1 Introduction

The function of a foundation is to transfer the structural loads from a building safely into the ground.

There are two basic types of foundations :

- Shallow foundations : those that transfer the load to the earth at the base of the column or wall of the substructural

- Deep foundations : those that transfer the load at a point far below the substructure.

2 Shallow Foundation

Most shallow foundations are simple concrete footings. A column footing is a square pad of concrete, with or without steel reinforcing that accepts the concentrated load placed on it from above by a building column and spread this load across an area of soil large enough that the allowable stress of the soil is not exceeded.

A strip footing or wall footing is a continuous strip of concrete that serves the same function for a loadbearing wall.

In situation where the allowable bearing capacity of the soil is low in relation to the weight of the building, column footings any become large enough that it is more economical to merge them into a single mat or raft foundation that support the entire building. Mats for very tall buildings are often 1.8m or more thick and are heavily reinforced.

3 Deep Foundation

Caissons

A caisson is similar to a column footing in that it spreads the load from a column over a large enough area of soil that the allowable stress in the soil is not exceeded.

It differs from a column footing in that it reaches through strata of unsatisfactory soil beneath the substructure of a building until it reaches a satisfactory bearing stratum such as rock, dense sands and gravels or firm clay.

A caisson is constructed by drilling or hand-digging a hole, belling out the bottom as necessary to achieve the required bearing area, and filling the hole with concrete.

Reinforcing is seldom used in the concrete except near the top of the caisson, where it joins the column of the superstructure.

A socketed caisson is one that is drilled into rock at the bottom, rather than belled. Its bearing capacity comes not only from its end bearing, but from the frictional forces between the sides of the caisson and the rock as well.

Pile

A pile is distinguished from a caisson by being driven into place rather than drilled and poured. Piles may be classified as either end-bearing or friction piles, according to the manner in which the pile loads are resisted. However, in actual practice, virtually all piles re supported by a combination of skin friction and end bearing.

The principal types of piles include timber, precast concrete, cast-in-place concrete, steel, composite and bulb piles.

Timber piles are inexpensive, easy to cut and splice, and require no special handling. However, maximum pile length is limited to about 30m, load-carrying capacity ability is limited and pile ends may splinter under driving loads. Timber piles are also subject to insect attack and decay.

Precast concrete piles provide high strength and resistance to decay. It is however heavy, and because of its brittleness and low tensile strength, special handling and driving is required. Cutting requires the use of pneumatic hammer, cutting torches etc.

Cast-in-place concrete piles (or shell piles) are constructed by driving a steel shell into the ground and then filling it with concrete. Shells driven into expansive soils should be filled with concrete as soon as possible after driving to reduce the possibility of shell damaged due to lateral soil pressure.

Steel piles are capable of supporting heavy loads, can be easily cut and can be driven to great depth. Common types of steel piles include H-piles and pipe piles. the principal disadvantage of steel pile is its high cost.

Composite piles are piles made up of two or more different materials. E.g. the lower portion of pile might be timber while the upper section might be a shell pile. This would be economical if the lower section would be continuously submerged (hence not subject to decay) while the upper section would be exposed to decay.

Bulb piles are also known as compacted concrete piles, Franki piles , end pressure-injected footings. They usually have a higher load capacity than do cased shaft piles.