy Loam Soils Loamy soils are usually very stable soil that shows little change with the increase or decrease of moisture temperature. The primary concern with foundations built on loamy soils is erosion. When soils underneath your foundation erode, they are less able to sustain the weight of a foundation. Because of the constant cycle of wet and dry periods that occur as the weather changes, certain types of soil can expand and contract indefinitely, subjecting your foundation to settling or expansive stresses that often cause damage. The "Active" Zone of Foundation Soils Your home is resting on many different layers of soil, each with different thicknesses and characteristics that can affect a house foundation. These soils have formed or been deposited over thousands of years -- some by water, some by wind, some by glaciers, and some by the contractor who built your home. Typically, soil layers gain in stability and load-bearing capacity with depth. Deep below these layers is a layer of bedrock - a layer composed of rock or stable, densely packed soils. The soil immediately under and surrounding your home is known as the active zone. This soil is most affected by changes in moisture and climate -- and the source of most foundation problems. The active zone may vary from a few feet below the surface to more than 30 feet below grade. How Does Foundation Settlement Occur? Foundation settlement occurs when the soil no longer properly supports the weight of your home. Three of the most common reasons for foundation settlement are drying and shrinking of soil, wetting and softening of soil, and poorly compacted fill soil. Drying and Shrinking of Soil Foundation soils experience most of their drying and shrinking from two common causes: Drought: Prolonged dry periods cause the soil to dry out. As we know, when clay dries out, it shrinks. Soil shrinkage beneath a foundation has the same effect as soil settling: It usually causes a section of the foundation to crack and settle into the void or hollow area where settlement has occurred. Maturing Trees: The root system of a tree can be up to twice the size of the tree's canopy. If a tree's branches extend over your home, there's a good chance that they extend under your house as well, drawing moisture up from the soil and causing it to shrink significantly Wetting and Softening of Soil The soils around your foundation experience wetting and softening primarily for two reasons: Heavy Rain & Flood Conditions: As clay soil gets wet, it holds on to water and becomes very soft. This soft soil can be weak, causing the home to shift. Poor Drainage: If water is allowed to stand or "pool" next to your home, the soil will absorb the water and swell. As it does, it can lead to bowing walls and cracks in the foundation. Poorly Compacted Fill Soil To level a site where a foundation will be built, builders sometimes bring in loose soil from another location to fill depressed or hollow areas. This "fill" soil can be looser than the dense, hard-packed virgin soils at the site that haven't been disturbed - possibly for centuries! The fill soil brought in by the builder has to be compacted thoroughly before a foundation is built on top of it. If the soil is not compacted well, it may begin to compress underneath the weight of your home, creating settlement problems that can damage your foundation.