Anchor Types in Sheet Piles

8/9/2019 Anchor Types in Sheet Piles

1/36


2/36

W LL

SYSTEMS

Different

wall systems can be used

as i l lus t rated in

Fig

1 depending

on the

soil conditions.

In

Fig

l a

i s

shown a conventional anchored sheet

p i le wall. The

lateral earth pressure on the

wall is

t ransferred

to

the ground anchors through wale beams,

normally

U-, H

or

I -

beams.

Soldier

p i le

and lagging construction

is

shown in

Fig

lb. This support

method,

also cal led

Berliner wall

construction,

is

commonly

used

in

the

United

States

and

in Europe

mainly

in sand, s i l t or·

gravel

above the

ground

water

level. The method i s not sui table in soft

clay.

The soldier pi les

or

beams,

usually

H-piles or

channels, are driven or

placed

in predr i l led holes and

grouted. The spacing of the pi les i s normally 1.0 to

2.0

m Lagging wooden boards) i s placed during the

excavation between the flanges of the soldier pi les .

t.Jale beam

' 1 - _ _ . , ~ - - ~ - e e l

:Sheet f'


3/36

reduce

the

costs

since

the

anchors can

be

placed

between

the two

channels.

Rails

are

used

as la teral support in

Fig

ld.

The

spacing is usually 0.2

to

0.3

m

This support method

is

mainly used in

stony

or blocky soi ls above the

ground

water

level.

The ra i ls are

often

placed in

predri l led holes when the content of stones or boulders

is high since the

ra i ls cannot be

driven. The rails

are often bri t t le

due

to

the low

duct i l i ty

of the steel

{high strength steel ) .

They

are

diff icul t to

spl ice by

welding. Therefore, bolted

joints

are often used.

In

dry

sand above

the

ground

water

level

plywood

boards

are sometimes

placed

between the ra i ls to contain the

sand. In

s t i f f medium to

s t i f f

clays or in s i l ty

soils, the soil is normally protected

by shotcrete

as

i l lustrated

in

Fig

le.

The reinforced

shotcrete

arches

transfer the

lateral pressure

from

the soi l

to the

ra i l s The

thiclmess of

the

shotcrete

is normally

about 50

mm

Also bored piles can

be

used as lateral support in deep

excavations as illustrated in Fig l f

In soft

clay the

piles

should overlap while

in

medium

to s t i f f clay

overlapping is not required.

The

distance between the

piles can be relatively large. The unprotected area

between

the piles is

often covered

by

shotcrete.

Overlapping

bored piles,

so-called

contiguous bored

piles,

are common

in Singapore

also

in

soft

clay

as

foundation for

high r ise

buildings

and

as lateral

support.

ANCl ORS

AND

STRUTS

Different support systems can be used

for a

deep

excavation in soft clay or

s i l t

as

illustrated in

Fig 2

depending on the soil

and

ground water conditions

and

on the

size {width,

length

and depth) of

the

excavation.

The choice of support

·system

depends mainly on the

costs, on

restr ictions

a t

the worksite, on available

equipment in the area and on the experience

of

· the

consultant

or of

the

contractor. For

example

adjacent

buildings

may be

damaged

by excessive settlements i f

a

cantilever

sheet pile

wall

is

used to

support a

relat ively deep

excavation.

Also water mains, sewer

lines

and

heating ducts

can

be

damaged

by

the

resul t ing

large settlements ~ lateral displacements. Excessive

settlements

can

also

be

caused

by

the

instal la t ion

of

the

anchors as well as by the driving of piles inside

the excavation. Struts may therefore, be chosen

instead of ground anchors to reduce the risks. The

settlements

can

be

reduced further

by preloading

the

struts or

the

ground anchors. If

the anchors

are le f t

permanently

in the

ground

they may interfere with

future construction

such

as

the

driving of

sheet piles.

However

different

anchor

systems have been developed

during the last few years which can be removed after

use and where the settlements caused

by

the

installation of the ground

anchors

will be small.

The lateral earth pressure behind a. cantilever sheet

pile wall {Fig 2a) is

resisted by

the

passive

earth

pressure

below

the

bottom of

the excavation while

for

an anchored

or

strutted

sheet

pile

wall

the

lateral

earth

pressure i s resisted by ground anchors or by

st ruts as shown

in

Fig 2b

and

2c, respectively. Ground

anchors

or

st ruts are normally required

in

soft clay

when the

depth of

the excavation

exceeds

2 to 3 m

In

a

large and

wide

excavation

the

length of the

struts

will

be

large i f the s tru ts

are horizontal. They

had

to be braced to prevent buck ing as can be seen in

Fig 3. The

st ruts

will , however

interfere with

the

1517

Anchor

;...,.___.Dd' el: con

I

D ~ l l . e c t - o l ?

\_ /

I

:·

:

······

IF ; == Ji l ·

.

· · · ·

c) ;5/:ru.H:-ed s h e ~ : l : ?-de. walt

Fig 2

Support systems

Ground anchor

work in the excavation and

reduce

the efficiency.

Horizontal bracing is common in Singapore.

The anchors or

the

s tru ts can either be

horizontal

or

inclined. In

narrow

deep cuts horizontal

s tru ts

are

used

while

in large

and

wide

excavations the

·struts are

often

inclined.

The inclined s t ru ts are

generally

supported

a t

the

bottom

of

the

excavation by a concrete

slab

or by

separate

individual concrete footings.

I t

should be

observed

that

the

inclined

s tru ts

or

anchors

will cause an

axial

force in

the

sheet pi les which

affects the s tabi l i ty of the wall.

A number

of

different

ground

anchor systems using bars,

wires or

strands have

been

developed

during the las t 20

years as described by e.g.

Hanna (1982). A

relatively

high

pressure is often used in sand or s i l t for

the

grouting

of

the

tendons in order to enlarge the hole so

that a bulb is formed around the

tendons

within the

g r o u t ~

section,

the fixed anchor length. The

tube-a-manchette method can be used especially in sand,

gravel and

rock to control

the

grouting.

The

bore hole

Cl3Jl be

enlarged

mechanically in s t i f f clay, using

a

special cutt ing device in order to increase the tensile

r«:sistance of

the ground

anchors.

Also, H-beams have

been us.ed as ground anchors in Sweden in very

soft

clay.

The

pull-out

resistance

is

high due to the

large

surface

area.


4/36

Fig 3

Braced sheet

pile

wall

Rods bars) are normally used when the load in the

anchon i s relatively

low,

less than

about 400 kN,

while

cables

wires or strands) are

uti l ised

as

tendons

when

the

load exceeds about 400 kN. The

anchor

rod or

wires are often

prestressed

in order to reduce the

horizontal d i a p l n t a and the deforme.tions of the

wall and thus the settlements during the excavation.

Ground

anchors

are mainly

used for

temporary structures

because of

th

risk of corrosion of

the

tendons or of

the

anchor rods. The corrosion can be reduced for

per.enent anchors

by enclosing the

tendons and by

introducing

a

fluid

between

the

covering and the

tendons. Also cathodic

protection

can be used.

A

recent

e v e l o ~ n t is expander

bodies. This

new

type

of anchor

consists

in principle of a

folded

thin

steel

sheet,

which

can

inflated

in-situ

thrQU h

the

inject ion

of

c . e n t

arout

as shown in

Fig

4 Broms, 1987). The

expander ·

bodies can either

be

driven into

the

soil or

placed in

predrU

led cued holes depending

on

the sotl

::

)mny

a Pfactm rrl

il onchor

··

Expender

bodies

conditions.

The volume of

the

grout required

for

the

expansion and the

pressure

should

be measured in

order

to

check the ul

ttmate resistance.

The me.ximum

grout

pressure in grenular

sotl

ta 3

to

4 IIPa. The main

adventage

with this new type of ground

anchor

ta that

the

size and the shape of the anchors are

controlled.

In

Sweden.

the

L1n48

and the

JB methods where

the

casing

i s

provided with

a

sacrif ic ia l dr i l l ing bi t are

used for

the dri l l ing of the

boreholes.

