P 1793- Negative Skin Friction in Piles and Design Decisions

8/10/2019 P 1793- Negative Skin Friction in Piles and Design Decisions

1/9

1 1111

Proceedings: Third

International

Conference on Case Histories

in

Geotechnical Engineering St. Louis Missour i

June 1-4 1993 SOA No 1

Negative Skin Friction in Piles and esign ecisions

M. T. Davisson

Consulting Engineer, Savoy, Illinois

SYNOPSIS

Negative sk in

f r i c t i on

behavior o f

p i l e

foundations

i s

descr ibed

for

condit ions of p i l e

usage

in

t he USA. Methods of de te rmining downdrag load and pi l e r es i s tance to downdrag are

explained.

Othe

Eactors

en te r ing

in to

design a r e discussed

such

as p i l e

load t es t ing

and

analysis ,

pi l e s t ruc tura l

s trength,

f ac to r s of safe ty ,

p i l e

d r i v ab i l i t y ,

and reduct ion

of

downdrag

loads. Observat ions

from

seven

~ n p u l i s h e d

negat ive sk in

f r i c t i on

fa i lu res

a re

used for

i l l us t r a t ion .

INTRODUCTION

~ e g t i v e skin f r i c t i on NSF) loads

on

p i l e

foundat ions

(a lso

ca l l ed

downdrag)

have been

t ecorded by engineers for

a t

l e a s t the pas t 70

y-ears

(Chel l i s ,

1961). Foundat ion

engineer ing

t 'eference

works

have

descr ibed both the

phenomenon

and

the

forces to be r e s i s t ed

in

design

fo r

a t l e a s t the pas t 45

years

(Terzaghi

and Peck, 1948). Never theless , fa i lu res o f

p i l e

foundations caused

by nega t ive sk in f r i c t i on

continue to occur .

Of

the p i l e

foundat ions

t h a t

have fa i l ed because

of

NSF,

the

olde r case hi s to r i e s general ly

have

causes re la ted to the enginee rs ignorance of the

physica l phenomenon and/or a lack of knowledge

o f

the s o i l pr of i l e

and

per t inen t physical

proper t i e s o f

the

so i l . The wri ter

has been

ca l l ed upon

personal ly to inves t iga te seven

NSF

f a i lu res

over

the pas t

33

years,

none

of

which

have been published,

and s aware

of many othe r

f a i lu res

from

both the

l i t e r a tu r e and personal

communications. A s t r i k i n g

f ea tu re

of t he

more

recent

case

h i s t o r i e s of f a i l u r e i s t h e

involvement

of

engineers t r a ined

in geotechnica l

engineer ing. Thus, NSF

fa i lu res

a re occurr ing a t

t he

hands o f

engineers who a re

supposed to

know

how to prevent them.

The

foregoing

exper ience i s reason enough to

r ev i s i t

the sub jec t o f

negat ive

sk in f r i c t i on .

Although

the

elements of t he phenomenon are

considered

wel l known in

th e

profession,

perhaps

a di f fe ren t method o f express ion wi l l prove

helpful in t h e

fu ture

to

those deal ing

with p i l e

design.

The design process has been

chosen

as

the

organizing

framework

fo r the

discussion

given

herein. Further , t i s assumed t h a t an

adequate

so i l

boring program

i s car r ied out , t ha t the

borings are s u f f i c i en t l y longer

than

the

pi l e s ,

and t h a t enough i s known

about the

s t r eng th and

s t i f fn es s s of the

s o i l

mater ia l s .

Design o f

a p i l e

foundation

typ ica l ly

(but

not

necessar i ly) involves both a geotechnical and a

s t ruc tu ra l

engineer .

An

over lap may ex i s t

in the

areas

of

competence of the

two engineers ,

o r

1793

t he i r

competence

together may barely

cover

t he

required

subjec t

matter .

Clear ly,

a danger

ex i s t s

t h a t

an important

subjec t may f a l l i n to a

gap

between the

two

engineers , and can become t he

cause

o f a

fa i lu re .

The wri te r i s aware

of

severa l such instances.

The design

of p i l i n g general ly

involves both

s t ruc tu ra l

and geotechnical

concepts.

:rn t he

following discussion

reference

i s made to

many

subjec t s , inc lud ing so i l

p rof i l e

analysis , pi l e

s t ruc tu ra l

s t reng th , p i l e dr ivab i l i ty , ana lys i s

of

pi l e

load t e s t s , p i l e

group

behavior, load

t rans fer

analysis , fac tors

of

safe ty fo r

both

s t ruc tu ra l

and

geotechnical

matters , and

techniques of res i s t ing NSF. These

concepts

are

brought to

bear

on

design i ssues for p i l e s

subjec ted to negat ive skin

f r i c t ion .

NEGATIVE SKIN FRICTION

CONCEPTS

Pi l e s

typ ica l ly

a re

used

where

a r e l a t i v e l y weak

compressible

s o i l

layer ex is t s near

the

ground

surface .

Pi les

are then driven through t h e weak

layer and founded on or in a

re la t ive ly

s t rong

incompressible

layer .

The

purpose of d r iv ing the

pi les i s to control set t lement of t he supported

s t ruc ture by

t r ans fer r ing the s t ruc tu ra l load

to

the r e l a t i v e l y s trong incompressible s tra tum

( s t ra ta ) .

This simple function represents

by

fa r

t he l a r g es t use of pi l ing in the USA Simple

s o i l

prof i l es wi l l serve to introduce both the

concepts

and

t he

nota t ion

used

herein.

Norma

pi l e

serv ice

condit ions

where

negat ive sk in

f r i c t i o n i s not

opera t ive a r e

shown in Figure 1a

wherein t he

p i l e

i s subjected t o s t ru c t u ra

loading ,

Rs

cons i s t ing of dead D)

plus

l i v e L)

loads .

The

weak

compressible

so i l layer

( l aye

1) i s not se t t l i ng , and

so i l

reac t ions on t h e

pi l e

cons is t

of upward

f r i c t i o n

from both

s o i

l aye rs p lus t i p

r es i s tance

in layer

2.

