The causes of foundation settlement are rarely due to the design (or under-design) of the structure itself. More commonly, damage is caused as changes occur within the foundation soils that surround and support the structure. The following paragraphs are brief explanations for a few of the more common causes of foundation settlement.
Weak Bearing Soils
Some soils are simply not capable of supporting the weight or bearing pressure exerted by a building's foundation. As a result, the footings press or sink into the soft soils, similar in theory to how a person standing in mud sinks into soft, wet clay. In such cases, footings may be designed to spread the load over the weak soils, thereby reducing potential foundation settlement. However, the majority of settlement problems caused by weak bearing soils occur in residential construction, where the footings are designed based upon general guidelines and not site-specific soil information.
Poor Compaction
Placement of fill soils is common practice in the development of both commercial and residential subdivisions. In general, hilltops are cut down and valleys are filled in order to create buildable lots. Properly placed and compacted fill soils can provide adequate support for foundations. When fill soils are not adequately compacted, they can compress under a foundation load resulting in settlement of the structure.
Changes in Moisture Content
Extreme changes in moisture content within foundation soils can result in damaging settlement. Excess moisture can saturate foundation soils, which often leads to softening or weakening of clays and silts. The reduced ability of the soil to support the load results in foundation settlement. Increased moisture within foundation soils is often a consequence of poor surface drainage around the structure, leaks in water lines or plumbing, or a raised groundwater table.
Soils with high clay contents also have a tendency to shrink with loss of moisture. As clay soils dry out, they shrink or contract, resulting in a general decrease in soil volume. Therefore, settlement damage is often observed in a structure supported on dried-out soil. Drying of foundation soils is commonly caused by extensive drought-like conditions, maturing trees and vegetation (see next section), and leaking subfloor heating, ventilation, and air conditioning (HVAC) systems.
Maturing Trees and Vegetation
Maturing trees, bushes and other vegetation in close proximity to a home or building are a common cause of settlement. As trees and other vegetation mature, their demand for water also grows. The root systems continually expand and can draw moisture from the soil beneath the foundation. Again, clay-rich soils shrink as they lose moisture, resulting in settlement of overlying structures. Many home and building owners often state that they did not have a settlement problem until decades after the structure was built. This time frame coincides with the maturation and growth of the trees and vegetation.
Foundations closer to the surface are more often affected by soil dehydration due to tree roots than are deep, basement level foundations. As a general rule, the diameter of a tree's root system is at least as large as the tree's canopy.
Soil Consolidation
Consolidation occurs when the weight of a structure or newly-placed fill soils compress lower, weak clayey soils. The applied load forces water out of the clay soils allowing the individual soil particles to become more densely spaced. Consolidation results in downward movement or settlement of overlying structures
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