A pile is a slender structural member having very small area of cross-section relative to its length. It is a deep foundation where depth is greater than the width. It is used when shallow foundations cannot support the structure.
A pile foundation must fulfill the following requirements:
- The material itself must not be over-stressed
- There must be an adequate factor of safety against a shear failure
- The settlements must be within the tolerable limits
- It should not over stress the underlying soft strata.
Piles maybe required for a variety of reasons. For example, it is used to:
- Transfer to stronger and / or less compressible stratum
- Transfer loads in weak soil by friction between pile and soil
- Compact loose granular soils
- Provide foundation below scour depth
- Carry horizontal and inclined forces from bridge abutments and retaining walls
- Carry uplift forces
- Stiffen soils beneath machine foundations to control both the amplitude of vibration and the natural frequency of the system
Types of Pile
A pile may be classified in different ways. The classification is done on the basis of
- Pile function
- Pile material
- Method of installation
Based on Function
Based on their function, piles are grouped into three main categories. They are points bearing piles, friction piles and combination of friction and point bearing. The point bearing piles derive resistance entirely from base. The friction piles derive resistance by adhesion between soil and pile shaft.
Based on Material
Based on materials used, the piles are classified as timber piles, concrete piles and steel piles.
Timber Piles – They are made of tree trunk or sawed timber sections. They are light, easy to handle and sometimes cheap. They can be jointed together and can be provided with driving points. Timber piles are liable to decay and attack by marine borers and generally used below water table. They are mainly used as friction piles, but on occasions also as point bearing piles. They can be driven to a. length of 20 m and can have capacity up to 60 tons.
Concrete Piles – These piles can be pre – cast or cast – in -situ. They can be driven to large lengths. Pre cast piles are manufactured in a casting yard and cured properly. Piles are then transported to the place where they are installed. They can be driven to a length of 27 m and can have capacity as 100 tons.
Steel Piles – The most usual types of steel piles are rolled H sections piles and screw piles. They are used where hard driving is required. Pipe piles are driven open ended or close ended. They are often filled with concrete after driving. The rolled steel sections can be driven up to 36 m in length and can take load up to 170 tons. Screw piles can have length up to 24 m and working loads up to 250 tons are possible.
Based on Method of Installation
On the basis of method of installation, piles are grouped into three main categories. They comprise large displacement piles, small displacement piles and non displacement piles.
a) Large displacement piles
Pre cast piles – Performed solid or hollow closed at the bottom end driven into ground and in position. Timber and pre east concrete are solid types whereas steel tubes or box pile and concrete tube are hollow types.
Driven and cast – in – situ piles – Cast in situ pile is formed by driving a closed ended tubular section to form a void then filling the void with concrete whilst withdrawing the section. Examples: Various systems such as Franki piles, Vibro piles, Alpha piles and so on.
b) Small displacement piles
Steel sections – Steel sections including H – piles, open ended tubes and box piles
Screw piles – Piles are screwed-into the soil.
c) Non – displacement piles
Bored and cast – in – situ piles – A void is formed by boring and excavation, the void is filled with concrete. The sides of the void may be or may not be supported. The sides may be supported permanently or temporarily by casing or temporarily by drilling mud such as bentonite.