A

s t a t i c

compression load

t e s t to f a i lu re of t h e p i l e

( s l ippage of

the

pi l e

r e l a t i v e t o t h e

so i l ,

Figure

lb) r e s u l t s in a t o t a l

fa i lu re

load, Ruv

with t h e s o i l react ions ac t ing n the

sam

di rec t ion as for

the service

condit ion, but

a

ul t ima te

s o i l

r es i s tance values .


2/9

=D L

Rut

.-- t-

r--- t-

r

layer 1

r

layer 1

r

layer2

r

layer2

- - - - -

r r

a) Service- Normal

b) Test at Ultimate Load

Fig. 1 Normal Conditions Without Downdrag

For

the

service loading case

where

so i l downdrag

i s operat ive

(Figure

2a)

layer 1 subsides

r e la t ive to the pi le . Because of t he downward

so i l movement the

f r i c t ion

load

on

t he pi l e i s in

the downward

direct ion,

or negat ive to

t h a t in

Figure 1a where the so i l was not moving.

Therefore,

layer 1

i s

act ing

as

a

load

on t he

pi le . The to ta l load in the

pi l e a t

t he

junc t ion

of

layers 1

and

2 s

+

NSF, where NSF i s the

downdrag

load.

The so i l react ions

res i s t ing

the

load in

the

pi le consist of

the poin t

r es i s tance

and

the

skin f r ict ion,

both from l ayer 2 only.

R

8

+ NSF

r

a)

Service- Downdrag

~ N S

layer 1

layer 2

b) Tension Test NSF

Fig.

2

Pile Soil Condit ions

With

Downdrag

The

magnitude of

the

downward movement

of

t he

so i l re l a t ive

to

t he

p i l e required to produce

negative

skin f r ic t ion i s qui t e small . Movements

on

the

order of 0.1 inch wil l suf f ice . I f

a

condit ion exists where so i l

shear

on t he s ide of

the pile reverses

(changes

from

pos i t ive to

negative f r ic t ion) the

r equi r ed

movement may be

on

the

order

of

0.2 inches.

Obviously,

with

such

small required movements, many

s i tua t ions ex is t

where

negative

skin

f r i c t i o n s present .

Whether

or not

it

i s signif icant

depends on

a var ie ty of

matters

tha t

should become c l ea r subsequently.

794

The NSF load from

l ayer

1 can be

determined

di rec t ly by a s t a t i c tension t e s t t o f a i l u r e

(sl ippage of

the

p i l e

r e l a t ive to

the so i l ) fa r

a

pi l e extending only to t h e depth of

l ayer

1, as

shown

in Figure 2b. Note, however,

t h a t

a

compression load

t e s t to fa i lu re

fo r a f u l l

l ength pi l e

(as in Figure

2a)

would

behave j u s t

as for the

normal

case (Figure 1b) and would

exhib i t

t he

same

ul t ima te

load

Rut This occurs

because under

a

pi l e

compression t e s t

the

p i l e

s

forced downwards r e l a t ive to t he s o i l during the

shor t term

condi t ions

of

t he

t e s t resu l t ing

n

pos i t ive f r i c t i o n ,

desp i t e the lang

term tendency

fo r downdrag

under

se rv ice

condi t ions.

t should be apprecia ted t ha t the NSF load

determined

by

t he t ens ion load t e s t represen t s

t h e upper l imi t

to nega t ive

skin f r ic t ion. Pi les

occurr ing

in

groups

may

not

be subjected to as

high a load. Fur ther , t h e downward

movement

of

the so i l r e l a t ive to the

pi l e ,

and hence

downdrag

loading, may not extend

to the

f u l l depth o f

laye r

1, and a lower magnitude of

downdrag would

r e s u l t

than would

be the

case if

the fu l l depth

of

layer

1 were involved. These p o s s i b i l i t i e s

are

discussed subsequent ly .

The primary

p i l e des ign

c r i t e r i o n in the USA i s

genera l ly

the ul t ima te load capaci ty t ha t

provides

an adequate f ac to r

of sa f e ty with

respect to the applied loads.

Est imates

of

p i l e

se t t lement

a re not usua l ly performed except

fo r

f r i c t i o n

pi l e foundat ions .

Pi le

i ns t a l l a t i on i s

administered through plans , sp ec i f i ca t i o n s and

f i e l d

control to achieve a presc r ibed u l t ima te

load capaci ty . In some instances

e leva t ions a re

speci f ied to which

the

p i l e s must

penet ra te

as a

minimum to ensure t ha t

p i l e s are founded

i n the

desi red

bear ing

l aye r ; never the less , v e r i f i ca t i o n

is

based on

a pi l e load

t e s t

t ha t

r eveals

load

capaci ty .

Therefore ,

a discussion of t h e methods

of deal ing with load capaci ty and f ac to r s o f

safe ty

i s warranted.

FACTOR OF

SAFETY IN

PILE

FOUNDATIONS

The fac tors o f safe ty fo r p i l e s

under

both normal

usage

without

downdrag,

Figure l a , and fo r

condi t ions

involving

downdrag,

Figure

2a, wil l

be

defined where p i l e s are

ins ta l l ed

on o r

n

a

r e l a t i v e l y st rong

incompress ib le s o i l

l ayer . The

def in i t ion used here in for pi l e f ac to r o f safe ty

with

respect to

a s o l

bear ing capaci ty f a i l u r e

i s t he r a t io of r e s i s t i ng forces to dr iv ing

forces.

In

normal pi l e

des ign

p r ac t i ce a f ac to r of sa f e ty

of two i s ut i l i z ed

when

p i l e load

t e s t s

a re

the

means

of

cont ro l l ing p i l e

load capaci ty .

U s ~ n g

Figure 1 and the terms def ined

above

fo r

normal

condi t ions

without

downdrag, t h i s

can be

expressed as:

2 (D + L) S

Rut

(Geot)

(1)

This

s imply says t ha t t h e p i l e ul t ima te load as

determined

by

t e s t

must equal

or exceed twice t h e

appl ied working (service) loads.

The

express ion

covers t he

geotechnical

(Geot) requirements

cons i s t ing of pi l e f a i lu r e (s l ippage) r e l a t ive

to

t he so i l ,

not

the s t ru c t u ra l

(Str) requirements

of

the

pi le .


