Civil Engineering Reference
In-Depth Information
0.6
0.8
0.6
P
g
/nq
p
= 1
0.5
0.36
0.36
0.25
0.25
0.4
0.16
0.16
0.3
a
gr
= 0.09
0.2
0.1
L/d
eq
= 1.5
0
0
0.2
0.4
0.6
0.8
1
Ratio of pile capacity to total load,
nq
p
/P
t
Figure 5.27
Load sharing between raft and pile group.
These results have been confirmed by other parametric studies, which have consid-
ered uniform loading of the raft (Prakoso and Kulhawy, 2001; Viggiani, 2001). For
many buildings, however, a significant proportion of the load is applied at the edges
of the raft through the outer walls of the building. Reul and Randolph (2004) (see also
Randolph, 2003) have considered such a case, with a relatively extreme load distri-
bution with 50% applied as a uniform line load around the edge of the raft, and the
remaining load uniformly distributed over the central 25% of the raft (representing
the load applied by the central core of the building containing lift shafts, etc.). Even
for that case, it was found that differential settlements (for thin rafts), or bending
moments (for thick rafts) could be minimized using central pile support.
As an example, application of the approach outlined for a complete raft, the foun-
dations for a 16-storey block of flats built on London Clay are considered. The
building, at Stonebridge Park in the London borough of Brent, was founded on a
pile group consisting of 351 bored piles, each 0.45 m in diameter and 13 m long,
capped by a pile cap in direct contact with ground (see Figure 5.28). The pile cap
and piles were extensively instrumented to measure contact stresses between the raft
and the ground, and the loads going into typical piles in the group. Results of the
field study, and implications for the design of such foundations, have been presented
by Cooke
et al
. (1981). A numerical analysis has also been reported by Padfield and
Sharrock (1983).
The shear strength profile for the site may be approximated by the straight-line
relationship
c
u
=
100
+
7
.
2
z
kPa
(5.31)
where
z
is the depth in m below foundation level (2.5 m below ground level). Adopting
a ratio for
G
c
u
of 200 (Simpson
et al
., 1979) for the London Clay, the shear modulus
profile is given by
/
G
=
20
+
1
.
44
z
MPa
(5.32)
