What is pile foundation?

A concrete pile is basically a long cylinder in reinforced concrete that is buried in the ground to act as a stable support for the structures that will be built on top of a platform supported by one or more of said cylinders. The set is known as foundation piles or deep foundations.

Pile foundations are mainly required for tall buildings as the design loads are too large to be supported on spread footings. Sometimes, when soil has low SBC or the soil is clayey, a raft or a pile foundation may be the solution.

 Piles can by classified in 3 ways:

1. The way in which the load is transferred on to the soil.
2. Method of Installation.
3. Material of construction.

 

Classification of Piles based on load transfer

 

Point Bearing Piles

If bedrock or hard strata is present within reasonable depth, piles can be extended to rest on the bedrock or penetrate required depth into the hard strata. In this case, the ultimate bearing capacity of the pile depends entirely on the underlying rock/strata.

 

Friction Piles

In a friction pile, the load on pile is resisted mainly by skin friction along the pile shaft. Pure friction piles tend to be longer than point bearing piles. Resistance is a function of the shaft area in contact with the soil. In cohesion-less soils, such as sands of medium to low density, friction piles are often used to increase soil density. When hard strata are not available at reasonable depth, end bearing piles are uneconomical. For this type of subsoil condition, piles are driven through the softer material to required depth.

 

Classification of Piles based on Method of Installation

 

Driven or displacement piles

They are usually pre-cast before being driven or hammered into the ground. This category consists of driven piles of steel or concrete and piles formed by driving tubes or shells which are fitted with a drive shoe.

 

Bored piles

These piles require a hole to be drilled first and into which the pile is then formed usually of reinforced concrete.

 

Types of Piles based on Materials

 

Timber piles

• Timber piles are made of tree trunks that are driven with narrow end as a point
• Maximum length: 30~40m; Optimum length: 10-20m
• Max load for usual conditions: 40 MT

 

Advantages

Low initial cost compared to concrete piles, permanently submerged piles are resistant to rot/decay, handling is easier, these are best suited as friction piles.

 

Disadvantages

Splicing is difficult, piles are vulnerable to damage especially while hard driving, piles are vulnerable to decay unless it is below permanent water table as is usually the case. if subjected to alternate wetting & drying, life can be short.

 

Steel piles

These are commonly used in marine and other structures. Maximum length is practically unlimited. They have a comparatively larger load capacity.

 

Advantages 

Larger capacity, best suited for end bearing, easy to splice, small displacement, able to penetrate through light obstructions

 

Disadvantages

• Required corrosion protection to the metal.
• May be damaged/deflected by major obstruction

Concrete Piles

• Concrete piles may be precast, prestressed, cast in place, or composite construction.
• Precast concrete piles may be made using ordinary reinforcement or prestressed steel.
• Precast piles using ordinary reinforcement are designed to resist bending stresses during transport & bending moments from lateral loads and to provide sufficient resistance to vertical loads and any tension forces developed during driving.
• Prestressed piles are formed by tensioning high strength steel prestressed cables. It is common to have higher-strength concrete (M35 to M55) in prestressed piles Read more........