Whenever construction workers begin work on a new building, they must first assess where and how they will build the foundation. This is a structure, commonly made of concrete for homes, that transfers the weight of the building onto the earth below. There are different types of foundation designs and each serves a different specific purpose, but generally, every one works to transfer the weight load of a structure to the soil beneath.
Most small and medium homes are built upon a shallow foundation. These are usually comprised of concrete strips that are laid about 3.3 feet (1 meter) beneath the soil, or of a single large concrete slab that is also set about 3.3 feet (1 meter) beneath the soil. When applicable, it will extend beneath the frost line. When engineers design the foundation of a building, they must keep in mind how much the soil will settle beneath it, as well as how much weight will go on top. If calculated incorrectly, the foundation may fail and place the entire structure in peril.
Larger buildings use a deep rather than a shallow foundation. A deep foundation uses long pylons of steel or concrete to penetrate beyond the weaker surface soils into the deeper and more stable soils or bedrock beneath. The load from the walls above is transferred deep into the earth, thereby providing support for the intense weight above. Much like the considerations for a shallow foundation, engineers must consider weight and settlement, as well as scour — water eroding soil beneath the structure.
For structures being built in colder climates, engineers must consider frost heaves as well. Frost heaves occur when moisture in the soil freezes, thereby changing the density of the building's support. A frost heave can cause damage to the foundation, thereby compromising the structural integrity of the entire building. Drier, warmer climates are not entirely exempt from such worries, however; certain soils will expand and contract when moisture is added or taken away, and engineers must factor in such movement when considering where and how to lay a foundation.