Pile Design and Construction Practice, Fifth edition

Pile groups under compressive loading 249
D should be taken as the penetration depth of the piles into the bearing stratum. Values of
the load inclination factors i c, i q, and i � are shown in Figure 5.10 in relation to � and
the effective breadth B� and length L� of the foundation. Simplified values where the
horizontal load H is not greater than V tan ��cB�L�, and where c and � are the parameters
for cohesion and friction respectively of the soil beneath the base are given by the following
equations:
i c � 1 � H
2cB�L�
i q � 1 � 1.5H
V
i � � i 2 q
(5.11)
(5.12)
(5.13)
Equation 5.13 is strictly applicable only for c � 0 and � � 30 � but Brinch Hansen advises
that it can be used for other value of �.
The base of an equivalent block foundation, i.e. pile toe level, is usually horizontal but
where piles are terminated on a sloping bearing stratum, the base of the block can be treated
as horizontal at a depth equal to that of the lowest edge and bounded by vertical planes
through the other three edges (Figure 5.6). The base factors b c, b q and b � are unity for a
horizontal base.
It is evident from the foregoing account of the application of the Brinch Hansen equation
that it is not readily adaptable from its original use in the design of relatively shallow spread
foundations to deep pile groups subjected to appreciable transverse loading. In such cases it
is preferable to use a computer program which can simulate interaction between the piles
and the surrounding soil and can give a visual display of the extent of any overstressed zones
in the soil below the group. Further aspects of group behaviour under transverse loading are
discussed in Section 6.4.
Equation 5.1 ignores friction on the sides of the block foundation. The contribution of
side shear is only a small proportion of the total where piles are taken down through a weak
soil into a stronger stratum. In cases of marginal stability side shear resistance can be
calculated as the shear resistance on a soil to soil interface on the sides of the group.
Where piles are installed in relatively small numbers there is a possibility of excessive
base settlement if two or more piles deviate from line and come into near or close contact at
the toe and the toe loads are concentrated over a small area. While failure would not occur
if the safety factor in end bearing was adequate, the settlement would be higher than that
which would occur when the piles were at their design spacing. This would lead to differential
settlement between the piles in the group. A safeguard against this occurrence is the
adoption of a centre-to-centre spacing of piles in clay of at least three pile diameters, with a
minimum of 1 m. BS 8004 recommends a centre-to-centre spacing for friction piles of
not less than the perimeter of the pile or for circular piles three times the diameter. Closer
spacing can be adopted for piles carrying their load mainly in end bearing but the space
between adjacent piles must not be less than their least width. Special consideration must be
given to the spacing of piles with enlarged bases, including a study of interaction of stresses
and the effect of construction tolerances. German practices for driven piles and for bored
piles are shown in Figures 5.11 and 5.12 respectively, (see also BS EN 1536 and EN 12699).