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c o r r e s p o n d i n g r e l a t i o n s h i p f o r p i l e
h e a d s e t t l e m e n t . T h e s e
figures s h o w t h a t th e m a x i m u m l o a d i n t h e p i
le a n d t h e pi le
h e a d s e t t l e m e n t c o n t i n u e
to
increase w i th increas ing l ive load .
W h e n
t h e
a p p l i e d
l ive l o a d
is
a p p r o x i m a t e l y e q u a l
t o t h e
d e a d
l o a d , th e m a x i m u m l o a d e q u a l s th e applied
load:
that
i s , the
d r a g
force d u e t o t h e g r o u n d s e t t l e m e n t i s r e d
u c e d s u c h t h a t
t h e
m a x i m u m
l o a d
i s now a t t he
p i l e h e a d .
T h e
p i l e h e a d
sett lement a lso becomes similar
to the
settlement thatwould
h a v e
occurred i f the grou nd se t t lement ha d not been imp
osed.
F r om
a p r a c t i c a l v i e w p o i n t , i t w o u l d a p p e a
r t h a t , a t l e a s t i n t h e
e x a m p l e considered
th e
amount
o f
live
load
that
w o u l d . n e e d
to be a d d e d to rel ieve th e nega t ive f r ic t ion e f fec
ts i s fa r
g r e a t e r
t h a n w o u l d n o r m a l l y
be
a l low ed. Thus
i t may be
c o n c l u d e d
t h a t
nega t ive f r ic t ion effects a re unl ike ly to be comple te
ly
r e m o v e d w h e n n o r m a l m a g n i t u d e s
o f
l i v e l o a d
a re
a p p l i e d .
30
25
20
15
10
5
Applied load =
1- 5
MN
Corner
pile
- group :
- - Centre pile - group
Single pile
20
40 60 80 100 120 140 160 180 200
Ground
surfacesettlement: mm
Fg 18.Pile settlement against
ground
surface
settlementfor
various
piles
ingroup
6.3.
Group
effects
I t
is
b e c o m i n g r e c o g n i s e d t h a t g r o u p e f fe c t
s
m a y b e b e n e f i c i a l
in r e l a t i o n t o t h e e f fects o f n e g a t i v e s k
in f r i c t io n . T o e x a m i n e
th e g e n e r a l n a t u r e
o f
g r o u p e f fects ,
t h e
p rogram PIES
h a s
been
u sed to a n a l y s e a g r o u p o f n ine p i les , a s s h o
w n in F i g . 17 , w i th
t h e g r o u n d
p r of i l e
b e i n g t h a t o f t h e e n d b e a r i n g c a s e s h o w
n in
Fig.
2 .
E a c h p i l e
is a s s u m e d to h a v e a l e n g t h of 25 m and to be
su bjec t ed
to a
l o a d
o f
1 - 5
M N , t hus g iv ing a n o v e r a l l f a c t o r o f
sa fe t y
o f a b o u t 2 a g a i n s t g e o t e c h n i c a l f a i l u
r e . A g r o u n d s u r f a c e
s e t t l e m e n t o f 200 mm is
t h e n i m p o s e d
o n t h e
pi les, dec reasing
f rom a
m a x i m u m
a t t h e
s u r f a c e
to
zero
a t 2 0 m
d e p t h .
T h e
i n d u c e d
p i l e
l o a d s a n d s e t t l e m e n t a r e e x a m i n e d f o r
t h e c o r n e r
a nd
cen t re p i les of the g roup, and a l so for a s ingle iso la
ted p i le .
F i g u r e
1 8
s h o w s
t h e
c o m p u t e d p i l e h e a d s e t t l ement
a s a
f u n c t i o n
o f t h e
g r o u n d s u r f a c e
settlement. I t can be seen that
a
th e
p i l e h e a d s e t t l e m e n t s i n c r e a s e (b u t
a t a
d i m i n i s h i n g
r a te ) w i t h i n c r e a s i n g
so i l
s u r f a c e s e t t l e m e n t ;
b
t h e c e n t r e p i l e s e t t l e s m o r e t h a n th e c o
r n e r p i l e
c )
b o t h
p i l e s
i n t h e
g r o u p s e t tl e c o n s i d e r a b l y m o r e t h a n
a
s i ng l e i so l a t ed
p i le .
