Structural Settlement

Differential settlement is the term used in structural engineering for a condition in which a building’s support foundation settles in an uneven fashion, often leading to structural damage. All buildings settle somewhat in the years following construction, and this natural phenomenon generally causes no problems if the settling is uniform across the building’s foundation or all of its pier supports. But when one section of the foundation settles at a faster rate than the others, it can lead to major structural damage to the building itself.

Differential settlement is not usually a sign of carpentry construction flaws, although some people view it that way. Instead, the phenomenon results when the soil beneath the structure expands, contracts, or shifts in an uneven fashion, causing the foundation to settle at an uneven rate. Thus, the villain is not the carpentry construction practice, but rather the prior evaluation and preparation of the building site itself and the construction of the foundation.

Causes

Uneven settlement of a foundation is always caused by some form of shifting of the soil beneath the foundation, but this shifting can take place for several reasons.

Soils with weak bearing capacity. Some soils are weak and highly compressible by nature, and buildings erected on such soils require special footings to spread the load over a wider area. This tends to be an issue about which local building architects are well familiar, and it is generally addressed during the excavation and construction of the foundation.

Poorly compacted soil. Building sites for commercial or residential structures often consist of land that has been artificially leveled and filled for ease of construction. When properly compacted, this fill soil can provide a perfectly solid base for supporting foundations, but when not compacted, the soil may settle and compress unevenly under the foundation, leading to structural damage.

Changes in soil moisture. Soil that is either too dry or too wet can cause foundation settlement. When moisture builds up, soils saturate and lose their load-bearing capacity. Dry soils shrink in volume. Either situation can cause uneven settling of the foundation. Soil moisture changes can come about due to prolonged drought or by mature trees and other plantings that draw moisture from the soil. In rare instances, leaking in subfloor heating, ventilation, and air-conditioning ductwork can affect the soil moisture beneath the foundation.

Trees and vegetation. Large trees, shrubs, and other vegetation planted along a building’s foundation or close to it can gradually draw the moisture out of the soil and cause it to shrink. This situation is more common with shallow foundations than with basement-level foundations that extend down many feet. When foundation settlement begins to occur several decades after construction, the soil has likely shrunk because large trees are drying out the soil.

Soil consolidation. The weight of a building on the underlying soil, especially fill soils that were added just prior to construction, will naturally compress the soil. Clay soils, in particular, become very dense as moisture is squeezed out. As the soil consolidates and shrinks, the foundation settles downward, a movement that can cause cracks and other structural damage.

Vibration. Vibration of the soil from seismic activity or even from nearby road traffic can cause soils to settle or shift unevenly, leading to structural damage to the building.