A foundation is the base of a structure or building. It is the part that provides support to the overall load of the building and prevents over-settlement or movement of the finished structure. Foundations spread out the overall weight to do this. Although there are several types of foundations, most can be broadly classed as either deep or shallow.
Shallow foundations, or spread footings, are built close to the surface. If ground conditions will not affect the load-bearing capacity of the footing, then shallow footings are used. Shallow foundations include pads, strip footings and rafts.
Deep foundations are built below the ground, at depths of 9 feet or more. This type of foundation transfers the load below ground to a more suitable material if the soil at the building site is unable to support it. Types of deep foundations include piles and caissons.
Shallow Foundation Types
Pad foundations might be circular, square or rectangular pads, used to support a specific point load such as a column. Typically pad foundations are made of a block or slab that may be stepped if the load must be spread from a heavy column. They are usually shallow, but deep pad foundations can also be used. Strip foundations support a line of loads such as a wall or a row of columns placed too close to use individual pads. Raft foundations spread the load from a structure over a large area and are used when structural loads such as columns are too close together to use pads. Raft foundations consist of a concrete slab that covers the entire building area and may be reinforced by ribs or beams.
Deep Foundation Types
Piles shift foundation loads through the soil to deeper soil or rock. They're used when it is beneficial because of cost or soil conditions to transmit loads beyond the depth of a shallow foundation. Piles are also used to support loads in regions where extreme weather such as high wind or rain would affect the foundation's load-bearing ability and are classed as end-bearing, friction, settlement-reducing, tension and laterally loaded piles. Caissons are made above ground and then sunk below ground by removing the material from inside the caisson.
Foundation Selection Criteria
The type of foundation used for any given structure will depend on a variety of elements. Location, climate, soil and the purpose of the structure play a vital part in the type of foundation used. When deciding on a deep or shallow foundation, a number of engineering factors are also considered. The builders must consider the type of load transfer needed, the quality of soil at the building site and the weight of the building when complete, as well as the predicted limits of movement over time. If the load-bearing capacity of the soil is high and the soil is very dense, then settlement isn't as much of a concern as when the soil is loose. A shallow foundation would be suitable in this case. Shallow foundations are often used for residential buildings. Deep foundations are used when soil is loose or will shift under heavy loads. These foundations are commonly used in coastal areas or building sites with sloped ground.