3/9

The foregoing expression

can be modified

account for

downdrag

condi t ions

Figure

2) .

NSF

i s included with normal loads,

express ions

are:

to

I f

the

2 D + L + N S F ) ~ R u t N S F Geot) or,

2)

2 D + L) + 3NSF Rut Geot)

3)

Note

tha t the l e f t s ide of Equation 2 rep resen ts

loads, whereas

the

r igh t

s ide

i s the p i l e

res is tance .

From

Figure

2a

t

can

be

seen

tha t

only

l ayer

2

provides

upward so i l

reac t ions

to

the p i le loads;

the

magnitude

of

the

upward

reac t ions i s equal to Rut

from

the compression

t e s t

Figure 1b) , less the

f r i c t ion

from layer 1

which

i s

equal to

the

downdrag,

NSF,

from the

tension t e s t Figure 2b). Thus, i f

NSF

i s

t rea ted s imi la r ly to normal

loads ,

t appears n

the design

expression

with

a

coeff ic ient

of 3

Equation 3) . This

occurs

because of the

reversa l in d i r ec t ion of so i l f r i c t ion n

layer 1

for the compression load t e s t

compared to

the

service

condi t ion.

The

foregoing expressions

should

help engineers understand the magnitude

of

loads tha t must be

res i s ted ,

and where t h a t

res is tance i s

loca ted, namely, the bearing

layer

layer 2 of

Figure 2a) .

Experience with t yp ica l so i l pro f i l e s shows tha t

NSF loads

can

eas i ly

be

equivalent

n magnitude

to normal

dead

p lus l ive loads. This doubles the

required

load carrying capaci ty and

leads to the

purchase of twice the load carrying capac i ty n

the p i le foundat ion than would

be

the case

without downdrag. However, t h i s does

not

mean

tha t costs

are doubled. Engineers often t ry to

mitigate t h i s cos t increase

by

cut t ing the fac tor

of safe ty n t h e i r analyses; th i s sub jec t bears

examination.

In the preceding normal analysis of p i le

foundations a fac tor of safe ty of

two

i s

implied.

In r ea l i ty ,

nei ther

the loads nor the p i le

res is tances

are known,

and are

sub jec t to

natura l

var ia t ions

t ha t

would

be

t rea ted

s t a t i s t i c a l l y

i f

suff ic ient data ex is ted . In an ef fo r t to be more

ins ight fu l ,

and

to

be

consis tent

with

supers t ructure design, load and res is tance fac tor

design (LRFD) techniques

are

used here in to

examine the issue. In

such

systems,

a

s ing le

overal l

fac tor

of

safe ty

i s not used. Instead,

safety i s provided

by

mult iplying t he working

loads serv ice loads) by load fac to rs

greater

than

1) , and ul t imate

s t ruc tura l

res is tances by

s t rength reduct ion fac tors ~ - f a c t o r s , less

than

1) for purposes of design.

TWo

systems

of

LRFD

are cur rent ly in use in the

USA, namely

those

put for th

by

ACI American

Concrete Ins t i t u t e )

and AISC

American Ins t i tu te

of Stee l Const ruct ion) . The load fac tors in use

by

AISC (

1 .

2D

+

1 .

6L)

are

genera l ly

consis tent

with ASCE American soc ie ty of Civ i l Engineers,

Standard 7-88)

standards,

but these are

undergoing heavy cr i t i c i sm, espec ia l ly the load

fac to r fo r

dead load.

on the

other hand, the ACI

load fac tors

1.4D

+

1.7L)

are general ly

accepted

by designers , and have a

30

year h i s to ry

of

usage. Because the

uncer ta in t ies

in

concre te

design are

more

analogous

to

so i l / p i l e problems

than those

of

s tee l

design,

the

wri te r pre fe rs

to

follow the genera l procedures of ACI. This has

meri t a l so

because

p i les

are

almost always

1795

embedded

in a concre te

p i l e

cap designed by AC

ru les (ACI 318), and

the

in te r face

with

p i l

design i s

thereby f ac i l i t a t ed .

Appl ica t ion of the

LRFD

concept to

pi les

leads

t

two expressions Davisson, 1989),

one

tha

re la tes

to

the

p i l e s t ruc tura l ly

Str) ,

and

th

other to the geotechnical

p i le / so i l

capaci t

Geot)

.

The s t ruc tu ra l express ion fo r norma

condi t ions

without downdrag is

1.40

+

1.7L

Pn

Str)

4

The 1.4

and 1.7 coef f ic ients

are

load fac tors fo

dead

and

l ive loads ,

respect ively , i s

s t rength

reduct ion

factor ,

and

Pn

i s

the

nomina

u l t imate ca lcu la ted load

(ACI

uses

the term

s trength)

fo r the p i l e column

with a s t a t e

minimum

design eccentr ic i ty of load. As used

n

design, t he fac tored

loads must

be

less

than

o

equal

to

the nominal ul t imate s t reng th of th

s t ruc tu ra l member

reduced

by

a ~ - f a c t o r

to

account for possible

unders trength r e l a t i ve t o

nominal

values. Comparison with the fac to r o

safe ty

concept i s not precise, but

can b

approximated. I f

t i s

assumed tha t

dead

loa

equals l ive load, the

load

factors

can

b

averaged resul t ing n a value of 1.55.

The

r a t io

of

r es i s t ing

s t reng th to

appl ied

service

loads

by

simple

algebra becomes

equal

to

1.55/. Strengt

reduct ion fac tors

are given

below fo r s tee

and

concre te

as used in AISC

and

ACI

LRFD

procedures :

Material

Struc tura l

s tee l

Reinforced

Concrete

Pres t ressed

Pla in

0.85

0.70

Concrete 0.65

Timber

See

FHWA Report

Davisson, e t al , 1983)

The

bes t analogy i s the fac to r of safe ty in

re in fo rced

concrete

columns which become

1.55/0 .7 ,

o r 2.21.

