Foundation Settlement
Settlement is the downward movement of a building (or building components) to a point below its original position. Foundation settlement is usually the result of the shifting or compaction of the underlying soil, often due to construction on non-virgin (previously disturbed) soil or backfill or changes in soil conditions and moisture content.
Most houses experience some degree of settlement within the first few years after construction; however, in most cases this movement is not structurally significant. Settlement cracks most often develop in the foundation or house slab, masonry veneer, ceilings and walls. Small or hairline cracks are often due to minor settlement, or thermal movement (expansion and contraction), or as a result of changes in the content of construction materials, rather than significant foundation movement. Cyclic or seasonal movement may or may not represent a significant concern.
In severe cases of foundation movement, operational problems with windows and doors may become apparent. Plumbing lines or mechanical equipment can also be affected. Although every situation is different, a crack is generally considered to be of a structural nature when it approaches or exceeds one-quarter inch in width. Smaller cracks may also represent a concern, particularly if there are multiple cracks; however they may be shrinkage cracks, which rarely warrant concern. On the other hand, new or enlarging cracks in an existing structure are more likely an indication of a potentially serious structural issue.
The effects of settlement are often more noticeable and possibly of greater concern in a house with a full concrete slab foundation or masonry veneer construction. Reinforced slab construction allows for limited movement of the slab without damage. Masonry or masonry veneer walls supported by a slab may not be capable of the same range of movement and can crack even without evidence of foundation movement.
Improper construction methods or design deficiencies, alone or in conjunction with other conditions, account for many forms of foundation movement. Building on soil with voids or on fill soil that has not been properly compacted will almost always result in foundation movement. Many cases of settlement are caused by a contractor over-excavating for the foundation footing and then improperly backfilling.
Settlement that occurs strictly due to normal (or slight) soil compaction generally reaches a point of stability. This explains why in some cases even what is considered a substantial crack might not necessarily be indicative of an ongoing or future concern. On the other hand, if the soil the house is built on contains material (organic or man-made), which contains voids or is subject to ongoing deterioration, settlement is likely to occur at some point in the future, and will continue until full compaction is reached.
Less frequently, unpredictable and generally more substantial settlement occurs due to mining operations, collapse of limestone caverns, frost heave (in northern climates), and similar phenomena. Settlement of the soil over a large area is called subsidence. It is due to the compaction of subsurface soil layers, primarily sand and clay. It is usually caused by a subterranean withdrawal of water (and oil and gas) from an underground aquifer. Since it occurs over a broad area and at a relatively slow rate, its effect is long-term.
With expansive clay soils, there is also a concern that foundation movement may not just appear as settlement but also as upheaval. Clay soil can cyclically shrink or swell with changes in soil moisture levels. This in turn causes cyclic foundation movement and an increased chance of significant damage occurring to the foundation or other components of the house.
Homes built in areas with expansive clays need continual monitoring and maintenance of soil moisture to ensure relative uniformity around the perimeter of a house. The lack of a ground cover, uneven lawn watering practices, changes in subsurface water level, and other factors can lead to uneven drying out of the soil. If this happens, unless the foundation was designed specifically for such conditions, settlement will occur. Conversely, if the soil becomes saturated and swells, upheaval of the foundation can occur. This may be due to a shift from a period of dry to wet weather, over-watering of foundation plantings, or plumbing leaks.
Foundation concerns need to be evaluated by an engineer or other qualified specialist. In some cases, long-term monitoring, soils studies, and borings may be required to determine the cause and necessary corrective measures.
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