Leaning Tower of Pisa  
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Differential settlement is a deceptively destructive failure that commonly occurs when the building site has not been properly analyzed. Differential settlement will occur when “the soil beneath a structure cannot bear the weights imposed. The settlement of a structure is the amount that the structure will ‘sink’ during and after construction,” [a]. A well-known example of differential settlement is the Leaning Tower of Pisa, which leans due to insufficient bearing capacity of the soil beneath one side of the tower [b].

Common signs that differential settlement has occurred would include: windows and doors sticking, roof and basement leaking, bricks or walls cracking, walls bowing, bulging or leaning, drywall separating, and more. For a full list of common symptoms of differential settlement click here. Common causes associated with differential settlement include: (1) drought conditions – particularly an issue with clay based soils that are able to expand with an increase in water content, and contract during a drought; (2) undermining of the foundation – poor drainage can cause soils to be eroded; (3) sinkholes – naturally occurring phenomenon resulting in a collapse of the supporting soil under the foundation. [c]

Differential settlement can be prevented by carefully inspecting the site before construction and using preventative measures. Certain soils primarily composed of sand or rock may not require a pier system, but any soil that is susceptible to expansion and contraction, such as clay soils, would benefit from a pier system such as the Resistance Pier System or Helical Pier System. Another concern may be Earthquakes such as in California or Japan; if the site is on or close to a fault line the earth itself can move by inches or even feet during an earthquake leading to differential settlement of the site. Many locations around the world must take advantage of seismic design principles.

The most dangerous aspect of differential settlement is that the failure mechanism is not visible, and thus is not easy to catch after the building is constructed. Each site should be carefully evaluated before construction begins.

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