A correct ion should be mad

fo r the ef fec t of the

minimum

design eccentr ic i ty

incorpora ted

i n to

column design

to

produce

number

d i rec t ly comparable to pi l ing

desig

prac t ice ; when t h i s correct ion i s

made

t he r esu l

i s a value of

2.54. Thus,

the factor of safe t

of

2

used

n

p i l e foundations i s s igni f icant l

less than

s t ruc tu ra l engineers

would use unde

more

favorable condi t ions n the supers t ructure

Geotechnical engineers have

been

more bold

t he i r

prac t ice

than

s t ruc tura l

engineers.

Hence

lower

geotechnica l

fac to rs

of

safe ty

for p i l i n

as

proposed

by some engineers

are

not warrante

because

they would increase an already i l log ica

imbalance n

design

of

supers t ructure

an

subs t ruc tu re .

From the

geotechnica l s tandpoint ,

the

followin

equation

can be

wri t ten :

Geot) 5

Rut

i s t he

load

a t

fa i lure in

a

s t a t i c

compressio


4/9

p i l e load t e s t , and i s

t he

geotechnica l

s t r eng th reduct ion f ac to r

appl icable

to

compression p i l e load t e s t s . The

wri te r

has had

one occasion to

evaluate

8

,

and

f ac to r s

varying

from

0.7 to 0.8

were

determined.

The p a r t i cu l a r

pro jec t where the

evaluat ion

took

place

involved

an unusually large

number

of t e s t s , and

pred ic tab le

so i l s t r a t i f i ca t ion .

I t

i s d i f f i c u l t

for

t he

wri te r to imagine

general ly

using a

higher value than 0.8; for most pro jec ts

a

value

0.7

or

lower would probably be experienced.

I f

Equation 5 i s examined for

t he geotechnical

global f ac to r of safe ty as was explored above

for

equat ion

4,

the following resul t s

are tabula ted:

Load

Condition

0.7

0.75 0.8

Dead 2.00

1.87 1

75

Live 2.43

2.27

2.13

Dead=Live

2.21

2.07 1.94

Thus,

t he

f ac to r

of

safe ty

of

2

used

in

normal

pi l e

foundation prac t i ce appears

to

corre la te

with the most

favorable

( l eas t variable) s o i l

condit ions when examined by

LRFD

techniques . The

wri ter o f fer s Equation 5 with the

caveat t ha t

a

~ g v a l u e

higher

than 0. 8 should not used. Further

ins igh t can be

obtained by examining LRFD

techniques with

downdrag

included.

DOWNDR G

ND

LRFO

Negative skin

f r i c t i on can

be

accommodated

in

LRFD

design

techniques for

both

s t ruc tu ra l and

geotechnical condi t ions. The

s t ruc tu ra l

equat ion

i s :

1.40 + 1.7L +1.4NSF ~ ~ p n

(Str)

(6)

Note t h a t NSF

has been

assigned a load f ac to r o f

1.

4

when determined. from tension

load

t e s t s ,

which the

wri ter

recognizes as t he

lowest

f ac to r

t ha t can be applied in uniform

so i l

st ra ta(same

as

dead load) . ACI 318,

however,

t r ea t s ear th

loads with a f ac to r of 1.7; therefore, others can

argue

for

a

higher

factor .

I t

would be

appropria te

to

use a higher

load

f ac to r i f NSF i s

determined

by

calcu la t ion or

othe r

ind i rec t

means,

or i f

t he

so i l

s t r a t i f i c a t ion i s

nonuniform.

The

companion

geotechnical

equation i s :

1.40 + 1.7L +

1.4NSF

8

Rut - (1+a)NSF

(Geot)

7)

The new i tem in

Equation

7 i s a , which accounts

fo r

t he var ia t ion

in NSF, and

i s

used to

obta in

an upper

bound on

NSF; it i s l ike ly to have a

value o f o.

2

a t t h e

lowest..

The express ion

r equ i res the

fac tored

loads ( l e f t

side)

to be

l e s s

than

t he

ul t imate p i l e load reduced by the

NSF

( r igh t

side)

1796

It i s recognized t ha t

could have

bee

mult ipl ied by the q u n t ~ t y


5/9

aware of t he i r

importance. These

top ics are

examined

n the

following

discussion.

PILE LOAD TESTS

AND

ANALYSES

The foregoing equations t r ea t ed p i le

ult imate

loads from

load

t e s t s

as

though there was

universal

agreement on

how

to perform the t e s t s

and analyze

the

r e su l t s .

This s

not

the

case .

Load

t e s t s are usual ly performed according to

ASTM D1143, which

spec i f i es how

to

se t

up and

perform

compression load

t e s t s .

However,

the

s tandard loading method

i s

not

always

sa t i s fac to ry because t can

i nh ib i t

in te rp re ta t ion of the data due to creep and

consol ida t ion def lec t ions a t t r i bu t ab le t o

the

slow

r a t e

of loading and l a rge load increments.

The wri ter suggests t ha t

quick

load t e s t s are

needed

to

ar r ive a t su i t ab le data;

t h i s

i s

avai lab le in

ASTM D1143

as an opt ion, but

t must

be

ca l l ed out

exp l i c i t l y

in the job

specif icat ion. Also,

the ASTM spec i f i ca t ion

leaves analys i s o f the

data

to

the

engineer ' s

profess ional

judgement.

Given

quick

load t e s t data

from

a

p i l e

compression t e s t , a method of analys i s must be

selected

for

purposes of

determining Rut The

method

se lec ted herein

i s

t ha t

developed

by

the

wr i ter (Davisson, 1972).

Resul ts of

p i le tension t e s t s

on

shor t p i les

extending

to

the bottom of the subsiding l ayer

are

u t i l i z ed in the foregoing

equat ions.

ASTM

D3689

covers

the

se t

up

and

performance

of

tension t e s t s , but

otherwise

has the same

features as ASTM D1143

for compression t e s t s .

The

quick

load

t e s t

opt ion must

be speci f ied , and

analysis of the data i s

spec i f i ca l ly

excluded

from

the standard.

The

analysis of tension

p i le load t e s t s to

fa i lu re (s l ippage

r e l a t i ve

to the soi l ) in

cohesive so i l s of ten leads

to

in te rp re ta t ions

t ha t

do

not vary

widely

from one engineer to

another.