Also

different

eccentric dri l l tng methods have been

developed

e.g.

Odex,

Exler

and

Alvik

to facili tate the ins tal lat ion of

the casing and to reduce

the

costs. An

addi ttonal

method

ts the In-Situ Anchoring

Method where

the anchor

rods

are

used

as dr i l l

rods during the dri l l ing

of

the

boreholes.

castng

is

not required. However. the

allowable

load 1s

relatively

low

for

this

type

of

anchor and the

method

is

therefore

relatively

expensive.

The chosen method of ins tal lat ion of

the struts

and of

the

anchors

affects

both

the

total lateral earth

pressure

as

well as the earth pressure distribution.

When relatively st i f f struts

are

used,

the

lateral

earth

pressure can be

considerably higher than the

active Rankine earth pressure

particularly

close

to the

ground surface

while

a t

the

toe the lateral earth

pressure can be lower than the active Rankine earth

pressure.

The

reason

for

this

difference is

the

relatively small

lateral

deflection of the sheet pile

wall close to the ground surface during the

construction

since

the struts are normally wedged and

pre loaded.

1518

A

certain

small

la teral

deflection

i s

required to

110biltze the shear strength of the soil behind the wall

and

to reduce the lateral

earth pressure.

In dense

sand

a la teral

displacements of

O.OSX of

the depth

of

the excavation

s

normally sufficient

to

reduce the

lateral earth pressure to

the

active Rankine earth

pressure. When the sand

is

loose

the

required

lateral

deflection i s approximately

0.2X of

the

depth.

A

~ c h

larger deforaation i s

required in soft clay.


5/36

DFSIGN PRINCIPLFS

The

following

four steps

are normally followed

in

the

design of

a

sheet pi le wall

:

o

Evaluation of the magnitude and the

dis t r ibut ion

of the lateral ear th pressure behind the sheet

pi le wall

o Calculation

of the

required penetration

depth

o

Determination

of

the

moment

distr ibution in

the

sheet piles

o

Estimation of the axial

force

in the ground

anchors or in the

st ruts

Extensive investigations are

normally

required in

the

f ie ld and

in

the

laboratory to determine the

depth and

the

thickness

of the

diflerent soil

s t r a t a and of

the

underlying rock as well

as

their

strength

and

deformation propert ies

as

indicated,

for

example, in

the Brit ish

Code

of Practice

CP2001). Penetration

tests are mainly

used

in cohesionless soils sand

and

gravel) in order

to

estimate

the re la t ive

density,

the

angle

of

internal fr ic t ion and

unit weight.

Cone

penetration tests (CPT)

and weight soundings

WST)

are

preferred

belore the

standard penetration

test

(SPJ')

because of the uncertainties connected with

this

testing

method.

However,

representative

samples

are

obtained

with SPJ' so that the

soi l

can

be classified.

The size

and

the

shape

of

the soil part icle are

important as well as the gradation since these

parameters

affect

the frict ion angle of the soi l .

The

driving of

the sheet piles are affected by stones

and boulders

in

the soi l .

The stone

and

boulder

content of the

dif ferent

st ra ta and the dif f icul t ies

that ~ y

be

encountered during the

driving

of the

sheet

piles

can

normally be evalauted from weight (WSf) or

ram soundings (DP) or from cone penetrat ion tests

(CPT) . Driving tests with ful l

s ize

sheet piles

may

be

required lor large jobs.

Stress

wave measurements

can

be

helpful to determine the

driving

resistance

and

the

elliciency

of

the

driving.

I t

is

also important to

determine

the

location

and

possible variat ions o£ the

ground water level.

For

anchored

or strut ted walls the depth of an y so l t

clay or

ai l t

layers below the bottom of the excavation

and the variation

of the

thickness of these layers

are

particularly

important since

the stabi l i ty of

the wall

depends

to a

large

extent on

the passive earth

pressure

that

can develop

a t

the toe of the

sheet

pi le

wall.

The

depth

to a

l irm layer below the bottom of the

excavation can usually be

determined

by penetrat ion

tests .

Also

seismic methods

can

be used.

Penetration

tests especial ly cone penetrat ion tests

(CPT) and wei ht

soundings WST) are useful in cohesive

soi l s in order

to

determine the sequence and the

thickness

or

the

different layers. The undrained shear

strength

of

the clay i s

normally

evaluated by f ie ld

vane tests. Undisturbed samples

obtained preferably

by

a

thin-walled piston

sampler

are

usually

required

when

the shear

strength of

the

soil

i s

evaluated

in

the

laboratory

by,

for example;

unconfined compression,

fall-cone or laboratory

vane tests.

Undrained t r iaxial

teats are

often

used to

determine the

undrained shear

strength of

s t i f f

f issured clay.

The

water

content,

the

l iquid and plast ic l imits or

the

clay

should

alao

be - . u r e d . Drained

t r iaxial or direct

shear

tests

are

required for heavily overconsolidated clays in

order

to evaluate

+d

o r + .

The difference

between the

1519

two angles is usually

only

a few degrees.

An

estimate

of

the long

term

ground

water level

and

the changes

that may occur with

time

is also necessary

.

Percussion

dr i l l ing and

coring are normally

required

in

rock.

The

quality

of

the rock can

often

be

estimated from

the

dr i l l ing

rate . The

compressive and

tensi le strengths

can be determined by unconfined compression and or

point load tests.

The

condi

t iona of

the adjacent structures

should

also

be

investigated

dilapidation

survey).

The type of

foundation spread footings, raf t

.

or

pi les)

i s

important since i t can affec t the

choice

or support

system .

C a u ~ t ol

.fa/lure

IOilurt m t h o m ~ m

/Odurt

o

r

iruloron

'I

.

\

. .

a.

;:otlure

ofmiddlt

-slrul o/ anchor

:;j

b.

Oilurt

of

/()l.«r

r

t n r l

or

anchor

.

.

c.

11oment Capt;lCI:fy d

~

n s u l k c i ~ n l ol lhe

top.

d .

o m ~ n l

C O j X l ~ td

r

risullictirrl -the

.

c ~ r r f r e

.

e.

Penefroft

on depth

r

nd moment

co-

.

p::zdf L

_o : t n ~ u l f i -

I

P Ctenf

•

,Piosftc

h,n

1

e

>

-

lOtlure r slrut or

anC. IJo,n

Fig

5

Failure

mechanisms


6/36

In the design o£

anchored

or braced sheet

p i le

walls

t

is preferable

to use character is t ic strengths

and

characteris tic loads

which

takes

into account the

uncerta int ies

connected with

the

determination

o£ the

shear strength o£ the soi l

or

o£ the rock and the

loading conditions. A design s t rength rd

=

fii Tm

i s

used in the calculat ion

of

the la teral ear th

pressures

where

fk i s

the character is t ic strength

of the so i l

or

the rock

nd

''m i s a

par t ia l ·

£actor of

safety

la rge r

than 1.0 . External loads

are

treated in a

similar way.

A

design load

Fd

=

Fk T£

where Tf i s

a

par t ia l

coeffic ient

and Fk

is the character is t ic

load,

i s then

used in

the calculat ions of the la tera l

ear th

pressures. The probabi l i ty that the character is t ic

load

will

be

exceeded in the

f ie ld

should not

be

greater

than

5%. The fa:ilure

mode

or fai lure

mechanism

and the deformation required to mobilize the peak

resistance of the

so i l should

a lso be

considered

when

the required

par t ia l

£actor o£ safety is evaluated as

well as cracks

and f issures . A

s ta t i s t i ca l analysis o£

tqe tes t resul ts

may in some cases

be helpful .

A global factor of safety F s is often used in the

design of both

anchored

and s t ru t ted sheet

p i le

walls .

A

value

o£ 1.5 on Fs i s often chosen for cleys wi th

respect to

the

required penetrat ion

depth in

order to

prevent

fai lure

by

ro tat ion

of the

sheet

p i le

wall

about the anchor level . For

cohesionless soi l s

a

global

factor of sa fe ty of 2.0 i s

normally

required.

