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INFORMATION SERIES: Foundation Settlement

Settlement is the movement of a building (or its components) to a point below its original position. It is usually the result of differential or uneven foundation movement caused by compaction of the soil below it due to construction on non-virgin (or previously disturbed) soil or changes in soil conditions or moisture content. The HouseMaster Technical Staff advises that settlement in an isolated house should not be confused with subsidence. Subsidence is a downward movement of the ground surface over a large area due to the irreversible 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.	Settlement is often more noticeable and of greater concern when it develops in a house with concrete slab and masonry veneer construction. The engineering design of a slab allows for limited up and down movement. However, masonry veneer walls supported by the slab are not capable of the same range of movement and may crack even without significant foundation movement. Construction methods or design deficiencies, alone or in conjunction with other conditions, account for many forms of settlement. Typically, this type settlement most often occurs when a contractor over excavates a site and then must backfill or when a house is constructed on a landfill. If this fill soil is not properly compacted or contains voids, post-construction settlement may occur. Settlement which occurs strictly due to normal (or slight) soil compaction generally reaches a point of stability. This condition explains why in some cases even what is considered a substantial crack might not necessarily be indicative of a future concern. On the other hand, if the fill material contains elements (organic or man-made) which are subject to ongoing deterioration, future settlement will continue to occur until full compaction is reached. Less frequently, unpredictable and generally more substantial settlement might occur due to fault movement, mining operations, collapse of limestone caverns, frost heave (in northern climates) and similar phenomena. With expansive clay soils, there is also the concern that foundation settlement may not be permanent as the soil could continually shrink or swell with changes in soil moisture levels. This could cause cyclic foundation movement and an increased chance of damage occurring to the foundation, slab or other components of the house. Uneven drying out of the soil around the foundation can occur due to the lack of natural ground cover (e.g., driveway, patio and pool decks against foundation), uneven lawn watering, changes in the subsurface water level or other factors. As a result, unless the foundation was designed for this change, settlement will occur as the soil dries out. With this type soil, there is also the possibility for upheaval of the house foundation or slab. This is usually caused by varying soil moisture levels from the perimeter of the house to the interior areas under the house. If the interior area soil moisture level increases due to an uneven slab plumbing leak or inadequate crawl space ventilation, the soil could expand and force any beam or pier upward and cause upward movement of the center of the house. Uneven saturation of the soil around the perimeter of the house could also cause the soil to swell and damage the foundation.

Most houses experience settlement within the first few years after construction; however, in most cases it is not structurally significant. Settlement cracks most often develop in the foundation or house slab, masonry veneer, ceilings or walls. In severe cases there may be operational problems with windows and doors or plumbing lines or mechanical equipment may be damaged. Although every situation is different. cracks are generally considered to be of a structural nature when they approach or exceed one quarter inch in width. Small or hairline cracks are often due to minor settlement, or thermal or moisture changes in construction materials rather than significant foundation movement, especially if they recur seasonally. New or enlarging cracks in an existing structure, however are more likely to indicate a potential structural concern.

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