The building codes for constructing a foundation generally require a foundation capable of carrying all loads and stresses and relaying the load onto the supporting fill soils. The selected fill soil must support the foundation and footings
The International Building Code requires that foundations not exceed the load-bearing capacity of the supporting soil. Loose soil types, such as sand and silty gravel have significantly lower load-bearing capacities than more solid soil types like crystalline bedrock.
Building codes have additional requirements for constructions made in unstable soils. Foundations must be constructed at greater depths in locations prone to shifting or moving soils. Constructions made on expansive soils must account for the impact that the volume change may exert on the foundation. Building codes require the soil to be removed or the foundation to be constructed to penetrate the expansive soil to stabilize the building.
Foundation footings are constructed against the supporting soil. Footings support the foundation, distribute the load, and protect against settling or changes in the foundation over time. For single-story buildings, the International Building Code requires the footing to be constructed at a minimum depth of 12 inches below the ground surface into undisturbed soil and at a minimum width of 12 inches.