There are mainly two categories of foundations .First is open footings & second is pile foundation.
Here we are going to explore through the realm of Pile foundations.
Concept of a foundation:
For stability,the entire load of a building, needs to be properly supported on the substrata. This load comprises of dead load ,live load & wind load (added with seismic & snow loads in specific cases)
This transferring of load can be done either through open foundation ( which requires digging up to the hard strata) or it can be done through piling which does not require any pit.
Let us know piling :
Piling is a method in which the entire load of the building is transferred to the sub-strata by means of cylindrical concrete block placed in the soil. There is no need to do an open pit .
Overall process of piling :
1. A vertical cylindrical hole is made in the soil by any of the 3 methods explained later.
2.Pile reinforcement is inserted into the hole and placed in position.
3.Concreting is done in the pile.
Why Piling ?
Following situations shall illustrate the Piling as a good viable option:
- If the hard strata is quite below from the surface it is not practically feasible to dig an open pit up to that pit. There are safety hazards involved and also the cost of excavation and taking out the soil becomes quite expensive.
- Piling is also a good option if the nature of the underlying strata is not homogeneous. For example if there is a reclaimed land or filled up plot , then it is not possible to correctly estimate the safe bearing capacity of the soil below.
- In cases where ,the land is marshy of filled up with water, it is very difficult to do open foundation.
Types of Piles:
By virtue of the way in which the cylindrical hole is made in the soil, the Piles can be classified as :
1.Hand augured Piles :
This type of piling is manual process. The boring is done manually and it is called ” Auguring “. This type of pile can be taken upto a maximum of 12’00” and can be of diameter from 6″ to 10″. The capacity of such piles can be enhanced by providing one or more bulbs on the stem of the piles. these bulbs are technically known as Under-reams and hence such piles are also called as ” Under-reamed piles.”
2.Hammered Piles :
Here the soil is penetrated by the repeated strokes of a free falling hammer. there is a three legged stand which is erected exactly above the piling location. A dead weight is lifted by a winch through a pulley tied on the top of the stand. The winch lifts the dead weight above the ground.When the winch clutch is released, dead weight falls from an height of 15′ on the ground and penetrates inside the soil. Repeated strokes by the hammer takes the hole deeper to the required depth.
3.Bored with rotary cutters :
A rotary cutter bores the soil up-to desired depth.The cutter is mounted on a rig which can be easily moved to the desired location.
Which pile to use ?
The hand augured piles are the cheapest whereas the rotary piles are the costliest of the three types mentioned.
However, Due to lesser strength , their use is limited to the boundary wall columns or for buildings with 2 floors only.
Hammered and rotary piles can be designed for any capacity by increasing the Pile diameter.
Limitations of each type of piling:
Hammered type of pile is unsafe when there are old buildings in the vicinity. There may be safety risks involved due to the vibrations caused by the repeated hammer impacts on the ground.
Although the rotary piles are quite efficient and safe , their per meter boring cost of is 4 times as compared to hammered piles .
Relevant terms :
Cluster of piles :