This occurs because

a

constant

load

with increasing def lec t ion i s

often observed,

s ignifying

fa i lure .

Another procedure i s

to

use

the wri te r ' s method, o r modify t by dele t ing the

term re la ted

to the base width of

the

p i l e , but

otherwise applying the

of f se t

method. On the

other hand, tension

t e s t s lead

to

cracking

in

re inforced

and pres t r essed pil .es; t h i s presents

special problems in analys i s

because

of the

d i f f i cu l ty of determining e l a s t i c p i l e

def lec t ions .

Also,

t e s t s

t ha t

are inf luenced

by

a

granular

stratum may

r e su l t in load versus

up l i f t diagrams for which fa i lure s not obvious.

The engineer

wil l

have to

exercise judgement in

analyses

of

such

cases.

The foregoing discussion

applies

to driven p i l e s .

Piles i n s t a l l ed

by d r i l l i n g

behave

di f fe rent ly

and must be analyzed

taking

those di f ferences

in to

account. The di f ference manifests i t s e l f

primari ly in compression. Driven

p i l e s compact

the mater ial below the p i l e t i p thus

increasing

so i l s t i f f ne ss .

As a

consequence, the

def lec t ion

needed to

produce

p las t i c behavior a t the t i p i s

markedly lower

than for

dr i l l ed

p i les . The

s t i f f ne ss

of the

s o i l a t

the t i p of dr i l l ed p i l es

may ac tua l ly have been degraded

by

the

i n s t a l l a t i on

process.

1797

I f

the wri te r ' s offse t method of analys i s s

appl ied t o d r i l l ed

pi les ,

the

term

conta ining the

width of the

p i l e

may need to

be

mult ipl ied by a

f ac to r varying from

2 to

6. This

i s because

research on dr i l led

piers

shows t lat

t i p

def lec t ions of 2 to 5

percent

(Reese and O'Nei l l ,

1988)

of the base width are required

to

reach

ult imate

load compared to

l es s

than

1

percent for

driven

p i l e s .

OBSERVED DOWNDRAG

BEHAVIOR

Observations in the

f ie ld on

p i le

foundations

t ha t

did not f a i l , plus observat ions of fa i lures

where negative

skin

f r i c t ion

was

the prime

cause,

provide a basis for the following discussion.

NSF can develop

dur ing

const ruct ion resu l t ing in

more load on the pi le than the designer thought

would be

present . This has been observed for

pipe p i l e s in sand where vibrat ions from dr iving

of adjacent pi les

caused

se t t lement of

the sand

leading to downdrag on the pi les . Measurable

downward movement of the p i l e tops

resul ted.

After

dr iving

was

complete

no fur ther so i l

se t t lement mechanism exis ted. Either the p i l e s

car r ied

the subseq.uently

added

superstructure

load within

t he i r

ult imate

load

capaci ty ,

or a

s l i gh t downward movement

(perhaps 0.1 inch)

of

the p i le t i p rel ieved

any

excess

load from

downdrag.

Another al ternat ive mode of

behavior

i s t ha t downward p i l e

movement caused

by

the

superstructure load simply rel ieved the act ing

negative skin

f r ic t ion. I t s

easy

to

imagine

many other s i tuat ions

where

s imilar behavior

occurred.

Some of the per t inen t fea tures of the seven case

his tor ies

about

which the wri ter has personal

knowledge have

been

summarized

in Table 1.

The

outward manifesta t ions of fa i lu re were excessive

se t t lement of the s t ruc tu res in

a l l

cases.

However,

two of the

cases

also involved co l lapse

of the

s t ruc tu res (Cases 1,

2}.

TABLE 1.

Unpublished

case

Histor ies

of

Fai lu re

Case Time Type

F i l l

Geot.

History

Period

Pile

Placed

Engineer

1 1950-60

Timber Yes

No

2

1960-70 Timber

Yes

No

3

1970-80

Timber No

Yes

4 1980-90

Pipe

Yes

Yes

5 1980-90

Pipe Yes

Yes

6

1980-90

Timber Yes

Yes

7

1980-90

PCPS Yes

Yes

Notes:

1} Large so i l se t t lements

in a l l cases.

2)

Structures a l l se t t led excessively .

3)

Structure

col lapsed

in

Cases

1,2.

4)

Pi le fa i l ed s t ruc tu ra l ly , Cases #1,2,6 .

5

PCPS

= Precast Prest ressed concre te .

When set t lement of the so i l i s a long term

occurrence

because

of :

1)

a

f i l l ,

2

dewatering,

3) vibra t ion , o r } other long term mechanism,

so i l set t lement can

more

than compensate for any


6/9

tendency

of

the

p i le

to

s e t t l e away from the

dragdown

load. Thus,

in such

instances,

the

downdrag load i s cont inuously establ ished. All

seven

of

the

case his tor ies

in

Table 1

are

of

t h i s type.

Soi l

se t t lements

occurred over a

long

period of t ime

resul t ing

in ground surface

se t t lements o f

several

inches

to several feet .

In th ree

of the

four

case

his to r ies

( 1,2,6,

Table

1) involving t imber pi les it i s cer ta in

t ha t a number of the pi les

broke (fai led

s t ruc tu ra l ly ) . The

other

cases

in

Table

1

involving pipe 4, 5) and

prestressed

concrete

( 7) did

not

experience s t ruc tu ra l fa i lu re , but

se t t l ed excessively . In one of

the

cases 6,

Table

1) involving t imber

pi les the so i l

se t t l ed

15 to 24 inches, and pul led some

of

the pi l e s

downwards

f ree

from

the s t ructure.

Of the seven case his tor ies in Table 1, f ive

involved

f i l l s placed

as p a r t

of the const ruct ion

of the f ac i l i ty . These

f i l l s

were the driving

force

in

causing

so i l se t t lement .

For one

s t ruc ture

on

t imber pi les tha t ult imately

collapsed,

an adjacent

f i l l

was

placed perhaps 20

to 30 years af te r the o r ig ina l const ruct ion, thus

i n i t i a t i ng so i l se t t lement . The

f ina l case ( 3,

Table

1)

involved long term dewatering which

caused

so i l

se t t lements

over

a

wide

area .