LATERAL EARTII PRESSURE

Possible failure mechanisms of

anchored

or

s t ru t t ed

sheet pi le walls supported

a t

several levels are shown

in Fig 5 . Failure may

occur

when the anchors or st ru ts

rupture

or buckle

Figs Sa, 5b

or 5c} or when the

moment capaci ty

of

the wall bas been exceeded 5d, 5e

or 5f . The deformations of

the

sheet pi les

during

the

excavation affect both the magnitude and

the

distr ibution or the la tera l earth pressure behind the

wall. The

la teral

ear th pressure can

be

considerably

lower than the active Rankine

earth pressure between

the

support

levels

due

to arching

when

the

la tera l

deflect ions of

the

wall are large. At

the s tru t

or

· anchor levels the la tera l earth pressure can be

considerably

higher

that the active Rankine

ear th

pressure. as pointed

out by

e.g.

Rowe 1957}.

The earth pressure dis t r ibut ion for temporary

structures in clay i s shown in Fig

6. This

distr ibution is in principle the same as that proposed

by Terzaghi and

Peck 1967}.

A

trapezoidal ear th

pressure dis t r ibut ion

can be used

in the calculation of

the force in

the anchors and in the st ru ts as well as

of the required penetrat ion depth. The la tera l ear th

pressure i s assumed to be [pH -

4c

] above the

bottom

of the excavation when the deptn uof

the excavation

exceeds 4cu/p nd 0.35pH when the depth

i s less than

4cu/p.

Below.

the

bottom of

the

excavation the

net pressure ,

the di fference in the la teral ear th pressure

on

bo th

sides of the wall i s (pH - Ncbcu }

where

Ncb i s the

bearing capaci ty factor o£

the soi l with respect

to

bottom heave. This factor depends on

the

dimensions of

the excavation depth, width and

length).

The net

pressure wil l

be

negative and

contribute

to

the

stabi l i ty when pH < Ncbcu and positive when pH >Ncbcu.

1520

Fig

6

b

Design of anchored and braced sheet p i l e

walls in sof t

clay

I t i s proposed to

use

the net pressure below

the

bottom

of

the excavation

a t

the

design

instead

of the

coeffic ient m as proposed by

Terzaghi

and Peck ( 1967}

to

take

into

accoWlt the increase

o£

the s tru t or

anchor loads when the

shear strength

o£

the

clay i s

low

below the

bottom

of

the excavation compared with· the

to tal

overburden

pressure . A similar calculat ion

method has been proposed by Aas 1984) and by

Karlsrud

1986).

t bas

been

assumed

in

the calculat ion of the

net earth

pressure that the

adhesion

ca) along the sheet pi les

corresponds

to the

Wldrained shear

strength of the clay

cu). The

bearing capacity

factor Ncb wil l be reduced

when ca

< cu.

For an inf in i te ly .long excavation Ncb =

4cu when

ca

= 0, a reduct ion by about 30%.

A relat ively large la tera l deflect ion

i s

required to

develop

the passive la tera l earth pressure in front of

the

wall and

thus the

net

pressure when

the

shear

strength of

the clay i s low. Adjacent bui ldings can be

damaged by

the

resul t ing large

settlements.

I t may

therefore be

advisable for sof t clay to

use

a lower

la tera l earth pressure than the

net

pressure

in

the

calculat ion of the

required

penetrat ion

depth.

The

to tal

la tera l

ear th pressure when

the

depth

of

the

excavation

i s less than the c r i t i ca l

depth

4cu/p

corresponds

approximately to the la tera l ear th pressure

a t rest (K

0.7 to 0.8}. This earth

pressure

may

be

used in the design

of

permanent structures

in sof t

clay.

The

preload in the anchors and in the

s t ru t s

should preferably be adjusted periodically especially

in sof t clay to compensate for creep

and

consolidat ion

of

the

so i l behind the wall .

In

a

heavily overconsolidated

clay i t i s important that

the la tera l

ear th

pressure

is

s u f f i i ~ t l y

high

close

to

the

ground surface to el iminate any tens i le s t resses

in the soi l nd

to

prevent cracking of the clay.

Vert ical tens i le

cracks

may

reduce the shear s t rength

of

the

clay

and

increase

the

la tera l pressure

when the

cracks

are f i l l ed with

water

af ter a heavy rainstorm.


7/36

BO'ITOM

HEAVE

In the design of a

strutted

or

anchored

sheet pi le

walls

in

soft clay

1

failure

by bottom heave had to be

considered as illustrated in Fig

7.

The

part

of the

sheet

piles that extends below the bottom

of the

excavation in Fig

7a must resist a

lateral earth

pressure

that

depends on

the depth

of thll excavation

and on the

undrained

shear strength of the clay.

t

is

proposed to use the net earth pressure as shown

in

Fig

6

for

the

part

of the sheet

pi le

wall

that

extends

below the

lowest

strut level. This

part

of the

wall functions as a

cantilever

which

carries

the

load

caused by the lateral earth

pressure

behind

the

sheet

piles. This load

is

partly

resisted

by the passive

earth

pressure between the two sheet pile

walls.

The

passive

earth pressure

is

affected

by

the distance

B) between

the two walls. f this distance is less

than approximately

the penetration depth D)

then

the

passive

earth

pressure

a t

the bottom of the

sheet piles

can

be evaluated from the relationship

CT

=

2 c Dp 2 c DIB

p u u

{1)

When the

distance B between

the

sheet

piles exceeds

the

penetration depth D B>D) i t

is

proposed

to evaluate

the

passive

earth pressure from

the

following

relationship Janbu, 1972)

2)

where

= ca cu. t should be noticed that

the passive

undrained shear

strength

as determined

from

tr iaxial

extension

tests should be used in

the

calculations.

This shear strength

may be

lower

than that determined

by

e.g.

field

vane tests.

A load

factor

equal

to 1.0 has been used with respect

to the. unit weight of the soil

and the

water. In

the

soft clay below the bottom of

the

excavation the net

lateral

pressure

is

[ ~ f q

pH

1

- pw w - N c b c u / ~ m ] where

Ncb

is the stabil i ty factor with

respect

to

bottom

heave Fig 8). In the intermediate sand layer

the net

pressure

will

be

positive

and contribute

to

the

stabil i ty

of

the

wall.

The

lateral earth

pressure will

to a

large extent

depend on the pore water pressure in

this layer.

A

2.0

m thick unreinforced concrete slab will be

cast

below water a t the bottom after excavation down to the

required depth to prevent heaving when the water level

in the excavation is lowered.

f the

adhesion

ca) along the sheet piles

corresponds

to the

undrained

shear strength

cu)

of the clay, then

CT =2.83

c

+ Dp

p u

3)

When

the

penetration

depth

is

large

compared

with

the

width

B,

the

passive pressure between

the

two rows

will

normally be

larger than the outside earth

pressure

and

the

sheet

piles will be supported at least

partly

by

the passive earth

pressure

between the two walls.

The uplif t pressure

a t

the

bottom

of the

sheet

pi le

wall

depends on the depth of the

excavation

H, the

penetration depth D. the

undrained

shear strength of

the clay as well as on the shape of the excavation

1521

....:JL.. _

,

17

1 J.

~

~ £ + ~

8

~ t t t H i

-

I

U J I I ~ c u

c{( "" 5 0 tlb

a

Eorlh -"':f5ut'

didn"bulion

h. Boll-om heare

Fig

7

Design

of

braced

sheet

J?ile

walls

in soft

clay

B/L).

The uplif t pressure at

the

bottom

of the

sheet

piles Fig

7b)

can be

evaluated from

the

equation

{4)

where Ncb is a stabi l i ty fe,ctor {Fig

8)

which

can be

determined from the

following

relat.ionships Bjerrum

and Eide, 1956).

Ncb =

5

{1

+ 0.2 HIB) {1 0.2

B/L)

when HIB 2.5 and from

Ncb =

7.5

{1 + 0.2 B/L)

when HIB >

2.5.

