Groups of Piles

Group of piles means when we have more than 1 pile in a row. Many factors influence the pile group stability. The major factors are Geometry of the group, soil conditions and direction of loads.

The efficiency factor E_g is defined as the ratio of the ultimate group capacity to the sum of the ultimate capacity of each pile in the group. It is this factor which is mostly used to express the ultimate load considerations.

Now
E_g = [2(BL+WL)f_s+BW_g]/nQ_u
where
B=Breadth of block of soil
W=Width of block of soil
L= Length of block of soil.
S = spacing
n= number of piles
f_s is the average peripheral friction stress of block
Q_u= single pile capacity.

The study shows that for cohesive soils E_g>1 if S is more than 2.5 pile diameters
For cohesionless soils E_g>1 for the smallest practical spacing.

On field, we take the minimum pile spacing for conventional piles in the range of 2.5 to 3D.

Design of rock sockets is conventionally based on

where
Q_d= allowable design load on rock socket
d_s= socket diameter
L_s= socket length
f_R= allowable concrete rock bond stress
q_a= allowable bearing pressure on