The

dewatering

occurred soon

a f t e r

the s t ruc tu re was

original ly

const ructed.

Although the wri ter has

not

provided case

his to r ies to

support

the following so i l -

set t lement-causing mechanisms, t he i r logic

i s

easy

to grasp. I f a

so i l

deposi t i s natura l ly

underconsolidated,

NSF

can be

expected

on pi l e s

dr iven in to

or

through such a deposi t . Likewise,

so i l f i l l

does

not have

to

be

the load

t ha t

in i t i a tes so i l

se t t lement ; it

could be an area

adjacent

to a pi l e supported

s t ructure used

fo r

storage of mater ials .

t

i s the weight of the

mater ia l tha t

i s

important, not the const i tuents .

In

geographical

areas

where

regional

subsidence

i s

occurring

because of

underconsol idat ion,

pumping

of water o r oi l

etc.

special problems

exis t

for

p i le foundat ions .

Mexico

City

represents an area where such

problems

occur to

an extreme, and because o f the la rge amount o f

const ruct ion, several

techniques for

dealing

with

the problem have

been

employed. t

i s

highly

recommended t ha t

designs for areas undergoing

regional subsidence have the benef i t of s tudied

loca l

experience which

may

reveal important i tems

in addi t ion to downdrag t ha t must

be

accommodated

in

design,

such as

apparent emergence

of the

bui ldings from the

ground

i f

the general

area

subsides within the depth of the pi les , and

problems with

connection o f

u t i l i t i e s .

Observations

are

given

here in

about

the

types of

er ror s made by engineers in

the

seven case

his to r ies in Table

1. Categor ies

of

engineer

er ro r

are offered below for contemplation. Some

of the p ro jec ts involved

more than one of

the

categories . The two oldes t case his tor ies

involved ignorance

of the downdrag

phenomenon.

Other causes

are :

Fai lure

to an t i c ipa te e f fec t

of

future

dewatering

(1)

1798

Fai lu re to an t i c ipa te

ef fec t

of

adjacent

ground

loading (1)

Improper analys i s

of downdrag (3)

Fai lu re to penet ra te

adequately

in to the

bearing l ayer

(2)

The numbers

in parentheses represent the

t o t a l

number of case hi s to r i e s

to

which

the

cause

applies .

The

above

list

r e f lec t s

poor ly on

the

competence

o f the geotechnica l profession consider ing

t ha t

in f ive

o f

the seven

cases downdrag was

spec i f i ca l ly iden t i f i ed

by

the engineers as a

problem dur ing

design.

Never theless ,

t h i s

paper

i s

offered

for those who

seek

some guidance

in

deal ing with the problem of downdrag on

pi l e s .

PILE STRENGTH ND DRIVABILITY

In current prac t i ce , p i l e s t ruc tura l sec t ions are

se lec ted

using

al lowable s t r e s s design.

Typically

the

al lowable

s t resses imply a f ac to r

of safe ty of 2.5 to 4

fo r

concre te , calcu la ted

as

the

r a t io o f

the concre te s t r eng th , f c , to the

allowable

s t r e s s .

Similar ly , for s t e e l

the

implied

fac to r

of

safe ty

i s 3.

However,

for

competi t ive reasons the

s t ee l

i ndus t ry has

aggressively promoted an al lowable s t r e s s of 50

percent of yie ld ( implied f ac to r o f safe ty of 2) ,

and some bui lding codes permit t h i s with

r e s t r i c t i ons .

Timber

presents

a much more complicated

s t ruc tu ra l mater ia l ,

and

i s not easy

to

summarize

succinct ly .

The

t imber indust ry has promoted

allowable s t resses for t imber pi les fa r beyond

values

the wri te r

deems

reasonable.

The

allowable s t resses promoted by

the

t imber

indust ry

for

t imber pi l e s (1200

ps i for yellow

pine,

f i r

and oak) may

not

inc lude a formal

factor of safe ty

NFPA,

1982).

Most engineers

are unaware

of

th i s . fac t .

Further,

most

engineers

are

unaware

t ha t

t imber

pi l e s

under

long term

sus ta ined

load

lose

40

percen t o f

the i r

shor t

term

s t r eng th

in

a period of 10 years .

More r e a l i s t i c allowable

design s t resses

are

on

the

order

o f

600

to

800

psi ; these s t resses

and

load t r ansfe r analyses lead

to

maximum t imber

p i le loads in the range of 20 to 30 tons, which

i s recognized

in the

indust ry as acceptable in

the

absence

of both hard

driving

and obst ructed

ground condi t ions. A thorough review of t imber

pi l e s t r eng th

i s

presented

in

a Federal Highway

Adminis tra t ion

repor t (Davisson, Manuel,

and

Armstrong,

1983);

t h i s r epor t a l so covers the

strength

o f s t ee l

and concre te p i les .

Pile load

t e s t s

are commonly

loaded

to a t l e a s t

twice

the serv ice load, and the

p i l e must

be able

to

sus ta in

such

loads

s t ruc tura l ly .

This has

presented

problems in

t es t ing H-pi les for designs

with

an

al lowable

s t r es s of 50

percent

of y ie ld

because

of loca l buckl ing pr io r to

a t t a in ing

twice des ign load. Because it i s des i rab le

to

t e s t pi l e s t o beyond twice design load to reveal

the u l t imate so i l /p i l e

load,

p i le s t rength must

be suf f ic ient for t h i s purpose. Concrete pi les

have

seldom presented a problem in

t h i s

regard,

and

the

same

i s

t rue

fo r

concreted pipe

p i les .

Another

s t r eng th

top ic

af fec t ing

se lec ted fo r

the p i l e

a

project

s t ruc tura l

i s pi l e


7/9

r

vabi l i y . This

s

a

concept

t h a t can be

hought o f as t o t a l

punching

force under a

ammer.

I t

s a

problem n

s t r uc tu r a l

dynamics

nvolving

so i l

res i s t ances and

other

harac te r i s t ics , and s bes t t rea ted with

roperly performed wave equation analyses of

p i l e

r iv ing (Davisson,

1972,1975). A thorough

iscuss ion i s not poss ib le here.