5)

{6)

This

upl i f t

pressure had to

be resisted by

the

weight

of

the

soil below·the bottom of

the

excavation

and

by

the

adhesion ca of

the

clay along

the

sheet piles.

7)

In

the

calculation

of the required penetration depth is

is

advantageous to

use

load

factors

( ~ f )

and partial

safety

factors_

c ~ m > as

mentioned previously.

The

proposed

design method

is

i l lustrated in

Fig

9a for·

a

braced

sheet

pi le

wall.

The

sheet

piles

have

been.

driven

through

sof t

marine clay upper

Marine

Clay, M)

into an

underlying

intermediate layer with sand

{F1).

Below this intermediate layer is a second layer with

soft

marine clay Lower Marine Clay·, M). The shear

strength of the

clay is

low.

t is

anticipated that the excavation of the f i l l

and

the soft clay

will

be carried out below water

in order

to

prevent

failure

of the excavation by bottom heave


8/36

1tab/illy fador

{

6

/0 r 1

>/

v

,

8 . . ..

6

::\: ---c

B;i

-o

4 ty; ·/ o

(Circa/or

or:yuorJ

I-/

0 :-_..___...___.......__ _J

0 I Z 3 -¥

j

Pofto lf ./J

;o /ure

b.

boffom ht CIYe

a l f . e ~ B;errum I i d ~

95f}

Fig 8

Stabil i ty factor Ncb

due to the very low shear strength of the clay. The

water

level in the

excavations will be

kept a t

or

above

the

ground level in order

to

increase the stabili ty

of

the excavation.

Bored

piles

are used to

support

the

bottom slab. The

piles will

be installed before the

s tar t

of the excavation and provided with a permanent.

casing to prevent necking

of

the concrete during the

casting

because

of the low shear strength of the ~ l a y . ·

The

earth

pressure-distribution when the

excavation

has

reached the maximum depth is shown in Fig 9a. The

lateral

earth

pressure

above the bottom of

the

excavation

c o r r e s p o n d ~ to [ ~ f q + pH

1

- 4 c u / ~ m J where

is

a

load

factor and is a

partial

factor of ·safety.

The

upl i f t

pressure on the concrete

slab

will

vary.

A

higher upl i f t

pressure q

1

) is

expected

on the

slab

next to the two sheet pi le walls compared

with

that

q

3

)

at the

center

of

the slab as shown in Fig 9b and

Fig 9c, respectively.

The upl i f t pressure q

1

in Fig 9b depends on the total

overburden pressure ~ f q

+

pH

1

) outside the sheet

pi le

wall

at the

level

of

the

concrete

slab, on the lateral

resistance of the sheet piles on the shear strength

~ ~ r .

·H,·H tt J

Fig

9a

Proposed design method for a strutted sheet

pi le wall i·n soft clay

1522


9/36

r

Fig

9b

Bottom heave (upper cla:y layer)

of the cla:y cul

and

on the

s tabi l i ty

factor

'Ncb This

upl i f t

pressure

will

act

on a

s tr ip with

a

width

that

corresponds to the depth

of

the clay layer

below

the

concrete slab.

The

s tabi l i ty

niDDber for the excavation

(DIL

=

0.58)

i s

5.9 when the excavation i s long compared with the width

{BIL 0 as can be seen from

Fig

7. However, a

relat ively

large

deformation

will

be

required to

mobilize

the average·

shear strength

of the cla:y. A

partial

factor

of

safety of

about

1.4

is required

to

l imit the maximum wall movement to 1 of the excavation

depth

{Mana and Clough, 1981).

.

The uplif t pressure within

the

center par t . of the

excavation

can be

estimated

as

shown in Fig

96. This

upl i f t

pressure

q

3

will

be

lower than that

next

to

the

two sheet pile walls (q

1

) because

of

the

relatively

high shear strength

or

the lower

marine

clay (cu2).

The overburden pressure

a t

the

bottom

of the fluvial

material Fl

depends on the average uni t

weight of the

soi l above

th is

layer.

The

confining

pressure q

4

below

the bottom of the

intermediate layer

{Fl) a t the

centre of

the excavation

can be

estimated

from the equation.

{8)

where lQs

i s the

total skin fr iction

resis tance

per

unit

length

along

the

sheet piles

and the piles

in the

marine clay

and

in the F1

material

{fs l and

1523

r

etay l1)

Fig

9c

Bottom

heave

(lower clay

layer)

respectively) and B

is

the

total

width

of the

excavation. The adhesion {ca) along

the

sheet pi les

and the pi les in the sof t clay is

estimated

to O.Scu'.

where cu is

the

undrained shear

strength

as

determined

by e.g. f ie ld

vane

tes ts . I t i s suggested

thai:

the

unit

skin

f r ic t ion resis tance in

the

sand

{Fl) can be

taken as

1 .of qc'

where qc is the cone

resis tance

as

determined by cone

penetration

tests

(CPT).

I t

has

thus been assumed that the total skin f r ic t ion

resis tance

along

the pi les and the sheet pi les can

be

distributed uniformly over

the

total width

of

the

excavation.

SHEET PilE WALLS SUPPORTED BY INO..INED ANQ.IORS

An

anchored

sheet p i le walls may fa i l when the ver t ical

bearing capacity

of

the sheet pi les

is

exceeded

as

i l lust ra ted

in

Fig 10

in the

case the

anchors

are

inclined. The inclined

anchors

produce a ver t ical

force

_in the sheet

piles

which may cause the sheet

piles

to

se t t le

i

the embedment depth is

not

sufficient .

A

settlement {6v) will also

cause the wall

to move outwards («\) a distance 6v tan a

where

a is

the

incl inat ion of

the

anchor

rods

or of the cables

a t

the level of the

anchor

(Fig 10). The incl inat ion of

soi l

anchors in so i l is

often 20

degrees

while for rock

anchors

the

incl inat ion

is normally 45

degrees. The

inclination

can

be increased

in

order

to reduce the

length of the anchor

rods or

of the cables and thus the

cost.

The ver t ical component of

the

anchor force along

the

sheet

pi les

i s therefore, .often

higher when the

sheet

pi les have been d;riven into :rock compared with


10/36

a. Ru ure m e h o m ~ m

;:5

y _

v :5 :5t.i?

h.

fOrce j22ly f-or;.

Fig 10

Vert ical

stabi l i ty

of sheet pi le wall

with

incl ined

anchors

o Anchorecf ~ h l

f- lt wall

b

/}raced

:5hel "t

;Q It?

wall

so i l

anchors because of th e difference in inc l ina t ion

of the

tendons.

The sheet pi les can general ly be

driven

to a

higher resistance

when

competent rock i s

located

close to

the bottom

of

the excavation and rock

anchors

are

used. I t i s

then

re la t ive ly easy to

re s i s t

the high vert ical force in the sheet

pi les

When the depth to

rock

or to a layer

with

a

high

bearing

capaci ty is relat ively

-large and

soi l anchors

had

to be

used

then

t i s dif f icul t to re s i s t

the

vert ical component of

the

anchor force

by

adhesion

or

by

f r i c t ion

along the

sheet

piles

I t

may

then be

more

economical to

reduce the

inclination of

the anchors and

to

increase the

length

of the anchor rods or of the

cables. Then the length

of

the

sheet

pi les

c n

be

reduced because

of

the reduced axia l

force.

Figs

l l a and l lb

i l lu s t ra te

the forces act ing.

on

a

braced

and anchored

sheet p i l e walls in clay.

respect ively.

The normal

force

Nand

the shear

force T

T

is

proport ional

to

the

active undrained

shear

strength of

the

so i l cu)

act

along

the

assumed

fa i lure

plane. The weight

W) of

the s l id ing soi l wedge is

approximately the

same for the two cases .

The force

Ca)

along the

sheet

pi les

depends on the adhesion

{ca)

between

the

sheet pi les

and the clay

below

the bottom

of

the

excavation.

