Geotechnical Engineering

DHARM

PILE FOUNDATIONS 677

16 .6 Pile Groups

A structure is never founded on a single pile. Piles are ordinarily closely spaced beneath struc-
tures; consequently, the action of the entire pile group must be considered. This is particularly
important when purely friction piles are used.

The bearing capacity of a pile group is not necessarily the capacity of the individual pile
multiplied by the number of piles in the group; the phenomenon by virtue of which this dis-
crepancy occurs is known as ‘Group action of piles’.

16.6.1Number of Piles and Spacing

Usually driven piles are not used singly beneath a column or a wall because of the tendency of
the pile to wander laterally during driving and consequent uncertainty with regard to centering
the pile beneath the foundation. In cases where unplanned eccentricities result, failure may
occur either at the connection between the pile and column or within the pile itself. Hence,
piles for walls are commonly installed in a staggered arrangement to both sides of the centre
line of the wall. For a column, at least three piles are used in a triangular pattern, even for
small loads. When more than three piles are required in order to obtain adequate capacity, the
arrangement of piles is symmetrical about the point or area of load application.

Representative pile group patterns for wall and column loads are indicated in Fig. 16.12.

S S

S

(b) 3-pile group

S

×

(a) For wall (c) 4-pile group

S

S ××

(d) 5-pile group

× 5S

S

S

(e) 6-pile group

S

S ×

(g) 8-pile group

×

S

S

S

S×

S

S

(h) 9-pile group

S

×

(f) 7-pile group

S

S S

S

Fig. 16.12 Representative pile group patterns
Column and wall loads are usually transferred to the pile group through a pile cap,
which is typically a reinforced concrete slab structurally connected to the pile heads to help
the group act as a unit (Fig. 16.13).

The requirement for group arrangement of driven piles does not apply to bored piles.
Drilled shafts can be installed quite accurately. A single large-diameter drilled shaft pile is
commonly used to support columns in residential buildings. This may be used when the three-
pile configuration yields unnecessarily extra load carrying capacity in the case of driven piles.