A pile is a long slender foundation member, made either of timber, structural steel or concrete which might be cast-in-situ or driven.
It acts as a structural member to transfer the load of the structure to a required depth in deep foundation to carrying a load which may be vertical or lateral or lateral plus vertical.
Pile foundations are used according to soil strata and load on the structure.
As for any heavy building with no hard strata in 20–30 meter & railway bridges where no hard strata are there, we use a pile foundation.
We use a pile foundation when:
- Compressible or weak upper soil layer.
- Presence of horizontal forces.
- Presence of Expansive Soils.
- Subjected to uplifting Forces.
- Soil erosion.
Types of Pile Foundation:
Classification based on material:
A pile foundation made of trees of about 30 cm in diameter is used as a heap; the trunk should be strong, durable and straight.
The head of the pile is covered with a steel cap to avoid scattering or spreading of the wood in the event that the pile is hammered into the ground.
An iron shoe is provided at the other end to facilitate insertion and avoid damage to the pile.
Pile foundation made of concrete are commonly used for deep foundation.
These piles are either pre-cast concrete piles or reinforce cast-in-situ piles.
Precast concrete piles are mostly circular or octagonal with a tapered edge to one side.
These are mostly pre-stressed piles with a length of over 10 meters.
Pre-cast piles can sustain a larger hearing pressure than a pile of wood.
A hole of the required diameter and depth in the ground is drilled into the concrete piles.
The reinforcement cage is then lowered into this hole and concreting is done.
Concrete piles can be used practically for every type of foundation subjected to medium and heavy loads.
Steel pile foundation are either a rolled steel section or a conical shape or it can be a sheet pile.
Classification based on supporting actions:
When the piles transfer the load of the building through the soft-soil layer to a suitable bearing stratum at greater depths, it is called a bearing pile, in which the pile remains on the stratum at greater depths.
When pile foundation moves the load only through the skin resistance without any end bearing then the pile are called friction piles.
Under-reamed pile foundation is commonly used in black cotton soils that swell or shrink when exposed to water or in large quantities upon drying.
It is a bored-in-situ stack that extends one or more or is provided with bulbs along its length known as under-rims.
Classification by size:
Pile foundation with ems section at the bottom are called taped piles.
These piles are used only to increase the density of the subsoil by driving these piles into loose soil.
Tapped piles are also called as compiled piles.
Advantages of Pile Foundation:
- The piles can be pre-prepared according to its length.
- The precast process reduces completion time.
- Can be installed over a very large area.
- Can be installed in very long lengths.
- We can use piles in a place where drilling is not required.
- The piles are very neat and clean.
Disadvantages of Pile Foundation:
- A pile gets quickly damaged when driving through the stones and boulders.
- Piles can be attacked by marine borers in saltwater.
- A pile cannot be above ground level.
- It is very difficult to know the actual required length in advance.
- Vibrations are produced when the piles are driving which affects the neighbouring structures.
- Piles require heavy equipment to operate.
- Piles do not have low drainage.
Uses of Pile Foundation:
As far as deep foundations are concerned there are numbers of types of deep foundations and pile foundation is among one of them.
The uses of Pile Foundation depends on the type of pile used, the intended function for which the pile is used.
The load which is to be applied on the pile and the type of material which is used for the construction of the pile; the following are the uses of piles:
End Bearing or Compressive Strength:
Sometimes we use the piles to achieve the required compressive strength in the soft soil.
In that case, we use the piles to transfer the load through that soft soil to a suitable bearing stratum by using the end bearing or toe bearing property of the pile.
To build a structure within the water and on the water river or canal bed; we have to build the foundation through the river bed and within the scour depth.
For the Ravi River in Pakistan, the depth of the pool is 30 to 35 cm below the bed.
In these types of situations if we go for shallow foundations then we may have to use cofferdam or some diversion which is very non-economic.
Tension or Uplift:
Pile foundation is usually used to carry the compressive nature of the load through the tip bearing or end bearing.
In the case of tall structures or towers, there may be tension that must be resisted by the piles.
For a tower carrying high power transmission lines; for example, wind thrust can create stresses that must be resisted by a tension stack.
Also, include the use of deep foundations or a thick raft that is sometimes informal.
Turbines and silos for the foundation of buildings supporting vibration equipment where vibrations are important and can cause failures.
There are two options that you can go for large scale blocks to absorb vibration and one is that mass block is very economical.
Sometimes piles are pushed into the weakened level of the soil to increase the bearing capacity of the soil.
Piles are called compile piles, and thus by using compile piles we can increase the bearing capacity of the soil.
To deal with any problem associated with soil or to lay a deep foundation, especially piles is a very good option when the time schedule is very tough.
They are fast in construction & are not difficult to design.
Frequently Asked Questions:
What is meant by Pile Foundation?
A pile foundation is defined as a series of columns constructed or inserted into the ground to transmit loads to a lower level of subsoil.
Which type of foundation is pile foundation?
Pile foundations are deep foundations.
What is a pile foundation used for?
Pile foundations transmit structural loads to deeper layers of firm soil in such a way that layers of soil or rock can sustain the load.
How are pile foundations installed?
The equipment fitted with a hollow-stemmed flight auger which is rotated into the ground to the required depth of pilling.
To form the pile, concrete is placed through the flight auger as it is withdrawn from the ground.
Pile foundation is commonly used when the soil conditions are insufficient to carry the design load of a project or when non-vertical loading is expected.