The inclination

and the magnitude

of the force R) in the

anchors

or in the s t rut s wi l l

however,

be

di ffe rent

I t

c n

be seen

from

the two force diagrams in Fig 11

that

both

the normal force

N

on the fai lure plane

and

the

passive

ear th pressure force

P which are required

p

for equilibrium wil l be · larger for an anchored sheet

pi le wall

when the

anchors are

incl ined

than

for a

braced

or a

st rut ted

wall when the

st ruts

are

horizontal . Thus

a larger

penetrat ion depth and a

higher passive

earth pressure will be

required for

an

anchored wall

where

the tendons are

inclined

compared

with

a braced

wall .

The s t ab i l i t y of

an

anchored

sheet

p i l e

wall

can be

expressed by the s t ab i l i t y

factor

Ncb defined by the

Fig

Stabi l i ty of anchored and braced sheet p i l e

walls

1524


11/36

equation (pHcr ~ f q

=Ncb

c u / ~ m where (pHcr ~ f q i s

the

total overburden

pressure a t the

bottom

of the

excavation

Her

is the cr i t ical depth and cu

is

the

undrained character is t ic shear strength

of

the

clay.

The total overburden pressure depends on the

c r i t i ca l

depth

of the excavation

Her ( the

maximum depth when

the

excavation i s

s t i l l

stable) . the

unit

weight of

the

soil

p

and

on

the surcharge load q.

The

stabi l i ty factor

Ncb

as

shown

in Fig 12 is

a

function of

the

inclination of

the

anchors (a), the

penetration

depth D) of the sheet

pi les below the

bottom of the excavation and the adhesion

(ca)

between

the sheet pi les and the clay. At = 1.0 the adhesion

p

corresponds

to the undrained shear strength

of

the soi l

cu.

At

=

0

the adhesion

is equal to zero. t can

be seen from

Fig

12 that the stabi l i ty

factor

Ncb

5tabtk- ylaclor ~ h

6 o

5.o

4 o

3 0

0

I 2

3

Fig

12

Stabi l i ty Factor Ncb

increases with

increasing value

on

and

with

increasing force

R

in the

anchors unt i l a

c r i t i ca l

value has been

reached. f

this

c r i t i ca l value i s

exceeded

then

Ncb will

decrease.

In order to simplify the calculat ions Sahlstrom

and

St i l le (1979) have

proposed

for sof t normally

consolidated clay

that the

stabi l i ty factor

Ncb should

be

taken

as

5.1

when the

sheet pi les

are

driven

to a

hard stratum so that the end

bearing capacity

of

the

sheet pi les will be

sufficient

to

resis t

the axial

force caused

by

the

inclined

anchors.

In

the case the

525

. :.

· :

·j

I

I

I /

iJ ?

/

/

a /0;/ure mtchan/517

N

Fig 13

Total s tab i l i ty of an anchored sheet p i l e

wall

sheet

pi les have not been driven to refusal in a

hard

layer and the ver t ical s tabi l i ty

of

.the wall

i s

low

then a value

on

Ncb of 4.1 should

be

used

in the

calculat ions.

The

s t ab i l i t y

may

be

reduced

especial ly in s i l ty clays

when

pi les

have

been

driven

close

to

an

exis ting sheet

pi le wall due

to

the remoulding of

the

soi l and the

resulting

increase of

the pore water

pressures

that

take

place

during the driving.

In

th i s case

a

value

equal to

3.6

on Ncb

c n

be used.

In

most

cases fai lure takes place in the undis turbed

soi l

between

the flanges = 1.0)

of

the sheet pi les

p

since

the

perimeter

area

i s

large. Usually a

layer

of

clay

will cling to

the surface

and come

up

together

with

the

sheet pi les

when they are

pulled.

The

length of the anchors should be sufficient so

tha t

the

s tabi l i ty

of the sheet

pi le

wall

wil l

be

adequate

with respect

to a deep-seated failure. In Fig 13

is

shown the forces act ing on

an

anchored

sheet

p i l e

wall

in a cohesionless soi l and the corresponding force

diagram. The rear face of

the

indicated

sliding wedge

had to res is t the la teral

earth

pressure

Pa.

The

required

passive

earth

pressure

Pp

req

a t

equi l ibrium

can

be determined

as shown in

Fig

13

(Broms, 1968)

which is

a

modification of

the

Kranz method which is


12/36

widely used in Germany and

Austria.

I t has

been

assumed

in the

analysis that

the critical failure

surface

is

located a/2) from the end of

the anchors,

where a is

the spacing of

the

anchors.

I t

has thus

been assumed that the

inclination of

the

failure

surface

behind

the

anchors

is 45°

+

1/2 ~ · . The main

advantage with

the

proposed

calculation method

is

i ts

simplicity.

I t

is

also necesary to check the .stability of

the

wedge

located above the fixed anchor length as i l lustrated ,in

Fig

14.

The

failure

surface has

been

assumed

to

extend

a distance a/2) from the end of

the

anchor block as

shown. The passive resistance of the soil

in

front of

the

sliding soil wedge

should be

sufficient to resis t

the lateral displacement

of the wedge. I t is

proposed

to use partial

safety

factors and

load

f,ru:tors in the

calculations.

a fcll1tlrt mechom m

w

,1 I

n

/

~

I

I=< ' I

I I

· ;

i I

Fig 14

Stability

of

anchor block

STRENGTH

OF

NCHORS

The design of ground

anchors

has

been

reviewed by

Littlejohn

1970).

The method

that

can be used

to

calculate

the

tensile

resistance of soi l

anchors is

illustrated

in

Fig

15.

The

ultimate tensile resistance

~ t

depends on

the

fr iction resistance Qskin along the

grouted part

of the

anchor

and on the

end resistance

Qend as expressed by

the relationship

9)

. :

•,

• .

Fig

15

Tensile resistance of ground anchors

The

displacement required to develop

the

maximum skin

friction is small,

a few

mm,

compared

with

the

relative

large

displacement

which

is

required

to

mobilize the

end

resistance.

In cohesionless soils sand and gravel) the pull-out

resistance sa)

depends

on

the effective

overburden

P

ressure a and on

the

friction

angle

.P

between

vo a

grouted part of

the

anchors and the soil

as

expressed

by

the equation

10)

The fr iction angle

.P

is

normally

assumed

to correspond

.a

to the angle of internal friction of the soil .P or .pd.

The coefficient K depends mainly

on

the .

relative

density of the soi l .

This

coefficient can for dense,

coarse

and

wellgraded sand

or

gravel be

as

high

as

2

to

3 due to the

dilatancy

of

the

soil . In loose fine sand

and

s i l t

the

coefficient

K

can

be

as

low

as

0.5. The

assumed value on K should be verif ied

by load

tests.

The

tensile resistance can

also

be

estimated from the

grout

pressure used

during the

installation of

the

anchors,

from the grout pressure

required

for the

expansion

of the

expander

bodies or from the

penetration resistance as

determined

by

e.g.

cone

penetration tests (CPT), standard penetration tests

(SPT) or

weight soundings {Wsr .

I t is proposed

to use the equations suggested by

Baquelin

e t al (1978) for

bored

piles to estimate the

pull-out resistance from the

maximum

grout pressure

p

.

The

tensile

resistance of the

anchors

grout

increases generally with increasing

grout

pressure

especially

in

hard rock

and

in

dense

s nd

and

gravel.

The capacity of

the

anchors wi 11

also

increase with

increasing length of the grouted zone, the fixed

anchor

length. In

s nd

and gravel there is , however, a

maximum

effective

length.

If

this

effective length

is

exceeded then there is no further increase of the

anchor force. The cri t ical length

is

about 6 m for

sand and gravel. Cyclic loading will , however,

reduce

this

length. The

fixed anchor

length

is

usually 3

to

6 m.

1526


13/36

According

to Baguelin e t al

{1978) the

net base

resistance of a

bored

pi le qend

can

be evaluated from

the limit

pressure

p

2

determined from pressuremeter

tests

(11)

where p

0

i s

the ini t ial

total horizontal

pressure

in

the grounct

a t the base of the pile

and

k is

a

coefficient

that depends on

the

embedment length and on

the

magnitude

of the

limit

pressure.

I t

is

expected,

however, that the limit pressure will

correspond to the maximum

grout pressure.

