DEEP FOUNDATIONS (PILE FOUNDATIONS)
In case of deep foundations, the piles are used to transmit the load of structure to the soil. The team pile foundations are used to describe a construction for the foundation separately or they may be placed in the form Of a cluster throughout the length of the wall. This construction is adopted when the loose soil extends to a great depth. The load of the structure is transmitted by the piles to hard stratum below or it is resisted by the friction developed on the sides of piles.
Piles are deep foundations. They are relatively long, slender members and are either driven into the ground or bored cast-in-situ. The pile foundations become one of the choices when shallow foundations are not able to meet the required objectives. Soils, which are not in a position to withstand the load from the superstructure be meeting the desired criteria of satisfactory foundation, i.e., no shear failure of the foundation soil and not exceeding the allowable settlement.
The most important factor in a pile selection is the soil condition and the type of piles I.e., driven pile or a cast-in-situ pile, selection will be governed by the soil type and its consistency if cohesive soil and its degree of compactness if cohesion less soil.
USES OF PILES
The situations which demand piles as foundations are as follows:
· The load coming from the structure is very heavy and the distribution of load on soil is uneven.
· The subsoil water level is likely to rise or fall appreciably. This may be seasonal or occasional variation.
· The pumping of subsoil water is too costly for keeping the foundation trench in dry condition.
· The construction of raft or grillage foundations is likely to be very expensive or is practically impossible.
· The firm bearing stratum exists at a greater depth. The piles up to 20 meters depths are common and under exceptional circumstances, they may even be taken to 30 meters depth. The piles are considered to be long when their length exceeds 30 meters.
· The timbering to excavations is too difficult to maintain the sides of the foundation trench.
· The pile foundation is to be adopted for the structures in the area where canals, deep drainage lines, etc. are constructed in near future.
· The structure is situated on sea-shore or river bed and the foundation is likely to be affected by the scouring action of water. Thus, the piles are useful for the marine structures.
· The piles are also used as anchors. They may be designed to give lateral support or to resist an upward pressure or uplift pressure.
· The piles are used as fender piles in the construction of docks, piers and other marine structures. A fender pile protects the berthing ships from damage.
TYPES OF PILES
These piles are broadly classified into the following two categories:
Load bearing piles
LOAD BEARING PILES
These piles bear he load coming from the structure. The piles are generally driven vertically or in near vertical position. When a horizontal force is to be resisted, the piles may be driven in an inclined position and such inclined piles are termed as the batter piles. The design of batter piles should be made by considering the fact that they will resist most, If not all, of the horizontal loading. If batter piles are used together with vertical piles, it may be assumed that part of the vertical load will be transferred to the batter piles also. The load bearing piles Amy resist the load by directly resting on a firm stratum or by friction developed at their sides. The former piles are known as the bearing or sustaining piles and the latter piles are known as the friction or pleating piles.
- Bearing piles: These piles penetrate through the soft soil and heir bottoms rest on a hard bed. Thus, they are end-bearing piles and act as columns or piers. The soft ground through which the piles pass also gives some lateral support and this increases the load carrying capacity of the bearing piles.
- Friction piles: When loose soil extends to a great depth, the piles are driven up to such a depth that he frictional resistance developed at the sides of he piles equals the load coming on the piles. Great care should be taken to determine the frictional resistance offered by the soil and suitable factor of safety should be provided in the design. The total frictional resistance of piles is obtained by multiplying frictional resistance of soil with the area of pile in contact with the soil. The circumference of pile when multiplied with the depth of penetration f pile n the ground will give the area of pile in contact with the soil. The total frictional resistance can be increased in the following ways:
o By increasing the diameter of he piles.
o By driving the pile to a greater depth.
o By making the surface of the pile rough.
o By placing the piles closely.
o By grouping the piles.