Ultimate-load considerations are usually expressed in terms of a group efficiency factor, which is used to reduce the capacity of each pile in the group. The efficiency factor Eg is defined as the ratio of the ultimate group capacity to the sum of the ultimate capacity of each pile in the group.
Eg is conventionally evaluated as the sum of the ultimate peripheral friction resistance and end bearing capacities of a block of soil
B=Breadth of block of soil
W=Width of block of soil
L= Length of block of soil.
For a given pile, spacing S and number of piles n,
Eg = [2(BL+WL) fs +BWg ]/nQu
where
fs is the average peripheral friction stress of block and Qu is the single pile capacity.
The study shows that for cohesive soils Eg>1 if S is more than 2.5 pile diameters D
For cohesionless soils Eg>1 for the smallest practical spacing.
A possible exception might be for very short, heavily tapered piles driven in very loose sands.
In practice, the minimum pile spacing for conventional piles is in the range of 2.5 to 3.0D. A larger spacing is typically applied for expanded-base piles.
Design of rock sockets is conventionally based on
group-piles
where
Qd= allowable design load on rock socket
ds= socket diameter
Ls= socket length
fR= allowable concrete rock bond stress
qa= allowable bearing pressure on rock
Load-distribution measurements show, however, that much less of the load goes to the base than is indicated .