The

objec t of

uch a study

i s

to ar r ive a t a

combination

of

i l e

hammer, hammer cushion, p i le cushion,

and

i l e , as a

system

tha t

can

drive through the

verburden

so i l s

and

cause

p i le

penet ra t ion

in to

he

bear ing

layer with enough

force

to develop

he

required

p i le load capaci ty

in

the bear ing

ayer (See

l ayer 2 in

Figure

2a) .

I t

i s poss ib le

hat p i l e penetrat ion aids

such

as

pr edr i l l i ng

i l l a lso

need to

be employed.

In

general ,

the

tore

concre te , s tee l ,

or

t imber n

the

p i l e

: ross-sect ion,

the grea ter i s the

axial

s t i f fnes s

,f

the p i le , and also the t o t a l punching force.

lowever, the ent i re driving

system

i s

involved,

.nd the hammer and cushion

components should

also

e

optimized

using

wave equation analys i s .

wo of

the

case

h is to r ies in Table 1 ( 4, 5)

.nvolve

th in -wal l pipe pi l e s

which have

a

e lat ively

small

cross-sect ional area

of

s t ee l .

~ h e ax ia l s t i f fnesses were too low to

al low

>enetration

in to

the

bearing

layer

with

:uff icient force to develop the required

load

:apaci t ies . Another

case his tory

( 7, Table 1)

.nvolved

pres t r essed p i les wherein

dr ivab i l i

ty

ras adequate. However, where pi l e s

were

furnished

.anger

than

required, they were overdriven to

tvoid p i l e cu t -o f f s , and performed

: a t i s fac tor i ly . Unfortunately, some pi l e s

were

:hor ter

than ac tua l ly

required and

were

mderdriven; they fa i led because of NSF. This

. a t te r case

his to ry

demonstrates

the

need to

levelop p i l e load capaci ty n the designated

>earing

layer .

~ h e foregoing case

hi s to r i e s

reveal

several

:ac tors t h a t

must

be considered in producing a

>ile

foundat ion

t h a t s successful

a f t e r

it i s

:onst ructed.

Discussion

and

guidance

on

severa l

>f

these

f ac to r s i s given

in the

paragraphs

tha t

:allow.

>ILE SHAPE AND

DOWNDRAG

load

t r ansfe r

analysis for the condi t ions on

.


8/9

A complete

method

of analysis consider ing t he

foregoing i s

given by Fel lenius (1988) . computer

software for the analysis has

a l so

been developed

(Goudreault and Fel lenius,

1990).

This

method of

analysis

concentrates

on so i l s t i f f ne ss

and

se t t lements ,

with

bearing capacity considered as

a

f ina l check.

Engineers

using the

method

should

have a

thorough

understanding of both

i t s

underlying theory, and also the

i tems

covered

herein.

MITIGATION OF DOWNDR G

The foregoing discussion

descr ibes downdrag

problems, means for

assessing

the magnitude of

downdrag

loads,

and

methods for designing to

r e s i s t the added loads with addi t ional pi l e

st rength

and bearing capaci ty .

Two

other

techniques are also avai lab le . The f i r s t i s

avoidance of

the

problem

by not

using

s i t e s where

downdrag

wil l occur,

t reatment

of

t he

subsiding

soi l , or removal

of

the subsiding

so i l ;

th i s

needs no fur ther

discuss ion.

The second i s a

physical means for

reducing

the magnitude

of

downdrag.

Predr i l l ing

through

obst ruct ing

f i l l s

( including

engineered f i l l s ) i s often done for t he purpose

of

al lowing

the

pi les to seek support in

underlying bearing

layers . This usual ly wil l be

accompanied

by

a decrease

n

downdrag,

even

i f

the

predr i l l ing

was

performed for a di f fe ren t

purpose.

Other

techniques involve a

s leeve

l i n e r

to

allow

the so i l

to se t t l e without causing

downdrag.

Bitumen

coating

of

pi les

i s the

t ~ h n i q u

receiving by far t he most at ten t ion 1n t he

l i t e ra tu re ; p i l es

are

coated with a

bitumen

layer

possessing rheological

proper t i e s

within

ranges

speci f ied

by

the

engineer.

The

pi les are driven

with

the

assumption t h a t the bitumen layer

remains

in tac t

in the subsiding layer .

Subsequently,

as

t he

downdrag

occurs,

the

drag

load

s l imited to the low values of shear n t he

bitumen layer. Where

the bitumen remains

in tac t ,

th i s

technique has

been very successful

n

reducing the downdrag load. t i s

also apparent

tha t the economics of

downdrag reduct ion

are

affec ted by the cos t

of

deal ing

with

t he bitumen.

A

research program

s nearing completion a t

Texas

A M Universi ty

on

use of bitumen coat ings

(Briaud,

1993), and repor t s

should soon be

avai lable

(1994)

from t he

sponsoring Federal

agency (National Cooperat ive

Highway Research

Program,

Washington, D. C.) .

Engineers

are

cur ren t ly

seeking

coat ings t ha t

have more des i rab le

proper t ies than bitumen.

Problems to

be overcome

are : 1

extending

the

temperature range

n

which

the

mater ia l may

be

appl ied

and

cured,

2)

handling and damage to the

coat ing,

3)

loss

of

t he coat ing during

i ns t a l l a t i on

in unfr iendly

so i l

condi t ions, 4)

assurances of permanence, and 5) cost .

t

s

t he

wr i t e r s opinion

t ha t where

a

coat ing

i s

to be

used for

downdrag

reduct ion, the

engineer

should

consider

proceeding as follows:

1 Design the

pi l e

foundation without the

coat ing.

1800

2

3)

4)

Design the p i l e foundation with the

coat ing.

Obtain pr ic ing fo r both designs in the

bid.

Be

prepared

to

s h i f t from t h e coated

pi l e

to

t h e

uncoated

p i l e

i schedule

and/or

weather

or

other

reasons

develop.

