anchor types in sheet piles
TRANSCRIPT
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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'
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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.
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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.
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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
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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.
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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
-
8/9/2019 Anchor Types in Sheet Piles
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
-
8/9/2019 Anchor Types in Sheet Piles
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
-
8/9/2019 Anchor Types in Sheet Piles
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
-
8/9/2019 Anchor Types in Sheet Piles
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
-
8/9/2019 Anchor Types in Sheet Piles
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
-
8/9/2019 Anchor Types in Sheet Piles
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,
-
8/9/2019 Anchor Types in Sheet Piles
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
-
8/9/2019 Anchor Types in Sheet Piles
15/36
-
8/9/2019 Anchor Types in Sheet Piles
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
-
8/9/2019 Anchor Types in Sheet Piles
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.
-
8/9/2019 Anchor Types in Sheet Piles
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)
-
8/9/2019 Anchor Types in Sheet Piles
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
-
8/9/2019 Anchor Types in Sheet Piles
20/36
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,...,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