Pe

= Pgrout

z

Pgrout <

12

)

where p t

is

the grout

pressure

a t the ground

grou

surface, Pgrout

is

the unit

weight

of the grout and z

is the depth.

For the case the

tensile

resistance

corresponds

to 70

of the ultimate bearing capacity of a bored pi le then

the end resistance of

the

anchors

can

be

calculated

from the

equation

Qend

=

0 ·7 k

Pgrout

Aend

(13)

where k is a coefficient that depends on the embedment

length and

on

the

magnitude

of the limit

pressure

and

Aend is the cross-sectional area.

The unit

skin

friction

resistance

fs

of

a pile in sand

or gravel will

normally

be 0.5% to

2 of

the point

resistance {Meyerhof, 1956). The

skin friction will

generally

increase with

decreasing

particle

size

and

increasing cone resistance.

It is suggested

for sand

and gravel that the skin friction resistance

should be

taken

as 1 of the

unit

end resistance.

For s i l t

2 is

proposed.

The total

skin

friction resistance Qskin of

the

expander

bodies

will

be

12 of

the

total

end

resistance

for

sand

and

gravel and

24 for s i l t .

Then

for

sand

and gravel

o

1

t

=

0.78 k p t A d

U

grou

en

(14)

where k iS a bearing capaicty factor which depends on

the

embedment

depth.

For

s i l t

= 1.

24

Qend =

0

·

86

k Pgrout

Aend

(15)

The

ultimate pull-out

resistance of the expander bodies

as determined

by Equs {14) and {15}

has been

plotted in

Fig 16 as a function of

the

maximum grout pressure. I t

can be seen that

the

tensile

resistance

increases

rapidly with

increasing

grout

pressure.

I t

should ·be

observed

that

the

depth

of the exp Ulder bodies

should

be

a t

least

eight

times

the

diameter. Otherwise

the

resistance will

be reduced.

The

tensile

resistance

can also be

calculated from the

penetration

resistance of different penetration

tests

such

as cone penetration

tests

(CPT) standard

penetration tests {SPI') and weight soundings (Wsr). A

comparison between the different penetration tests is

shown

in Table

I

for

cohesionless

soils

{si l t

sand and

gravel). For example, a standard penetration

resistance

(N

30

)

of

30

blows/0.30

m

in

a

medium

sand

Ten:>de rest stonce, QuU

;

MN

6.or------------------------------------

5 · ··

)

: Q ~ u

_:.

- ~ - - - .

/

/

/

/

/

/

0.01.;. ------- t - - - - . .1 - . . - :L--- - - - ---L-- - - . . . .._L_ J _

. J . . . . __ L__j

0 1

0.2. 04 06 as lo 2.o

4 o

.

0.3

0.5

0.

7

0.9

3 o 5 o

Ma umu rn ctrou t

Dressu re o , MPct

v ' / rgr-out:)

Fig 16

Pull-out resistance of Expander Bodies

corresponds

a cone

penetration resistance

of

about

10

MPa.

I t

should be

noted

that the results are

affected. for

example,

by

the

particle size.

the depth

below the ground surface

and

the location of the ground

water

level. For

s i l t sand ,md gravel the cone

penetration resistance

in MPa

is

approximately 0.2

N

30

•

1527

0.4 N

30

and

0.6

N

30

, respectively.

However, the result from the

weight soundings

are

a t

large depths (> 10 m} influenced

b y

the fr iction along

the sounding

rod

since a casing

.is

not used, while

a t

SPI'

the results are affected by the

method

used

to

l i f t

and to

release

the hammer. The

energy

delivered by a

free

falling

hammer i s

considerably higher

than that.

when

the

rope

and pulley method

is

used.

Load

tests

indicate that

the end

bearing·

capacity

corresponds closely

to the cone penetration

resistance

(CPT} within a zone that extends one pile diameter

below

and

3.75 pile diameters above the pile point (van

der

Veen and Boersma, 1952}.

In

cohesionless

soils

the

tensile

resistance

will be

lower

than

the end

bearing

capacity because

of the reduction of the

over-burden

pressure as

mentioned above. I t is therefore,


14/36

TABLE I

a>MPARISON BETWEEN DIFFERENT PENETRATION TESTS

after

Broms

and Bergdahl, 1982)

Cone Penetra

t ion Tests

(CPT),

Relative

Point Resistance

e n s ~ t y qs MPa

Very

loose

2.5

Loose 2.5 - 5

Medium 5 -

10

Dense

10 - 20

Very

dense

>

20

suggested that the tensi le resis tance of soi l

anchors

should

be

taken

s

70 of

the bearing

capacity

of an

equivalent pi le .

Test data indicate

also

that the tensi le resis tance of

the expander bodies

wil l

decrease

with

increasing

diameter. I t is , therefore ,

suggested

that the uni t

tensi le resis tance of 0.5

m and 0.8 m diameter expander

bodies

should

be

taken as

80 and

50 , respectively of

the resistanye of expander

bodies with 0.3 m

diameter.

The

net

end resis tance in clay can be estimated from

(16)

when

the

anchor is located a t least four

diameters

below

the

ground

surface.

Also

the skin

resis tance

(ca)

wil l

depend on the

undrained shear strength

cu

of

the

clay

s

a

c

a u

(17)

where

i s a reduction coefficient which decreases

with

increasing shear

strength.

I t

is

suggested that

should be taken s 0.8 for sof t

clays

(cu 50 kPa) and

s 0.5

for medium

to s t i f f clays

when

cu >50

kPa.

I t should be noted that the tensi le resis tance

wil l

gradually

increase with time af ter

the

instal lat ion due

to the reconsolidation of the clay.

Particulary

the

skin fr ict ion

resis tance

is

affected. About 1 to 3

months

wil l be

required

in sof t

clay to reach the

Ximum

resis tance

while

in medium to

s t i f f clay

the

calculated

tensi le

resis tance usually wil l

be

obtained

within a few weeks. In weathered

rock

and residual

soils a value 0 .45

C

i s CODIIIOnly USed.

The

tensile

·

resis tance

can

be

increased further by enlarging the

boreholes by

underreaming.

The

pull-out

resis tance of ground anchors

in

rock has

been

correlated

with the unconfined compressive

strength. The

allowable shear resis tance

is

often

taken

as

0.1 where

is the

unconfined compressive

Standard

Penetra

t ion

Tests

(SPT),

Penetration

Resistance

N

20

,

Weigth Sounding

Tests,

Penetra

t ion

Re.sistance

blows/30 em

Nw' ht /0 .2 m

4

4 -

10

10 -

30

3 0 - 5 0

>50

4

10

- 30

3 0 - 6 0

6 0 - 100

>

100

strength of small diameter

rock

cores. The

maximum

shear

resis tance

is normally

limited to

4

MPa.

HQwever,

the spacing

and

the

orientation of

the jo in t

in the rock can have a large influence on

the

pull-out

resis tance. The reduction of

the shear

resis tance

has

been

related

to

the

RQD-value of the rock. Failure of

rock

anchors

located close to the

ground

surface

(D

<

1.5

m) often occurs when a cone

of

rock i s pulled out

together

with

the anchor

rod

or the cable . The tensi le

resis tance will in that case correspond to the weight

of the rock

cone and thus

to the

uni t

weight of the

rock

mass.

SETil EMENTS AND LA'FERAL DISPLACEMENTS

Deep

excavations in sof t clay

can

cause settlements

around the

excavation. As a

resul t surrounding

buildings

can

be damaged. The

damage can be related

to

ei ther

the

angular distort ion, the relat ive deflection

(sagging and hogging) or

the

la tera l

deformation

of

the

building. Buildings are in general more

af fec ted

by

large re la t ive deflections or

by

large la tera l

deformations than by an angular distort ion. Structures

are also more sensi t ive

to

hogging

than to

sagging.

Buildings

located

close to

an

excavation are often

loaded

in compression while

buildings

located further

away are subjected to lateral tension (elongation)

and

. may

therefore

crack.