1-25 1- 2 5
1-25
@
1-25
A = 1- 5
MN/pile
Stable zone
Ground settlement
stiff clay) profile
F i g u r e 1 9 sh o w s t h e c o m p u t e d r e l a t i o n s
h i p b e t w e e n t h e
m a x i m u m l o a d i n e a c h p i l e a n d t h e g r o u n
d s u r f a c e s e t t l e m e n t .
T h e m a x i m u m l o a d i n c r ea s e s w i th i n c r e a
s i n g g r o u n d
se t t lement , and i s
less
f o r t h e c e n t r e p i l e t h a n f o r th e c o r n e
r
p i le .
T h e
r a te
o f
increase
fo r
b o t h
th e
g r o u p p i l e s
is ,
h o w e v e r ,
s i g n i f i c a n t l y l o w e r t h a n for a s i n g l e i
s o l a t e d
p i l e .
I t is not
u n t i l
re l a t iv e ly l a r g e g r o u n d s e t t l e m e n t s o c
c u r t h a t t h e l o a d s i n t h e
g r o u p a n d s i n g l e p i l e s b e c o m e s i m i l a r
. T h i s c h a r a c t e r i s t i c i s
c o n s i s t e n t w i t h t h a t f o u n d b y K u w a b a r
a a n d P o u l o s . 4
It
c a n
t h e r e f o r e
b e
c o n c l u d e d t h a t g r o u p
e f f e c ts m a y b e
b e ne f ic i a l
in
t e r m s
o f t h e
i n d u c e d l o a d s
i n t h e
p i l e s , e s p e c i a l l y
fo r
relatively
s m a l l m a g n i t u d e s o f g r o u n d m o v e m e n t .
H o w e v e r ,
a t
n o r m a l w o r k i n g l o a d s
t h e
p i l e h e a d s e t t l e m e n t
is
s t i l l
increased because of group ef fec ts .
ON LUSIONS
This paper h a s
demonstrated that
des igning p i les to a c c o u n t fo r
nega t ive sk in
f r i c t i on
r e q u i r e s t h r e e c r i t e r i a t o b e s a t i s f i
e d :
o v e r a l l
g e o t e c h n i c a l c a p a c i t y , s t r u c t u r a l c
a p a c i t y
o f t h e
p i l e
i tself , a n d
settlement
cont ro l . F o r this last cri terion, i t h a s been
s h o w n that s e t t le m e n t s c a n b e l i m i t e d b y
h a v i n g t h e l e n g t h o f
p i le
i n t h e s t a b l e ( n o n - s e t t l i n g ) z o n e s u c
h t h a t t h e r e i s a
f a c t o r
o f
safety o f
about
1-25 in
that zone against
th e combined
effects
o f a p p l i e d l o a d a n d d r a g l o a d d u e t o n e g
a t i v e s k i n
f r ic t ion . I f th i s con di t ion i s sa t i s f ied , then
th e se t t lem ent
Cornerpile- group
Centrepile- group
Single
pile
50 100 150
Ground
surface
settlement:
mm
20 0
Fig.
17. Pile g roup example
Fig.
19.Maximum loadagainstground settlementfor various
piles ingroup
7/25/2019 17 ICE - A Practical Design Approach for Piles With
Negative Friction
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reachesa limiting value and does not continue to increase if
theground continuestosettle.Asimple approachcan then
givean adequate estimation of the pile head settlement.
The
influence
of
other
factors on
induced drag loads
and
drag
settlements is also examined. It is found that the presence
of
residual
stresses
in a
pile tends
to
reduce
the
drag settlement
considerably,
especially
if the
pile
has a
relatively large
end
bearing capacity and
stiffness.This suggests that preloading
a
pilemay
have
a
beneficialeffect
in
reducing drag settlements.
The
application
of
live load
to a
pile does
no t
reduce
th e
total
load
in the pile, but rather reduc es the relative contribution
that
t he
drag load
makes to the
overall maxim um pile load.
Group
effects
aregene rally beneficialandlead to a
significantly lower rateo fdevelopmentofdragforce and drag
settlement with increasing soil settlement than is the case
for
an isolated pile.
ACKNOWLEDGEMENTS
The
author gratefully acknowledges
th e
valuable comments
o f
Patrick
K .
Wong
of Coffey
Geotechnics.
REFERENCES
1.
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B. H.
Recent advances
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Urban
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