Lecture 25 :
Pile Groups [ Section 25.2 :
Group Deficiency Factor () ; Block Failure Criteria ]
GROUP DEFICIENCY FACTOR ( )
For bearing capacity, = (Ultimate load capacity of group) / Sum of ultimate load capacity of individual piles) The aim of the designer is to make =100% or 1.0
However for sandy soils, > 1.0
This is because, the soils gets densified due to driving of piles in sandy soil and hence soil strength properties increase. Therefore while starting a design we start with
. Since remains same, therefore calculate based on the no. of piles to be used. Compare with In design take whichever value is minimum.
Alternatively, is expressed as function of spacing of piles (s) and equated with .
Calculate ‘s' for =1.0. Usually, ‘s' is taken as 3d to 8d depending on the soil and pile parameters. Lower the value of s, higher is the lower is the efficiency.
Fig. 5.43 Variation of efficiency of pile group with spacing of piles
) ; Block Failure Criteria ] 
) 
=100% or 1.0 
> 1.0 
. Since 
remains same, therefore calculate 
based on the no. of piles to be used. Compare with 
In design take whichever value is minimum.
is expressed as function of spacing of piles (s) and equated with 
. 
=1.0. Usually, ‘s' is taken as 3d to 8d depending on the soil and pile parameters. Lower the value of s, higher is the lower is the efficiency. 
