Civil Engineering Reference
In-Depth Information
5.3 Piled rafts
As discussed at the start of this chapter, where competent soil exists close to the ground
surface, a combined pile and raft foundation may provide the most economic form of
foundation. Early analytical work by Butterfield and Banerjee (1971) suggested that,
although the proportion of load taken by the piles would drop, inclusion of a ground-
contacting pile cap would have little effect on the stiffness of a pile group. This finding
has been substantiated by experimental work (Abdrabbo, 1976; Cooke
et al
., 1979).
It is also true that, in order to increase the stiffness of a raft foundation significantly,
piles need to penetrate to depths greater than the width of the raft, or to reach strata
that are substantially stiffer than the surface sediments.
Early numerical techniques for the detailed analysis of piled raft foundations,
including assessment of bending moments and differential settlements, have been
described by Hain and Lee (1978) and also by Padfield and Sharrock (1983) (although
treatment of the piles in the latter approach is only approximate). Approximate treat-
ments of mixed foundations are also available for rigid rafts using the programs
PGROUP and DEFPIG, discussed in section 5.2.3. Over the last decade, however, a
variety of analytical approaches of varying complexity have been developed specif-
ically for piled rafts (Poulos, 1994, 2001; Clancy and Randolph, 1996; Franke
et al
., 1994; Ta and Small, 1996; Russo, 1998; Katzenbach
et al
., 2000; Reul and
Randolph, 2003).
Design strategies for pile groups and piled rafts have been discussed by Viggiani
(2001) and Mandolini (2003). Viggiani (2001) has suggested that they may be divided
into two main categories:
1
'Small' pile groups, where the ratio or overall width
B
of the group to the pile
length
L
is less than unity. Piles are needed to ensure adequate bearing capacity,
and the pile cap (or raft) can easily be made sufficiently stiff to eliminate differen-
tial settlements. Even where the pile cap bears directly on the ground it will not
contribute significantly to the overall performance of the foundation.
2
'Large' pile groups, with
B
1, where the pile cap alone will usually pro-
vide sufficient margin against bearing failure, and will contribute significantly in
terms of transferring load directly to the ground. The design of such foundations
hinges more on limiting the average and differential settlements to a acceptable
level. Since for large rafts the flexural stiffness will be low, the location and
length of any pile support should be chosen in order to minimize differential
settlements.
/
L
>
A simple approach to evaluating the overall stiffness of a piled raft, and assessing
the load sharing between pile group and cap, has been suggested by Randolph (1983).
The approach is based on the use of average interaction factors,
α
rp
between the pile
and pile cap (or raft). Writing the raft stiffness as
k
r
and the pile group stiffness as
k
p
,
the overall foundation stiffness,
k
f
, is obtained from
k
r
1
α
rp
k
p
+
−
2
k
f
=
(5.27)
rp
k
r
/
1
−
α
k
p
