What is Pile Foundation?
The foundation which is provided in soils, incapable to transmit the structural load to suitable stratum by inserting relatively slender structural element called pile is known as pile foundation.
Pile foundation is a type of deep foundation. Pile made of steel, concrete or wood. A pile is either driven into the soil or formed in-situ by excavating a hole and filling it with concrete.
Why pile foundation is used?
Pile foundations are generally used when poor soil conditions extend to large depths and load to be supported is quite heavy. Pile foundation are best suited in the following conditions:
- The load transmitted from the structure is heavy and non-uniform.
- The sub-soil water is so high. Providing raft foundations are uneconomical.
- If it is not possible to maintain foundation trenches in dry conditions by pumping due to heavy inflow of capillary water.
- If structures are located on riverbed or sea-shore.
- When the strata just below the ground surface are highly compressible.
- When there is washout, erosion or scour of soil below shallow foundation.
- When there is the non-uniform settlement of the shallow foundation.
- When we have to transfer structural loads through deep water to firm stratum.
- When there is expansive soil such as black cotton soil, which swell or shrink as the water content changes.
Types of Pile
1. Based on material Used
Steel pile – Steel pile are generally either in the form of thick pipes or rolled steel H-Sections. Pipe steel piles are driven into the ground with their ends open or closed. Steel piles are provided with a driving point or shoe at the lower end.
Concrete Piles – Cement concrete is used in the construction of concrete piles. Concrete piles are either precast or cast-in-situ.
The reinforcement is provided to resist handling and driving stresses during construction. Precast piles can also be prestressed using high strength pre-tensioned cables.
Timber pile– Timber piles are made from tree trunks after free from trimming. The timber used should be straight, sound and free from defects.
Generally, below the water table, timber piles have a long life. However, above the water table, these are attacked insects.
The life of timber piles can be increased by preservatives such as creosote oils. Timber piles should not be used in a marine environment where these are attacked by the various organism.
Composite Pile – A composite pile is made of two materials. A composite pile may consist of the lower portion of steel and the upper portion of concrete.
A composite pile may also have the lower portion of timber below the permanent water table and upper portion of concrete.
As it is difficult to provide a proper joint between two dissimilar materials, composite piles are rarely used in practice.
2. Based on Mode of transfer of Load
End-Bearing pile – these types of piles transmit the load through their bottom tips. Such piles act as columns and transmit the load through a weak material to a firm stratum below.
If bedrock is located within a reasonable depth, piles can be extended to the rock. If instead of bedrock, a fairly compact and hard stratum of soil exists at a reasonable depth, the pile can be extended a few meters into the hard stratum.
The ultimate bearing capacity of the pile depends upon the bearing capacity of the rock. These types of piles are also known as point bearing piles.
Friction Pile – Friction pile do not reach the hard stratum. These piles transfer the load through skin friction between the embedded surface of the pile and surrounding soil.
These types of piles are used when a hard stratum does not exist at a reasonable depth. The ultimate load carried but the pile is equal to the load transferred by skin friction.
These piles are also known as floating piles, as these do not reach the hard stratum.
Combined end bearing and friction piles – These piles transfer loads by a combination of end bearing at the bottom of the pile and friction along the surface of the pile shaft.
The ultimate load carried by the pile is equal to the sum of the load carried by the pile point and the load carried by the skin friction.
Also, Read – What is Combined footing? – Definition and Types
3. Based on the method of installation
Driven pile – These piles are driven into the soil by applying blows of a heavy hammer on their tops.
Driven and Cast-in -situ piles – These types of piles are formed by driving a casing with a closed bottom end into the soil. The casting is latter filled with concrete. The casting may or may not be withdrawn.
Bored and Cast-i-situ piles – These piles are formed by excavating a hole into the ground and then filling it with concrete.
Screw piles – These piles are screwed into the soil.
Jacked piles – These piles are jacked into the soil by applying a downward force with the help of a hydraulic hammer.
4. Based on Uses
Load bearing pile – These piles are used to transfer the load pf the structure to a suitable stratum by end bearing, by friction or by both.
Compaction Pile – These piles are driven into loose granular soils to increase the relative density. The bearing capacity of the soil is increased due to densification caused by vibration.
Tension piles – These piles are in tension. These piles are used to anchor down structures subjected to hydrostatic uplift forces or overturning forces.
Sheet pile – Sheet piles farm a continuous wall or bulkhead which is used for retaining earth or water.
Fender piles – Fender piles are sheet piles which are used to protect the water-front structure impact of ships and vessels.
Batter piles – Batter piles are not vertical but driven inclined to resist horizontal and inclined forces.
Anchor piles – Anchor piles are used to provide anchorage for anchored sheet piles. Anchor piles provide resistance against horizontal pull for a sheet pile wall.
5. Based on the displacement of soil
Based on the volume of the soil displaced during installation, the piles can be classified into the following 2 categories
Displacement pile – All driven piles are displacement piles as the soil is displaced laterally when the pile is installed.
The soil gets densified. The installation may Cause heaving of the surrounding ground. Precast concrete pile and closed-end pipe pile are also known as high displacement piles. Steel H- piles are low displacement piles.
non-displacement piles– Bored piles are non-displacement piles. Since the soil is displaced when the hole is bored, there is no displacement of the soil during installation.
The installation of these piles causes very little change in the stress in the surrounding soil.
6. Under-reamed Pile
These are a special type of RCC piles provided with bulb near its bottom end and is known as single under-reamed pile.
For heavy load, more than one bulb may be provided which is known as multi under-reamed pile. This type of foundation is economical about 25% and can bear all adverse conditions.
So, friends, this is the complete information about pile foundation and types of pile foundation.
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