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Short Note on Group Action of PIles



 Group Action of Piles:
Piles are always used in groups. This is to ensure that the structural load from a member like a column or a wall lies within the zone of influence of foundation. If single driven pile is used as foundation, one cannot be certain that pile would be located centrally below the foundation element as quite often pile moves laterally during driving. Resultant eccentricity of loading may result in development of bending stress and pile may fail. So, minimum number of three pile is used under a column. The load is transferred to pile in group through a reinforced slab or beam called pile cap. When pile cap stand clearly above the ground level the pile group is called free standing pile group.
The pile spacing mainly controls the behavior of pile groups. The spacing should not be too small so that upheaval of ground surface takes place during driving into dense or incompressible material. On the other hand, if spacing is too large uneconomic pile cap may result. When driving pile in sand, it is advisable to begin at the center of a group and then proceed outwards to avoid problems of ground tightening.
The load carrying capacity of a group of piles is given by
 (Qu)g=Nqu𝑛
where,
(Qu)g= Load carrying capacity of pile group
N = number of piles
qu= allowable load per pile
n = group efficiency

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