Pile foundations may be classifies in three types, based on construction methods.
- Cast-in-situ piles
- Driven piles
- Driven and cast-in-situ piles
Cast-in-situ Pile Foundations
Cast-in-situ piles are concrete piles. These piles are of small diameter compared to drilled piers.
The construction process as below:
Phase 1: Drilling holes with rotary drilling rigs equipped with various drilling tools such as drilling bucket, drilling auger or core barrel.
Phase 2: After reaching the required depth, to install the reinforcement cage with the auxiliary winch of the drilling rig into the borehole.
Phase 3: pour the concrete through the tremie pipe into the borehole.
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 Pile Foundations
Driven pile foundations can be made from steel, timber, or concrete. These piles are prefabricated before being moved into construction site. When the piles are made of concrete, they are precast. These piles are driven by pile hammer.
When these piles are driven into the granular soils, they displace the equal volume of soil. This helps compact the soil around piles and hence densifiesthe soil. The piles which compact the soil adjacent to them are also called as compaction piles. This compaction of soil increases its bearing capacity.
Saturated silty soil and cohesive soil are not drainage-friendly. Thus these types of soil are not compacted while driven piles pass through them. Water needs to be drained to facilitate the compaction on the soil. Thus stresses developed adjacent to the piles aretaken by the water in cavities only, which consequently increases the cavity water pressure and decreasesthe bearing capacity of the soil.
Cast-in-situ and Driven Piles
Cast-in-situ and Driven piles have the advantages of both cast-in-situ and driven piles.
The installation procedures are as follows:
A steel shell same as pile diameteris 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 shell type piles.
The shell-less type is formed while the shell is being withdrawn and concrete is being poured in. In both types ofpiles, 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 classified as this type of pile. In some cases the shell will be left in place and the tube is concreted. This type of pile is commonly used in the water.