The locat ion of the building

within the settlement trough

around

an

open

excavation

is

thus

important.

1528

The

la tera l displacement of the soi l around

deep

excavations nd i t s effect

on nearby

buildings has

attracted so

far re la t ive ly l i t t l e at tent ion.

The

resulting la tera l movement can damage

buildings

close

to

the excavation and

other

structures . A tensi le

st ra in of only

0.1X

to 0.2X is

often

suff icient to

cause extensive

cracking

of ma.sona.ry

s t ructures .

E.g.

O Rourke

(1981)

has observed large la tera l

s t ra ins

behind

an 18

m

deep excavation. The

resulting

la tera l

displacements

were

high

enouih

to

cause

extensive

cracking of ma.sonary structures located

up

to

9 m

behind

the

excavation.

Some

settlements wil l

always occur

even

when

the

best

available construct ion teclmique i s been used

and

the


15/36


16/36

a.

J h ~ z d f i d e n t

j2.UlefMion do/ f_h

Pion

.

··

r / . 1 /1 <

.

.

.

. .

.....

:

...

z: <

b.

5fab/lr2nlon

wtl 6

~ l e e 1 / - f ? l e ' ~ .

Fig 18

Vertical

s tabi l i ty of

anchored sheet

pile

walls

Erosion may even

occur

below the boulders or

the

stones

i the surface of the cut is not protected by,

for

example, shotcrete. Drain holes will be required to

reduce the high water pressure that otherwise may

develop behind the

shotcrete

layer.

Fig

18b i l lus tra tes the case when

the ver t ical

stability

of

the

sheet

pile

wall i s not sufficient

and

the

vertical

force

in

the

sheet piles

from the

inclined

anchors will cause the sheet piles

to

set t le

The

vert ical

s tabi l i ty of the wall can be increased by

driving steel H-piles in

front of

the wall as shown.

The H-piles should be welded to the sheet piles so

that

the vertical force from the anchors

can

be

transferred

to the

pi les .

The

bearing capacity of the H-piles

should be

sufficient ly

high so that they will be able

to carry the vertical force.

IMPROVEMENT OF 1HE SI'ABILITY

IN

SOFT a..AY

Different

methods can e

used

to

increase

the s tabi l i ty

of braced or anchored sheet pi le wall in soft clay as

i l lustrated in

Figs

19

through

22.

Lime

or

cement

columns have

been

installed in Fig 19 in

front

of

or

between

the two rows of sheet pi les in order to

increase the average shear strenght

of

the

clay

and

thus

the passive

resis tance

of the

soil .

The

lime or

cement columns

can also

be installed in

such a way that they form a series of continuous walls

between

the two sheet pi le walls to keep them apart .

The

lateral earth pressure acting

on

the

sheet

pi les

below the bottom

of the

excavation

will then be

1530

transferred

through the walls. In th is case,

the

columns

will function as

an

additional level

of s tru ts

below the

bottom

of the

excavation.

The

required

spacing of

the 1 me

or

cement columns

depends on the

increase of the shear

strength

that can be obtained

with

lime

quick

lime) or with

cement.

This can be

investigated

in the

laboratory by mixing

the clay with

different

amounts

of lime

and

cement.

The optimum

lime

content

i s usually

6% to 10% with

respect to the

dry

unit weight.

About

15%

to

25%

cement

is usually

required in order to reach

the

required shear strength

of

the

stabilized

soi l .

Gypsum

in

combination

with

quicklime

can be beneficial

in organic soils.

The columns

will increase

the

average undrained

shear

strength of

the

soi l In soft clay the average shear

strength can usually be doubled i

the

0.5

m diameter

lime

or

cement columns

are spaced

1.4 to

1.5

m apart .

Lime or cement columns

can also

be placed

behind the

sheet

pi les

in order to

reduce

the

la teral earth

pressure acting on the wall.

The soil

a t

the ground surface has been excavated in

Fig 20

in

order to reduce the total overburden

pressure

at the bottom of the

excavation. The

reduction of

the

lateral earth pressure on

the

wall will be large below

the excavation

especial ly

when the

total

overburden

pressure a t

the bottom of the excavation is

approximately equal to Nc cu.

The

excavated soil can

be replaced by

light

weight

f i l l

e.g. expanded

shale,

slag or flyash. In the Scandinavian

countries

and in

Finland

sawdust,

bark and peat

are

often

used.

With

slag

or

flyash, pollut ion

of the

ground

water

might

become a

problem.

Also jet

grouting and

quick lime columns

can

be

used

to

increase

the s tabi l i ty

as shown in Fig

20

as

has been

the, case in Singapore. At

the

quicklime column method

v' '

t T rf

1---

E evcrlton

~ · r n e

or

Cem ent

Co ..u.rnn5

r ement coLumn.>

_j,

_ ~ m e o

I

-

•

i

·· ·

-

y

}

• •

:-

.. -

••••

-

•

.

• ••

•

J

•

.

y

•

•

.

..A

A

A.Uernative I

A l ~ e r n a f : L · v e [

v

Fig

19 Stabilization with lime or cement columns


17/36

Fig

20

Fig 21

et

Jrot.dt:mJ

or

~ w e i f me

coturnns

Qu,·cl:. [,·me.

coi..W??ns

Stabilization with

l ight-weight f i l l

grouting or quicklime

columns

je t

Stabi l iza t ion

with

Bakau pi les and embankment

pi les

1531

large diameter holes which

are

f i l led

with

quicklime

are used. At th is method, the expansion that takes

place

when

the unslaked lime

reacts

with

water

is

uti l ized.

The

method i s mainly effect ive in s i l ty

soi l s with a low

plast ic i ty

index where a small change

of the water

content

will have a

large

effect . on

the

shear strength. The effectiveness of the method is

however, reduced when the soi l is s tra t i f ied . Then the

expansion of the

quicklime

columns

wil l occur

faster

than

the consolidat ion of the

sof t so i l around

the

columns. As a

resul t

the soi l wil l

be displaced

and

heave

rather

than consolidate.

Embankment or Bakau p i l es are used in Fig

21

in

order

to reduce the la teral earth pressure

acting

of the

sheet

pi le wall. The pi les

will

carry

part of

the

weight of

the

clay due to the

fr iction

or adhesion

along

the

pi les . The

efficiency of

the

embankment

piles can be

increased

i f the

pi les

are

provided with

concrete caps

which will transfer

the weight· of the

soi l above the caps to the

pi les . P i le

caps

are

required especially when concrete

or steel p i les with

high

bearing capacity are used because of the large

length

required

to t ransfer the load from

the

soi l to

the pi les

though

adhesion

or

fr iction along the pi les .

The

transfer

length wil l be large because of the

relat ively

high

p i l e

loads

which

are required in order

to

make

the

method economical. Embankment pi les are

common

in

Sweden, Finland and Norway par t icular ly in

sof t clay. Bakau pi les are extensively

used

as

embankment

pi les

in

Southeast Asia.

They have

the

advantage

that the surface

area i s

large,

that

the

transfer

length

i s small and that they

are

cheap. The

diameter i s usually SO to

100

mm The maximum length

i s about 6 m f longer pi les are required

they

had

to

be spliced.

The

stabi l iz ing ef fect

of embankment pi les i s

equivalent to that caused by an increase of the uni t

weight of the soi l

below

the excavation bottom as

i l lust ra ted in Fig

22.

The equivalent uni t

weight ..,eff

of the soi l

when

the

embankment pi les

are used

to

stabilize an

embankment or slope can be estimated from

the equation

where d

=-diameter

of the

pi les

ca

= adhesion

of

the c lay along the pi les

a = spacing of the pi les

'f = uni t weight of

the

soi l between

the pi les

n

elCBIIIple where

an 7.

6 m deep excavation in

soft

marine clay was successfully stabi l ized with 6 m long

Bakau pi les has

been described

by Broms and

Wong

1985).

