Civil Engineering Reference
In-Depth Information
Figure 5.25
Use of settlement-reducing piles to minimize differential settlement.
L
r
Raft: Stiffness,
k
r
2
/
4
Area,
A
=
B
r
L
r
=
p
d
eq
Load,
P
t
B
r
d
eq
Piles: Length,
L
; Area,
A
g
Stiffness,
k
p
Capacity per pile,
q
p
Actual load,
P
g
Ratios:
K
pr
=
k
p
/k
r
a
gr
=
A
g
/A
p
gt
=
P
g
/P
t
P
t
∗
=
P
t
/nq
p
m
=
P
g
/nq
p
Zone of pile
support
L
Figure 5.26
Key geometric parameters for piled raft.
be similar to that of the pile group alone,
k
p
, hence
K
pr
=
k
r
is close to unity. For
moderate-sized rafts, they found that the pile group is most effective if the pile length is
between 1 and 2 times the equivalent diameter of the rectangular raft (
L
k
p
/
1to2).
Differential settlements were minimized by locating the pile support over the cen-
tral 20 to 30% of the raft (
a
gr
∼
/
d
eq
∼
2 to 0.3). For that arrangement, the optimum
design conditions to give essentially zero differential settlement are where the total pile
capacity,
nq
p
, is 40 to 50% of the total applied load
P
t
. As shown in Figure 5.27, for
that load range, the piles are loaded to approximately 80% of their ultimate capacity
(
P
g
/
0
.
nq
p
∼
.
8) and the pile group as a whole carries some 30 to 40% of the total
applied load (Horikoshi and Randolph, 1998).
0
