Structural Capacity of Pile

A pile is a slender structural member made of steel, concrete or wood. A pile is either driven into soil or formed in-situ by excavating a hole and filling it with concrete.
Feasibility as Foundation
When the soil at and near the ground surface is not capable of supporting a structure, deep foundation are required to transfer the load to deeper strata. One of the most common and most widely used deep foundation is pile. It is generally much more expensive than the shallow foundation and feasible only when a firm stratum is so deep that it can not be reached economically by shallow foundation.
Structural Capacity
Piles have to withstand handling stress during driving or installation and the necessary strength to transmit the load imposed on them to soil.
Axial Capacity
When a pile is fully embedded in soil with undrained shear strength greater than 10 KN/m2 , the axial carrying capacity of it should not be limited by its strength as long column. If the soil is weak ( undrained shear strength less than 10 KN/m2), it should be examine whether the shaft would behave as a long column. Where buckling may occur, its structural strength should be decreased to account for reduction in capacity. In such case, in determining effective length of a pile which is not safe against buckling by adequate bracing, the fixity condition imposed on it by the structure it supports and by the nature of soil in which it is installed, is used. 
Lateral Capacity
Lateral capacity of vertical single piles shall be the least of the values calculated on the basis of soil failure, structural capacity of pile and deflection of pile head.
Deflection calculations required horizontal subgrade modulus of surrounding soil. When considering lateral load on piles, the effect of other coexistent loads, including axial load on the pile, shall be taken into consideration for checking structural capacity of the shaft.
For estimating depth of fixity, established method of analysis shall be used, or lateral load test to at least twice the proposed design working load shall be made. The resulting allowable load shall not be more than one-half of the test load which produces a gross lateral movement of 25 mm at the ground surface.
All piles standing unbraced in air, water or soils not capable of providing lateral support shall be designed as columns in accordance with the provision of ACI code.

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