For piles in clay you could consider using the Converse-Labarre Formula to compute the effiency (%) of a specified group of piling compared to the performance on an isolated pile.
E = 1 - (a/90) x ((n-1)m + (m-1)n) / mn
Gees... Reminds me of writing equations in Fortran 2!
E = effiency of pile group
a = arc tan (d / s)
d = pile width
s = pile spacing, center to center
m = number of rows of piles
n = number of piles in each row
(Note: If I put some of my parentheses in the wrong place just search on Google for the formula - you'll find it.)