The

reason for the foregoing s

t ha t economy

s

of ten claimed for bitumen coated pi l e s

based

on

engineer

est imates a lone . In

t he r ea l

world,

cont rac tors (general and

p i l e

subcontractor ) are

the

exper ts on

pro jec t economics,

and t h e i r voice

should

be

heard

fo r the c l i e n t s benef i t . Steps

1

through

3 above provide the cons t ruc t ion

manager and/or

owner

with

maximum

information.

t s poss ib le t ha t the coated p i l e appears

lowest in

cos t

i f pi l ing ac t iv i t i e s

take place

n

favorable

weather.

However,

other matters may

dic ta te

a schedule

change to unfavorable

weather

for

coated pi l ing ac t iv i t i e s , and the schedule

change

may be

of

economic

benef i t

to

the p ro jec t

desp i t e the

cos t increment between coated and

uncoated

pi l e s .

SUMM RY

Fai lures

of

pi l e

foundations caused

by negat ive

sk in f r i c t ion cont inue

to occur. During

the

pas t

70 years the causes

of

fa i lu re

have

progressed

from lack

of

knowledge

of the downdrag concept by

t he

responsible

engineers

to

i nab i l i t y of

t ra ined

geotechnical engineers

to

achieve a s t ab l e design

even though they recognized t h e

problem.

Observations

from

seven unpublished case

his to r ies of downdrag fa i lu re i l l u s t r a t e the

poin t s

made

herein.

A review

of t h e various

geotechnical and

s t ru c t u ra l f ac to r s en ter ing in to

t he

design

of a pi l e

foundation

for downdrag

condit ions

has been presented. The

discussion

was predica ted

on

prac t i ce

in the US where most

p i l e

foundations

are

i ns t a l l ed to

be

es s en t i a l l y

non-se t t l ing

a f t e r

the oads are n

place . In

geographical l oca t ions where regional subsidence

s

act ive ,

other

problems may also

ar i se and need

speci f ic loca l s tudy and t rea tment .

A method of determining downdrag

loads

has been

presented and a l t e rna t ives considered. Downdrag

can

be

a very

se r ious load r e l a t ive

to

normal

supers t ruc ture

loading, and i s cause for

concern

about the s t ruc tu ra l s t r eng th

o f pi l e s . Timber

pi l e s have been

observed to

break under downdrag

condi t ions. The discussion

focused

on the

in terac t ion of

s t r uc tu r a l

and geotechnica l

pr inc ip les , and also the

in te rac t ion

o f

engineers

specia l iz ing

n

the two

disc ip l ines .

Guidance s

presented on

the

s t r uc tu r a l

problems.

Factors of safe ty

used

by s t ru c t u ra l and

geotechnical engineers

have been compared.

t

was found

t ha t

normal p i l e design

prac t i ce

u t i l i zes a lower

f ac to r o f

safety than does the

design

prac t i ce fo r the superstructure it

suppor ts , which i s i l l og ica l

consider ing the

re la t ive

unknowns

in each disc ip l ine . Lowering

geotechnical

f ac to r s o f safe ty i s not recommended

as

a

means fo r deal ing with downdrag. LRFD

techniques

were

introduced

as a thinking too l for

engineers , and allowed some assessment of fac to rs

of safe ty .


9/9

i l e compression and t ens ion

load

t e s t s and

roblems

were

rev iewed .

Spec i f ic

recommendations

ere made for quick load t e s t s accompanied by

a

ompatible

method

of ana lys i s . P i l e d r i v a b i l i ty

oncepts were i n t roduced

and

t h e

wave equat ion

na lys i s o f

p i l e dr iv ing was recommended a s a

se fu l t o o l t h a t could help t he engineer assure

ha t

s u f f i c i en t p i l e load

capac i ty

was developed

n

the

proper s o i l l aye rs . Fie ld

cont ro l o f

p i l e

n s t a l l a t i o n must

a l so

concen t ra t e on

achiev ing

dequate p i l e bear ing capac i ty

in t h e

designated

'ear ing

l aye r .

Problems

assoc ia ted with

p i l e

hape,

p i l e

b a t t e r ,

and

groups of p i l e s were

a lso

. i scussed .

:ethods o f mit iga t ing downdrag were discussed,

:onsis t ing

pr imar i ly

of

p r ed r i l l i n g , cas ing ,

or

' i umen

coa t ing . Spec i f ic

advice i s given

to

, es igners

cons ider ing bitumen coa t ing about

:oord inat ing p i l e design wi th

p r o j e c t

management

teeds.

CKNOWLEDG

EMENT

'he

wri ter i s indebted

to Dr.

David

Rempe and

Mr.

~ o y Armstrong for t h e i r

e f fo r t s ,

suppor t and

: r i t i c i sm over

many

years of

working toge ther .

~ E F E R E N C E S

CI

318, (1989},

Bui lding Code Requirements

for

~ e i n f o r c e d Concre te ,

American

Concrete

:ns t i tu te ,

Det ro i t .

,ISC, (1986), Load and Resis tance Factor Design

:peci f ica t ion

for

St ruc tura l S tee l Bui ld ings ,

~ e r i c a n

In s t i t u t e

of S tee l

Const ruct ion ,

:h icago.

,seE 7-88, (1988), Minimum

Design Loads

for

lu i ld ings

and

Other s t ruc tures ,

American

Society

>f Civi l

Engineers , New

York.

sTM

D1143,

(1987), Standard

Tes t

Method

for

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Under

St a t i c

Axial Compressive

Load ,

lmerican

Society for Tes t ing

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Mater ia l s ,

>hi ladelphia .

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D3689,

(1983), Standard Method of Test ing

:ndiv idual Pi l e s

Under

St a t i c Axial Tens i le

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lcGraw-Hill , New

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M.

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Fe l l en ius , B.

H., (1988},

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Pi l e s and P i l e Groups , Transpo r t a t ion Research

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TRB

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Goudreaul t ,

P. A., and Fel len ius , B. H., (1990),

Unip i l e , Vers ion

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Uniso f t ,

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L. C. , and O'Neil l , M.

W.,

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P 1793- Negative Skin Friction in Piles and Design Decisions

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Transcript of P 1793- Negative Skin Friction in Piles and Design Decisions