There are three types of pile foundations according to their construction methods:
1. Driven piles,
2. Cast-in-situ piles, and
3. Driven and cast-in-situ piles.
Driven Pile Foundations:
Driven pile foundations can be made from concrete, steel or timber. These piles are prefabricated before placing at the construction site. When driven piles are made of concrete, they are precast. These piles are driven using a pile hammer.
When these piles are driven into the granular soils, they displace the equal volume of soil. This helps in compaction of soil around the sides of piles and results in the densification of soil. The piles which compact the soil adjacent to it is also called as compaction pile. This compaction of soil increases its bearing capacity.
Saturated silty soils and cohesive soils have poor drainage capability. Thus these soils are not compacted when driven piles are drilled through it. The water have to be drained for the soil to be compacted. Thus stresses are developed adjacent to the piles have to be borne by pore water only. This results in increase in pore water pressure and decrease in bearing capacity of the soil.
Cast-in-situ Pile Foundations:
Cast-in-situ piles are concrete pile. These piles are constructed by drilling holes in the ground to the required depth and then filling the hole with concrete. Reinforcements are also used in the concrete as per the requirements. These piles are of small diameter compared to drilled piers.
Cast-in-situ piles are straight bored piles or with one or more bulbs at intervals are casted. The piles with one or more bulbs are called as under-reamed piles.
Driven and Cast-in-situ Piles
Driven and cast-in-situ piles have the advantages of both driven and cast-in-situ piles. The procedure of installing a driven and cast-in-situ pile is as follows:
A steel shell of diameter of pile is driven into the ground with the aid of a mandrel inserted into the shell. After driving the shell, the mandrel is removed and concrete is poured in the shell.
The shell is made of corrugated and reinforced thin sheet steel (mono-tube piles) or pipes (Armco welded pipes or common seamless pipes). The piles of this type are called a shell type piles.
The shell-less type is formed by withdrawing the shell while the concrete is being placed. In both the types of piles the bottom of the shell is closed with a conical tip which can be separated from the shell. By driving the concrete out of the shell an enlarged bulb may be formed in both the types of piles. Franki piles are of this type. In some cases the shell will be left in place and the tube is concreted. This type of pile is very much used in piling over water.