FOUNDATION TYPES AND USES

Foundation of a building is that part of the substructure which is in direct contact with and transmits loads to the ground. The substructure is that part of building which is below natural or artificial ground level and which supports the superstructure.

To select a foundation for a particular building some factors are to be considered

1.       The load to carry

2.       Nature of the subsoil

3.       The soil bearing capacity

4.       The soil behaviour under seasonal and ground level water changes.

When these are noted a foundation type can be select

TYPE OF FOUNDATION

The types of foundation are categories into two major classes which are

1.       SHALLOW FOUNDATIONS

2.       DEEP FOUNATIONS

SHALLOW FOUNDATION:   Shallow foundations are those founded near to the finished ground surface; generally where the founding depth is less than the width of the footing and less than 3m. These are not strict rules, but merely guidelines: basically, if surface loading or other surface conditions will affect the bearing capacity of a foundation it is 'shallow'. Shallow foundations (sometimes called 'spread footings') include pads ('isolated footings'), strip footings and rafts.
Shallows foundations are used when surface soils are sufficiently strong and stiff to support the imposed loads; they are generally unsuitable in weak or highly compressible soils, such as poorly-compacted fill, peat, recent lacustrine and alluvial deposits, etc.

·         STRIP FOUNDATION: it is a continuous strip serves as a level base on which the wall is built and is of such a width as in necessary to spread the load on the foundations to an area of subsoil capable of supporting the load without undue compaction. Concrete is the material principally used today for strip foundation as it can readily be placed, spread and levelled in foundation trenches, to provide a base for walls and it develops adequate compressive strength as it hardens to support the load on foundation. .....

·         RAFT FOUNDATION: it consist of reinforced concrete under the whole of a building. This type of foundation is described as a raft in the sense that the concrete raft is cast on the surface of the ground which supports it, as water does a raft, and the foundation is not fixed by foundation carried down into the subsoil.

·         Pad foundation: are used to support an individual point load such as that due to a structural column. They may be circular, square or rectangular. They usually consist of a block or slab of uniform thickness, but they may be stepped or haunched if they are required to spread the load from a heavy column. Pad foundations are usually shallow, but deep pad foundations can also be used.

DEEP FOUNDATION: are those founding too deeply below the finished ground surface for their base bearing capacity to be affected by surface conditions, this is usually at depths >3 m below finished ground level. They include piles, piers and caissons or compensated foundations using deep basements and also deep pad or strip foundations. Deep foundations can be used to transfer the loading to deeper, more competent strata at depth if unsuitable soils are present near the surface

·         PILE FOUNDATION: A foundation ( spread footing or grillage ) supported on piles is called a pile foundation. A pile foundation usually consists of a base of spread footing or grillage supported by piles at their bottom. Piles distribute the load of structure to the soil in contact either by friction alone or by friction combined with bearing at their ends.