CL507 Pile

Desig

n to

EC7

1

Pile D

esign

to E

uro

code 7

The d

esign of

piles is

covered

in S

ection 7

of EN

1997-1

, w

ith th

e v

alues

of th

e p

artial facto

rs tak

en fro

m A

nnex

A.

Sectio

n 7

M

ainly co

nsid

ers th

e b

ehavio

ur of

pile fo

und

ations u

nder

axial lo

ads.

Th

e u

se of

static lo

ad tests

is co

vered

as an

important

desig

n m

ethod

.

A

new

concept

is th

e u

se of

correlatio

n facto

rs

for d

eriving th

e

characteristic

com

pressiv

e and

tensile

resistances

of piles

either

from

static lo

ad tests

or fro

m g

round

test results

.

Limit

states

Cla

use

7.2(1)P

gives

eleven

limit

states w

hich n

eed to

be co

nsid

ered,

seven

are U

LS co

vering

geotech

nical o

r stru

ctural

failures

, and

four are

SLS, co

vering

excessiv

e settlem

ents.

Actio

ns

Th

e m

ost

frequent

actions to

be co

nsid

ered are

perm

anent

and v

ariable

load

s fro

m th

e supp

orted

structu

res.

A

xial (n

orm

ally v

ertical) and

transv

erse (n

orm

ally h

orizo

ntal) lo

adings

are u

sually

consid

ered sep

arately.

A

ctions d

ue to

gro

und

movem

ents also

need

to b

e co

nsid

ered. Th

ese

are v

ery sp

ecific to

piles. Typical

gro

und

movem

ents are;

V

ertical settlem

ents cau

sing d

ow

ndrag

i.e

. n

egativ

e sh

aft frictio

n,

U

pw

ards soil

displacements

causing

heav

e,

H

orizo

ntal soil

displacements

causing

transv

erse lo

adings.

Pile lo

ad tests

Th

e pile

desig

n m

ethod

s accepted

by E

uro

code 7

are all

based

, directly

or indirectly

, o

n th

e results

of static

pile lo

ad tests

.

This

is ob

viously

the case

when

the pile

desig

n is

based

on pile

load

tests o

n th

e sp

ecific site

.

CL507 Pile

Desig

n to

EC7

2

In

any case

, pile

load

tests are

required

where

there

is g

reat u

ncertainty

about

their

beh

aviour in

a p

articular situ

ation see

Clause

7.5

.

Cla

use

7.5

gives

a reco

mm

ended

test p

roced

ure

and interp

retation fo

r

static pile

load

tests.

Desig

n of

Axially

Load

ed Piles

This is

describ

ed in

Clause

7.6

, w

hich is

the co

re of

Sectio

n 7

.

Limit

states

Cla

use

7.6

.1.1(1)P

gives

the lim

it states

which

hav

e to

be ch

ecked

explicitly. Th

e first

three

are U

LS;

Th

e b

earing resistan

ce failu

re of

a single

pile,

Th

e b

earing resistan

ce failu

re of

a w

hole

found

ation,

C

ollapse

of a

supported

structu

re.

Th

e last

is th

e SLS

of th

e supp

orted

structu

re cau

sed by

displacements

which

exceed

an ag

reed lim

iting v

alue.

N.B

. Th

e U

LS b

earing resistan

ce failu

re of

a single

pile and

of a

whole

pile fo

und

ation co

rrespond

s to

the traditio

nal

bearing

capacity

desig

n.

W

e w

ill fo

cus o

n this

ULS

.

Partial

facto

rs fo

r pile

desig

n

Th

e sets

of p

artial facto

rs are

given

in th

e table

belo

w.

CL507 Pile

Desig

n to

EC7

3

Com

pressive

gro

und

resistance

(ULS)

U

LS is

check

ed u

sing th

e follo

wing

ineq

uality;

dc

dc

RF

,,

where

Fc,d is

the d

esign axial

com

pressio

n lo

ad,

and R

c,d is

the d

esign v

alue of

the co

mpressiv

e g

round

resistance

.

Th

e co

mpressiv

e g

round

resistance

can b

e d

etermin

ed fro

m static

load

tests, g

round

test results

(e.g

. sh

ear strength

tests), o

r dy

nam

ic lo

ad

tests.

Static lo

ad tests

Th

e trial

piles m

ust

be of

the sam

e typ

e and

must

be fo

und

ed in

the

same stratu

m.

Interp

retation m

ust

take into

account

the v

ariability of

the g

round

and

the v

ariability of

the pile

installatio

n.

