Types of Building Foundations

Foundations are generally grouped into shallow and deep categories, depending on how far they extend into the ground and how they transfer the building's load. The choice between them depends on soil strength, groundwater level, the weight of the structure and sometimes the depth at which stable ground can be found.

Shallow foundations

Shallow foundations sit relatively close to the surface and are used when the soil near the top is strong and stable enough to carry the building's load without excessive settlement. They are generally more economical than deep foundations and are the most common choice for houses and smaller buildings.

Strip foundations

A strip foundation is a continuous band of concrete laid under load-bearing walls. It spreads the wall's weight along its length and is one of the most widely used foundation types for residential construction, particularly where soil conditions are reasonably uniform and the building load is moderate.

Pad foundations

Pad foundations, sometimes called isolated or spread footings, are individual blocks of concrete placed under single points of load, such as columns. They are common in framed structures where the weight of the building is carried by a series of columns rather than continuous walls.

Raft foundations

A raft foundation is a single continuous slab that covers the entire footprint of a building, spreading the load over a large area. Rafts are often used where the soil is relatively weak or where differential settlement needs to be minimised, since the slab helps the structure move as one unit rather than cracking unevenly.

Deep foundations

When the soil near the surface cannot safely support a building, or when loads are very high, deep foundations are used to transfer weight down to stronger soil or rock layers further below ground.

Pile foundations

Piles are long columns, typically of concrete or steel, driven or bored deep into the ground until they reach a layer capable of bearing the load. Piles can work either by bearing directly on strong material at depth or through friction along their length within the surrounding soil. They are common for larger buildings, bridges and structures on poor or variable ground.

Caissons and deep piers

In some situations, large-diameter shafts or caissons are used, particularly where very high loads need to be carried or where conditions below the water table make other methods impractical. These are specialised solutions generally reserved for significant structures rather than typical residential building.

How the right type is chosen

Selecting a foundation type is not a matter of preference but of matching the solution to the ground. A geotechnical investigation, which typically involves test pits or boreholes and laboratory testing of soil samples, provides the information an engineer needs to recommend a suitable foundation. Factors considered include the bearing capacity of the soil, its tendency to shrink or swell with moisture changes, the depth of the water table and the presence of any unstable layers such as soft clay, fill material or, in some parts of South Africa, dolomitic rock.

Why this matters for existing buildings

Knowing which foundation type a building likely has can help explain certain patterns of cracking or movement. A structure on a raft, for example, tends to move as a whole, while one on strip footings may show more localised cracking if only part of the foundation is affected by soil movement. This background knowledge is useful context when investigating problems, even though diagnosing the actual cause still requires a qualified professional to inspect the specific building.