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The pile is a structural member that is made of either concrete, timber or steel. It is a small diameter column, which is driven or cast into the ground up to suitable depth. They are used to construct a foundation, which are deep and ofcourse which cost more than shallow foundations. Despite the cost, the uses of piles are needed to achieve structural safety. Particularly for large structure and weak or poor reclaim soils. The piles may be subjected to vertical loads, horizontal loads or combination of both.
Condition that requires the use of pile foundation:
01. Weak upper Soil:
When upper soil layers are highly compressible and too weak to support the whole load of the structure, piles are used to transfer the load to underlying bedrock or a stronger soil layer. When the rock is present at the reasonable depth below the ground surface, Piles are used to transmit the whole load to the soil gradually. The resistance to the load is derived mainly from the frictional resistance developed at the soil-pile interface.
02. Effect of horizontal forces:
When structure is subjected to horizontal forces, pile foundations resist by bending, while still supporting the vertical load transmitted by the structure. This type of condition is generally encountered in the design of earth water retaining structures and structure subjected to the wind or earthquake forces.
03. Presence of Expansive Soil:
The soil where shrinks and swells considerably with the change in moisture content are called expansive soil. Expansive soils may be present at the site of a proposed structure. Expansive soils shrink and swell depending on their moisture content which may increase and decrease. If shallow foundations are used in such condition, the structure may suffer considerable damage due to settlement of foundation. Hence, pile foundations are considered as alternative when piles are extended beyond the stable soil zone, Which is where shrinking and swelling doesn’t occur.
04. Subjected to Uplifting forces:
The foundations of typical structures such as offshore platforms, a basement of the buildings, transmission towers below the water table,etc. are subjected to uplifting forces due to subsoil water pressure. In such case, piles foundations are used to resist the uplifting force.
05. Soil erosion:
Piers and bridge abutments are usually constructed over the pile foundation to avoid the loss of bearing capacity that a shallow foundation might suffer because of soil erosion at the surface of the ground due to water flows.