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Note on Pile Group Efficiency



 Pile Group Efficiency
The efficiency of a group of piles is defined as the ratio of ultimate  bearing capacity of group to the sum of individual piles ultimate bearing capacity.

Efficiency (n) =ultimate bearing capacity of the group/(n x ultimate bearing capacity of a single pile in the group) x 100 %


                        𝑛=  (Qu)g/N qu
Where,
            n = group efficiency   
            (Qu)g= Load carrying capacity of pile group
            N = number of piles
            qu= allowable load per pile
The group efficiency depends upon the type of soil, method of installation, spacing, load per pile and the pile in the group. Its value for bearing or friction piles at sites where the soil strength increases with depth is found to be 1. For friction piles in soft clays the value on n is less than 1. When piles are driven in loose, sandy soils, the soil is densified during driving, and n >1 in such cases, It has been observed that if the spacing between piles is more than 2.5 times the pile diameter, the group efficiency is not reduced. Fleds Rule and Converse Labare Formula are generally used to determine efficiency.

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