Pile Foundation Analysis and Design

Normally, a pile belongs to a long cylinder with a strong material like concrete that is driven into the ground to provide a stable support for structures constructed on top of it.

Pile foundations are based on the following conditions :

When a layer of weak soil exists at the surface. This layer does not have the ability to support the weight of the building, therefore, the loads of the building should be circumvented this layer and transmitted to the layer of harder soil or rock that remains underneath the weak layer.

If a building has to sustain very heavy, concentrated loads, like in a high rise structure, bridge, or water tank.

On the basis of structural behavior, the pile foundations are classified as follow :-

End Bearing Piles: In end bearing piles, the bottom end of the pile stands on a layer of specifically sturdy soil or rock. The load of the building is transmitted through the pile on the sturdy layer as if this pile functions like a column.

The main objective is that the bottom end stands on the surface that is the junction of a weak and strong layer. So, the load circumvents the weak layer and securely transmitted to the strong layer.

Friction Piles: Friction Piles transmit the load of the building to the soil around the full height of the pile, by friction. It means, the whole surface of the pile, that is cylindrical in shape, functions to transmit the forces to the soil.

Also Read: Types of Piles for Pile Foundation

How Piles are used: Since pile foundations bear a lot of load, they should be designed cautiously. The engineer has to study the soil properly where the piles will be provided to make sure that the soil is not overloaded exceeding its bearing strength.

Each pile contains a zone of influence on the soil across it. Proper care should be adopted to place the piles far enough apart in order that loads are circulated uniformly over the complete bulb of soil that holds them, and not concentrated into a few areas.

Normally, the engineers have to group a few piles collectively, and top them with a pile cap.

A pile cap stands for a very solid cap of concrete that expands over a small group of piles, and acts as a base upon which a column is built up. The load of this column is then circulated to all the piles in the group.

Some vital points on pile foundations

Clear cover over reinforcement
Cl.6.11.4 50×2 = 100mm

Dia of vertical bar minimum
Cl.6.11.4 12×2 = 24mm

Helical reinforcement minimum
dia Cl.6.11.4 8×2 = 16mm

Clearance for operation of Tremie
Cl.6.11.4 4×20 = 80mm

Dia of Tremie pipe minimum
Cl.8.4 (c) = 200mm

Total dia of pile required
= 420mm say ~ 450mm