Building structures primarily consist of two parts: foundations and superstructures. The lowest parts of the structure which are in direct contact with the ground and subgrade transmit the loads of the structures to the ground are the foundations or substructures. The term superstructure is used to denote the part of the structure which is above ground level.
The foundations can be broadly classified into two categories, shallow and deep foundations.
Shallow foundations: The foundation which is constructed at a shallow depth is termed as shallow foundations.
Spread footing: To construct shallow foundations, a spread is given under the base of a wall or a column. If the footing contains steps or offsets, then it is known as the stepped footing. If the wall rests directly on foundation without any step then it is known as a simple footing.
The image below shows a step footing. The left half of the image shown the photo of the site condition and construction. The duplicated image on the right half shows the same step footing but with yellow markings indicating the layout of the steps in the footing.
Combined footing: A common footing provided for two or more columns is known as a combined footing. This type of footing is adopted when the exterior column is situated near boundary line of the plot and it is not possible to extend the footing symmetrically on both sides.
Raft/Mat foundations: These foundations are constructed mainly to increase the bearing power of soil when it has to transmit non-uniform load and the soil is of yielding nature such as soft clay or reclaimed soil. The raft foundations are generally adopted for public buildings, office buildings, school buildings etc.
Grillage foundations: The depth of grillage foundations are kept within 1m to 1.5m and the width is increased considerably to bring down the pressure on the soil within permissible limits.
Deep foundations
Deep foundations are adopted when shallow foundations are not able to meet the requirements.
Pile foundation: They are relatively long, slender members which are either driven into the ground or bored cast in-situ.
Caissons: A caisson is a structure which is sunk through ground or water to exclude water and semi-fluid material during the process of excavation of foundations and which subsequently becomes an integral part of the substructure. There are mainly three types of caissons:
Box caissons: A box caisson is open at top and closed at bottom. It is merely a variation of the suspended type cofferdam.
Wells: A well is a caisson which is open at top as well as at the bottom. It is constructed with a cutting edge at the bottom to facilitate its sinking to the soil.
Pneumatic caissons: A pneumatic caisson is open at bottom and closed at top. The compressed air is used to remove water from the working chamber and the foundation work is carried out in dry conditions. They are useful when it is not possible to adopt wells.
Coffer dams: A coffer dam is defined as a temporary structure which is constructed so as to remove water and or soil from an area and make it possible to carry on the construction work under reasonably dry conditions. They are particularly useful to facilitate pile driving operations, to place grillage and raft foundations, to construct foundations for piers and abutments of bridges, dams, locks etc.
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