Pile Design and Construction Practice, Fifth edition

244 Pile groups under compressive loading

its ultimate bearing capacity as determined for a given penetration by calculation, static
load testing, or an empirical method. Burland(5.3)has stated his strong opinion that this
requirement can result in grossly uneconomic foundation design, because it ignores the
capability of a raft to redistribute loads from the superstructure on to the piles forming the
group. Redistribution of loading can be permitted provided that

(1) The raft has sufficient flexibility (ductility) to perform this function without failure as
a structural unit
(2) The superstructure has sufficient flexibility to accommodate any resulting movements
in the raft
(3) The pile group has an adequate safety factor against failure or excessive settlement
when considered as an equivalent block foundation and
(4) Account is taken of the effects of ground heave or subsidence of the mass of soil
encompassed by the pile group during the construction stage (Section 5.7).

Burland recommends that redistribution should be effected by permitting piles carrying
the heavier loading to mobilize their ultimate resistance in shaft friction, thereby yielding
and transferring some of their load to surrounding piles within the group. This concept of
‘ductile foundations’is discussed further in Section 5.10.
In all cases where piles are designed to transmit loading as a group terminating in a clay
or sand stratum, whether or not some of the piles are permitted to yield, it is essential to
consider the risks of general shear failure or excessive total and differential settlement of the
equivalent block foundation taking the form shown in Figure 5.5.
The ultimate bearing capacity (ultimate limit state) of the block foundation as shown in
Figure 5.5 can be calculated by using Brinch Hansen’s general equation(5.4). This was
referred to in Section 4.3 with reference to the bearing capacity factor Nqin equation 4.16.
The complete Brinch Hansen equation as applied to a shallow spread foundation embedded
in soil with a level ground surface is

Q/A or qu cNcscdcicbcpoNqsqdqiqbq0.5 BNsdib (5.1)

Overall width Overall le

ngth

L

B

D

Figure 5.5Pile group acting as block foundation.