DEEP FOUNDATION: PILE FOUNDATION.

Shallow foundations are normally used where the soil close to the ground surface and up to the zone of significant stress possesses sufficient bearing strength to carry the superstructure load without causing distress to the superstructure due to settlement. However, where the top soil is either loose or soft or of a swelling type the load from the structure has to be transferred to deeper firm strata.

The structural loads may be transferred to deeper firm strata by means of piles. Piles are long slender columns either driven, bored or cast-in-situ. Driven piles are made of a variety of materials such as concrete, steel, timber etc., whereas cast-in-situ piles are concrete piles. They may be subjected to vertical or lateral loads or a combination of vertical and lateral loads. If the diameter of a bored-cast-in-situ pile is greater than about 0.75 m, it is sometimes called a drilled pier, drilled caisson or drilled shaft. The distinction made between a small diameter bored cast-in-situ pile (less than 0.75 m) and a larger one is just for the sake of design considerations.