Foundation Systems for Buildings
A building’s foundation is the structure which supports it in the ground. The forms and materials of building foundations vary according to ground conditions, structural material, structural type, and other factors. In most buildings, the foundation (or basement) wall does not have a significant role in carrying the structural loads from the tower above, but does resist the lateral load of the soil (and any water) that the basement is constructed in. Foundation systems are constructed of concrete almost without exception.
In some building systems, the floor slab is designed to assist in carrying some of the lateral loads of the building, and the floor slab thereby become an integral part of the framing system.
Foundations consisting of vertical structural members that are forced into the ground by impact (from a machine called a “pile driver”). Some early skyscrapers utilized wood piles, but steel and concrete became more practical at the beginning of the 20th century. Piles can be driven to bedrock, or more commonly, “to refusal” (that is, until underlying soil resists the pile being driven significantly further into the soil).
Caisson foundations are similar in form to pile foundations, but are installed using a different method. Caissons (also sometimes called “piers”) are created by auguring a deep hole into the ground, and then filling it with concrete. Steel reinforcement is sometimes utilized for a portion of the length of the caisson. Caissons are drilled either to bedrock (called “rock caissons”) or deep into the underlying soil strata if a geotechnical engineer finds the soil suitable to carry the building load. When caissons rest on soil, they are generally “belled” at the bottom to spread the load over a wider area. Special drilling bits are used to remove the soil for these “belled caissons”.
Mat foundations (also known as “raft foundations”) are a foundation system in which essentially the entire building is placed on a large continuous footing. Mat foundations found some use as early as the Nineteenth Century, and have continued to be utilized to effectively resolve special soil or design conditions. In locations where the soil is weak and the bedrock is extremely deep, “floating or compensated mat foundations” are sometimes utilized. For this type of foundation, the amount of soil removed and the resulting uplift (on the foundation) caused by groundwater is equalized by the downward forces of the building and foundation. Yet another variation of the mat foundation is to use it in combination with caissons or piles.
For spread foundation systems, the structural load is literally spread out over a broad area under the building. Spread foundation systems utilize one or more horizontal mats, or pads, to anchor the building as a whole or to anchor individual columns or sections separately. Spread foundations are also known as “footing foundations” and are a type of foundation often utilized in low-rise buildings.
Load-bearing Wall Foundations
Many building foundations, including most buildings that have basement levels, use slurry walls at the edges to hold out the surrounding earth. In very few cases, this slurry wall or another underground wall element becomes a major load-bearing part of a highrise building’s foundation. This foundation type is usually found in combination with one of the above types.
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