Other

methods· that

have been used

to

increa.Se

the

s tabi l i ty

with respect

to bottom

heave are shown in Fig

0 . The s tab i l i ty can be improved

by

driving a few

sheet pi les to a so i l layer with

high

bearing capacity

so

that part

of the

weight

of

the

soi l

can

be

carried

by

the

skin f r ic t ion along the

sheet piles. I t

is a l so

possible to

use

inclined anchors

in

order

to

increase

the ver t ical s tab i l i ty of

the sheet

pi le wall as shown.

This method

can

be

economical

if there i s a

concrete

slab

next

to the excavation. The s tab i l i ty can be

increased

as

well by placing the

bottom

level of s t ru t s

in trenches

below

the

bottom

of the excavation.

Thereby

the effect ive length of the sheet pi les below

the

lower

s tru t level will be reduced.


18/36

Fig

22

0

_. l o

0

0

J

v

1

i

c

v

v

i

y

y

y

Elevofti:m

Increase

a£

the equivalent uni t weight using

embankment pi les

FAILURE

OF A SINGLE ANCHOR

The

redis tr ibution

a£ the load

that

takes place when

one or

several

a£ the anchors or s t ruts

£ail

has been

investigated by St i l l e {1976)

and

by St i l l e and

Brems

1976).

In Fig

24 i s s ~ w the load redistribution

that was observed for an anchored

sheet

pi le wall a t

M8lntorp. Sweden in a

very

soft clay with an average

shear strength o£

18 kPa

when one

or

two a£ the

anchors

were

unloaded. For

thi s

sheet

pi le

wall

which

was

anchored a t two levels i t was

observed

that

the

maximum

increase of the load in the

adjacent

anchors was 9

when one

anchor

was unloaded and that the lOad

increased by an addit ional 8X when the load

in

a

second

anchor was

released. I t is

interesting

to note that

the

total increase of the load in al l anchors was

only

36 o£ the

in i t ia l

load in the unloaded anchor.

Thus

the total lateral earth

pressure

on the sheet p i l e wall

decreased by

64

of the in i t ia l

load in

the unloaded

anchor. When

the second

anchor

was unloaded then

the

total increase

of

the load in

the adjacent

anchors was

only

16

of the in i t ia l load in that anchor. Thus the

total

lateral

earth

pressure

on the wall decreased by

84 with respect to

the in i t ia l anchor

load.

The corresponding load

redis tr ibution

for a sheet p i l e

wall a t

Bergshamra,

Sweden with

three

anchor levels i s

shown in

Fig

25.

In

this

case

Panel.Bl)

the

maximum

increase of load in the

adjacent

anchors was to 35 of

the in i t ia l anchor

force

before

the

f i r s t

anchor

was

unloaded.

The to tal la teral

earth

pressure

on

the wall

increased by

32 with respect to

the

in i t ia l

anchor

load.

In a second panel Panel Cl) the maximum

increase

of the anchor. force in the

adjacent

anchors

was 14 with respect to the in i t ia l

load

when the load

in

one

of the

anchors was

released. In

thi s

case the

IL___.

5/:ru l:s

£n

{renches

heet

ptles

drt ven

c nto

/

. / :

<

5

::< :· /

a ~ r r t Y

t.aye;

ond

t

Fig

23

Inclined

anchors and lowering of the

strut

level

to tal la teral ear th pressure on the wall increased by

4

with

respect

to

the

load in

the unloaded

anchor

compared with

a decrease of

64

a t M8lntorp.

The

behaviour of

this

sheet Pi.le was thus different . This

difference in behaviour can be explained

by

the

difference in mobilized shear strength of the clay

behind

the

wall.

Fig 24

1532

·

~ z e a + a r m m a a e ; ; y : ; a ~ ~ ; ; ; ; : w ; ; o : i i A , a ; r ; & E O I

t

8 ·

Load redistribution

a t

Molntorp, Sweden a t

failure o£

one

or two

ground

anchors after

Sti l le , 1976)


19/36

Fig 25

Fig 26

Load redistribution

a t

Bergshamra, Sweden

at

failure of one or two ground

anchors

after

Stil le,

1976

fvlobd t zcrtt on o

; heqr

sirenJ- h

Load redistribution due to

mobilization of

shear strength

1533

The lateral earth pressure

acting

on a braced or an

anchored

sheet pi le

wall depends on the lateral

displacement

required to

mobilize the

shear strength

of

the soil

behind the wall

and on the

factor of

safety

used in

the design. The

wall will

deflect laterally

when the

load

in one of the anchors is

released

or the

anchor fails. The

increase

of the lateral

deflect ion

of

the

wall

is

generally .sufficient to mobilize the

shear strength of the

clay

along a potential failure

surfaces

behind

the wall as

illustrated in Fig

26.

A

relative small

deflection

is

normally

required to

develop the maximum shear strength of even soft clay

compared

with the displacement required to

develop

the

ultimate resistance of the anchors

or of

the struts.

In the case the factor of safety ini t ia l ly is

relatively high then only a small part of the available

shear strength will

ini t ia l ly

be mobilized. A

reduction of

the force

in one of the anchors will

then

mainly

increase the

average shear stress along

potential

failure surfaces in the

clay. In

this

case,

the

increase of the

load in the

adjacent

anchors will

be

small

and the

total lateral earth

pressure on the

wall

will decrease when one of the

anchors

is

unloaded

or fails

as was the case at Molntorp.

If on the other hand the factor

of safety

is low

and

the

shear strength of the

clay

has

been fully mobilized

before the release of the force in one of the anchors

then

the failure of one of the anchor will result in a

large increase

of

the

load

in the adjacent

anchors.

The

total

load

on

the sheet

pile

wall

m y

even

increase

when

the

peak strength of the

clay

has been exceeded

and the residual shear strength is lower than the peak

strength. This was the case at Bergshamra where the

total

force acting

on

the sheet

pile wall

increased

when

the load in

one of the

anchors

was released.

The

consequences

when one of the

anchors

fai l

will thus

depend to a large part on the chosen factor of safety.

If

a relatively

high

factor

of

safety has been used in

the

design 1.5) and only part of

the

shear strength

of the

soil will

be

mobilized a t

working loads

then

the

increase of the load in the adjacent anchors will be

small when one

of

the anchors fails. If on the other

hand the factor of safety

is close

to 1.0 then the

failure

of

one

of the

anchors

will

cause a large

increase of the

load

in the adjacent

anchors

which also

may

fa i l The

total lateral earth pressure

on the

sheet

pi le

wall

may

ev.en

increase

and cause

a

progressive

failure

of the whole wall {zipper effect).

SfABILI1Y OF

THE

BASE OF A SHEET PILE WAlL

Several failure of anchored

walls

have been occurred

in

Sweden

in

soft clay. In Fig 27 is shown an anchored

wall

constructed of large

diameter bored

piles {Broms

and Bjerke, 1973). The exposed

clay

between the piles

was

shotcreted during the

excavation.

Clay started to

flow into the excavation below the shotcreted part of

the wall

almost like

tooth paste

squeezed

out of a

tube

when the depth

of the

excavation was 5.5 m. Within a

few minutes the

excavation

was

f i l led with

soft

remoulded

clay

due to

the high

sensitivity

of the clay.

Failure

took

place

when

the

total

overburden pressure

at the bottom of the excavation was about 6 c where c

u u

is

the undrained

shear

strength of the clay as

determined by

f ield

vane tests. The

factor

6.0

corresponds

to the

stabi l i ty

factor Ncb This

type

of

construction

using

bored piles

and shotcrete is

therefore

not suitable for soft

clay

when the depth of

the excavation is large and

the

total overburden

pressure at the bottom

of

the

excavation

exceeds

N

be

c_ u


20/36

5ho-l:cretc-

,...,s.srn

Bored pde

.:5hotcre.f:e-

Sol : do j_,

c

4

12 /;Pet

A

Fig

27

Failure

of a vertical cut in soft.

clay

(after

Brems

Bjerke,

1973)

(Brems and Bennerma.rk,

1967). Steel

sheet piles

or

contiguous bored

pi les should have been used instead.

Several

fai lures have

also

occurred in

Sweden when the

sheet

pi les have been dr

Documents

Anchor Types in Sheet Piles