PILE FOUNDATIONBY

CHANDAN K B

Pile foundation is a special kind of deep foundation, where the depth of the foundation is much greater than the width of the foundation.

Today, pile foundation is much more common than any other type of deep foundation. It is used;

1.where the soil is compressible, 2.where the soil is water logged and 3.when stratum of required bearing capacity is at greater depth. The major uses of piles:

To carry vertical compression loads, To resist uplift loads To resist horizontal or inclined loads

What is Pile Foundation?Pile foundation is required when the soil bearing capacity is not sufficient for the structure to withstand. This is due to the soil condition or the order of bottom layers, type of loads on foundations, conditions at site and operational conditions.

Many factors prevent the selection of surface foundation as a suitable foundation such as the nature of soil and intensity of loads, we use the piles when the soil have low bearing capacity or in building in water like bridges and dams

A pile foundation consists of two components: Pile cap and single or group of piles. Piles transfers the loads from structures to the hard strata, rocks or soil with high bearing capacity. These are long and slender members whose length can be more than 15m.

Piles can be made from concrete, wood or steel depending on the requirements. These piles are then driven, drilled or jacked into the ground and connected to pile caps. Pile foundation are classified based on material of pile construction, type of soil, and load transmitting characteristic of piles.

TYPES OF PILE FOUNDATION

Based on the function;1. End bearing Pile2. Friction Pile3. Compaction Pile4. Tension Pile or Uplift Pile5. Anchor Pile6. Fender Pile and Dolphins7. Batter Pile8. Sheet Pile

Based on the material & composition;1. Concrete Pile2. Timber Pile3. Steel Pile 4. Composite Pile: Concrete & Timber, Concrete & Steel

Based on the method of installation;1. Driven Pile2. Cast-in-situ Pile3.Driven and cast- in- situ Pile

Classification of Piles:

Pile foundationPiles can be divided in to two major categories:

1. End Bearing Piles

If the soil-boring records presence

of bedrock at the site within a reasonable depth,

piles can be extended to the

rock surface

2. Friction Piles

When no layer of rock is present depth at a site,

point bearing piles become very long and

uneconomical. In this type of subsoil, piles

are driven through the softer material to

specified depths.

Function of pilesAs with other types of foundation, the purpose of a pile foundation is:

To transmit a foundation load to a solid groundTo resist vertical, lateral and uplift load

Piles can beTimberConcreteSteelComposite

General facts Usual length: 10m-20mUsual load: 300kN-3000kN

Advantages Corrosion resistanceCan be easily combined with a concrete superstructure

DisadvantagesDifficult to achieve proper cutoffDifficult to transport

Concrete piles

Pile Cap Reinforcement

Pile caps carrying very heavy point loads tend to produce high tensile stresses at the pile cap.

Reinforcement is thus designed to provide: ◦ Resistance to tensile bending forces in the bottom of the cap ◦ Resistance to vertical shear

Piles are commonly driven by means of a hammer supported by a crane or a special device known as a Pile Driver.

Hammers adopted for driving the pile are of the following types:1. Drop hammer2. Single acting hammer3. Double acting hammer4. Diesel hammer5. Vibratory hammer.

Pile Driving

What is ultimate load bearing capacity? What is allowable load and how is it determined?

Ultimate load bearing capacity of a pile is defined as the maximum load which can be carried by a pile and at which the pile continues to sink without further increase of the load.

The allowable load is the safe load which the pile can carry safely, which can be determined from ultimate load bearing capacity dividing by suitable F.O.S.

Load carrying capacity of Pile:

Negative friction is a down-ward drag acting on a pile due to the downward mo-vement of the surrounding compressible soil relativeto the pile.

It is shown in figure aside. Figure shows the pile pass-

ing through a recently con-structed cohesive soil fill. The soil below the fill is completely consolidated under its overburden pressure.

Negative Skin Friction

Negative Skin Friction will occur due to the following reasons:

When the surrounding compressible soilhas been recently filled.

If the fill material is loose cohesion less soil. By lowering the ground water which increases the

effective stress causing consolidation of the soil with resultant settlement and friction force being developed on the pile.

It also occurs when the fill is over the peat or a soft clay stratum.

Cont...

Factors governing the selection of pilesThey are:

Length of the pile in relation to the load and type of soil, Characters of structure, Availability of the materials, Types of loading, Factors causing deterioration, Ease of maintenance, Estimated cost of types of piles, taking into account the initial cost, life expectancy and cost of

maintenance and, Availability of funds.

Selection of PileThe selection of type of pile foundation is based on site investigation report. Site investigation report suggests the need of pile foundation, type of pile foundation to be used, depth of pile foundation to be provided. The cost analysis of various options for use of pile foundation should be carried out before selection of pile foundation types.

Unless the ground condition is rocks, for heavy construction and multi-storied buildings, The bearing capacity of soil at shallow depth may not be satisfactory for the loads on the foundation. In such cases, pile foundation has to be provided. The number of piles in a pile groups required is calculate from the pile capacity of single pile and the loads on the foundation. Piles are a convenient method of foundation for works over water, such as jetties or bridge piers.