Civil Engineering Reference
In-Depth Information
Randolph (1981) has shown that interaction factors for fixed-headed piles may be
estimated from the expression
E
p
G
c
1
/
7
d
s
1
cos
2
α
uf
=
0
.
3
ρ
c
+
β
(5.19)
ρ
c
and
G
c
are as defined in section 4.4.2, and
β
where
is the angle of departure that
the piles make with the direction of loading (see Figure 5.12). At close pile spacings,
this expression tends to overestimate the amount of interaction, and it is suggested
that, where the calculated value of
α
uf
exceeds 0.33, the value should be replaced by
2
27
α
uf
=
−
1
(5.20)
α
uf
The form of equation (5.19) shows that interaction under lateral loading decreases
much more rapidly with the spacing,
s
, than for axial loading. In addition, interaction
of piles normal to the applied lateral load is only half that for piles in line with the load.
As discussed above, the lateral response of a pile group will depend on the axial
stiffness of the piles in addition to their lateral stiffness, and general analysis of the
group is necessary. Only if rotation of the pile cap (and thus axial movement of the
piles) is prevented, do the piles deflect purely horizontally. For this case, it is possible
to think in terms of a 'deflection ratio',
R
u
, defined in a similar way to the settlement
ratio,
R
s
for vertical loading. Thus
R
u
, is the ratio of the average flexibility of piles in
a group, to that of a single pile, for lateral deflection under conditions of zero rotation
at ground level. Figure 5.13 shows this ratio for square pile groups containing up
to 121 piles at
s
3, for a range of pile stiffness ratios (represented by different
values of slenderness ratio
L
c
/
/
d
=
d
(see Chapter 4)). Typical values of deflection ratio
are significantly lower than for vertical loading and decrease with decreasing stiffness
(or slenderness) ratio. As for vertical loading, values for
ρ
c
=
0
.
5 are well below those
ρ
c
=
for homogeneous soil (
1), reflecting the lower interaction between piles for soil
where the stiffness is proportional to depth.
Limitations of the elastic approach to pile interaction under lateral loading have
been revealed from model pile tests carried out on a centrifuge by Barton (1982).
These tests showed that leading piles of groups embedded in sand carried a higher
proportion of the overall applied load than the trailing piles. This arises out of the
effects discussed in section 5.1.2, whereby the limiting pressure on a pile lying in the
wake of another pile is reduced. For pairs of piles at close spacing (
s
2), Barton
found that the applied load was shared 60% to the front pile and 40% to the back pile.
/
d
=
Figure 5.12
Definition of departure angle,
β
.