F

rom

the m

easured

com

pressiv

e resistan

ces R

c,m

, th

e ch

aracteristic

valu

e R

c,k is

obtained

from

;

{}2

min

,1

,,

/)

(;/

)(

mc

mea

nm

ck

cR

RM

inR

=

where

1 and

2 are

correlatio

n facto

rs related

to th

e n

um

ber

of piles

tested. Th

e reco

mm

ended

valu

es are

given

in A

ppendix

A, T

able A

.9.

Th

e d

esign co

mpressiv

e resistan

ce R

c,d is

obtained

by applying

the

partial

factors

given

in T

ables A

.6, A

.7 and

A.8

to th

e ch

aracteristic

resistance

Rc,k ;

sk

sb

kb

dc

tk

cd

cR

RR

RR

,

,,

,,

or

+=

=

F

rom

the tables

it can

be seen

that

partial

factors

can b

e applied

separately

to th

e sh

aft and

base

resistance

.

CL507 Pile

Desig

n to

EC7

4

Gro

und

test results

This is

based

on th

e m

odel

pile p

roced

ure

, w

hich co

nsid

ers th

e results

of

one o

r m

ore

gro

und

test p

rofiles, as

follow

s;

C

alculate th

e co

mp

ressive resistan

ce R

c,cal

by applying

the p

rediction

meth

od to

each p

rofile sep

arately, u

sing th

e follo

wing

equatio

n;

cals

calb

calc

RR

R,

,,

+=

This

gives

the p

redicted resistan

ce of

the pile

if it

was

exactly

at th

e

locatio

n of

the g

round

test p

rofile.

The ch

aracteristic v

alues

Rc,k , R

b,k and

Rs,k are

obtained

from

;

{}4

min

,3

,,

/)

(;/

)(

calc

mea

ncal

ck

cR

RM

inR

=

where

3 and

4 are

correlatio

n facto

rs related

to th

e n

um

ber

of test

profiles

. Th

e reco

mm

ended

valu

es are

given

in A

ppendix

A, T

able A

.10.

Th

e m

odel

pile p

roced

ure

can b

e u

sed w

ith D

A-1

and D

A-2

, b

ut is

not

applicable to

DA

-3 sin

ce this

involv

es applying

partial

factors

to th

e

characteristic

valu

es of

gro

und

strength p

arameters

.

CL507 Pile

Desig

n to

EC7

5

EN 1997

-1 D

esign

Exam

ples

These

desig

n ex

amples

are tak

en fro

m F

rank et

al.

Exa

mple

1 D

esign of

a pile

in co

mpressio

n fro

m static

load

test results

Th

e objectiv

e is

to sh

ow

how

the results

of static

load

tests sh

ould

be

interpreted

follow

ing E

uro

code 7

.

Th

e objectiv

e is

to d

etermin

e th

e n

um

ber

of d

riven

piles of

length 55

m

required

to carry

the applied

load

s.

O

nly U

LSs in

persistent

and tran

sient situ

ations and

accidental

situatio

ns are

consid

ered.

Th

e p

erman

ent v

ertical co

mpressiv

e lo

ad is

31 M

N and

the v

ertical

accidental

load

is 16

MN

.

It

is assu

med

that

there

is n

o n

eed to

check

any g

roup

effects o

r to

check

the SLS

.

A

typical soil

profile

consists

of 20

30

m of

very

soft clay

, and

muddy

sands, and

sands and

grav

els at

the b

ase lev

el.

Th

e results

of fo

ur static

load

tests o

n d

riven

piles w

ith different

embed

ment

lengths are

available

, as

show

n b

elow

.

CL507 Pile

Desig

n to

EC7

6

Exa

mple

2 D

esign of

a pile

in co

mpressio

n fro

m lab

orato

ry test

results

This

exam

ple p

resents th

e U

LS d

esign of

a single

bored

pile in

clay

(pile diam

eter B

= 0

.8 m)

using

the u

ndrain

ed sh

ear strength

cu.

Th

e ch

aracteristic v

alues

of th

e lo

ads are

a p

erman

ent lo

ad of

600 kN

and a

variable

load

of 300

kN.

Th

e d

esign aim

s to

check

the req

uired length

of th

e pile

, w

hich is

expected

to b

e 18

.5 m

.

Th

e g

round

investig

ation co

nsisted

of th

ree p

rofiles of

und

rained

shear

strength c

u, as

show

n b

elow

.