Pile Design and Construction Practice, Fifth edition

Pile groups under compressive loading 267
In the case of pile groups the width B is the width at the base of the equivalent raft as
shown in Figure 5.3. The Burland and Burbidge method was developed essentially for
shallow foundations and correlations with published settlement records given in their paper
were mainly confined to foundations where their depth was not greater than their width.
They state that the depth to width ratio did not influence the settlements to any significant
degree and hence a depth factor of the type shown in Figure 5.20 should not be applied.
However, a correction should be applied to allow for the foundation shape and for the thickness
of the compressible layer beneath the foundation where this is less than the depth of
influence, z I.
The correction factors are
1.25 L�B
Shape factor � fs � � (5.32a)
L�B � 0.25� 2
Thickness factor � fl � (5.32b)
Hs zI � 2 � Hs z I �
where L is the length of the loaded area (L�B)
B is the width of the loaded area
H s is the thickness of the compressible layer (H s�z I)
Burland and Burbidge state that most settlements on granular soils are time-dependent,
i.e. they show a long-term creep settlement and a further time correction factor is applied
using the equation:
ft � (5.33)
where t is equal to or greater than 3 years
R3 is the proportion of the immediate settlement which takes place in the loaded
area
R is the creep ratio expressed as the proportion of the immediate settlement that
takes place per log cycle of time
�t �i � � 1 � R3 � Rlog t
3�
Burland and Burbidge give conservative values of R and R 3 as 0.2 and 0.3 respectively for
static loading and 0.8 and 0.7 respectively for fluctuating loads.
Summarizing all the above corrections, the average consolidation settlement is given by
� c � f s f l f t [(q�� 2
3 �� vo) � B 0.7 � I c] (in mm)
(5.34)
The wide range of I c values between the upper and lower limit shown in Figure 5.26 can
cause difficulty in obtaining a reasonably close estimate of pile group settlements, particularly
where the group is underlain by medium-dense sands. For example, the average I c value for
a sand with an N-value of 10 is 6 compared with upper and lower limit values of 20 and 3
respectively, giving an upper limit of settlement of three times that calculated from the
average curve. However, in most cases piles are taken down to dense sands to obtain