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Piles are the columnar elements in the foundations which have function of transferring loads from super structure through week compressible strata or through water onto stiffened or more compact and less compressible soils. 

What are piles?

They may be required to carry uplift loads when used to support tall structures subjected to overturning forces from wind or waves. 

Piles used in marine structures are subjected to lateral loads from the impact of berthing ships and from waves. 

Combinations of vertical and horizontal loads are carried where piles are used to support to support retaining walls, bridge piers and abutments and machinery foundations.

Presentaiton on Geotechnical Considerations in design of Pile foundation [ppt]

When to use piles?

When the upper soil layer (s) is highly compressible and too weak to support the load transmitted by the superstructure, piles are used to transmit the load to underlying bed rock as shown in figure. 

When bed rock is not located at reasonable depth, piles are used to gradually transmit structural load to the soil. The resistance to the applied structure load is derived mainly from the frictional resistance developed at the soil-pile interface as shown in figure 

When subjected to horizontal forces, piles foundation can resist by bending, while still supporting vertical loads transmitted by the superstructure. This situation is generally encountered in the design and construction of earth retaining structures and foundation of tall structures that are subjected to high wind and earth quake forces 

In many cases, expensive and collapsible soils may be encountered at the site, the soil may extend to a great depth below the ground surface. Expansive soil swells and shrinks depending on increase or decrease of moisture content. The swelling pressure of such soils can be considerably high. Pile foundations are best alternate when piles are extended beyond the active zone, which undergo swelling and shrinking.

Foundations of such structures, such as transmission towers of offshore platforms and basement mats below the water table are subjected to uplifting pressure. Piles are sometimes used for these foundations to resist the uplifting forces.

Bridge abutments and piers are occasionally constructed over pile foundations to avoid the possible loss of bearing capacity that a shallow foundation might suffer because of soil erosion at the ground surface. 

Note: all pictures are in presentation

Classification of Piles

The British Code of Practice for Foundations (BS 8004) places piles in three categories:

• Large Displacement Piles
• Small Displacement Piles
• Replacement Piles

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