pseudo static analysis of piles subjected to late

7/21/2019 Pseudo Static Analysis of Piles Subjected to Late

http://slidepdf.com/reader/full/pseudo-static-analysis-of-piles-subjected-to-late 1/14

PSEUDO-STATIC ANALYSIS OF PILESSUBJECTED TO LATERAL SPREADING

Misko CUBRINOVSKI1 , Keni IS!I!ARA" #n$ !#%%& POULOS'

ABSTRACT

Soil liquefaction during strong ground shaking results in almost complete loss of strength andstiffness in the liquefied soils, and consequent large lateral ground displacements. Both cyclic

displacements during the intense ground shaking and development of liquefaction, andespecially post-liquefaction lateral displacement due to spreading of liquefied soils aredamaging for piles. Characteristics of the liquefied soils and loads on piles are significantlydifferent during the cyclic phase and subsequent lateral spreading phase, and therefore, it isnecessary to separately consider these two phases in the simplified analysis of piles. This paperdescribes a practical procedure for preliminary assessment and design of piles subected tolateral spreading, and addresses key parameters and uncertainties involved.

INTRODUCTION

There are several methods available for analysis of piles in liquefied soils includingsophisticated finite element analysis based on the effective stress principle and simplified

methods based on the pseudo-static approach. ! rigorous effective stress analysis permitsevaluation of seismic soil-pile interaction while considering the effects of e"cess pore pressureand eventual soil liquefaction on the pile response. #hereas the predictive capacity of suchanalysis has been verified in many studies, its application in engineering practice is constrained

by two requirements, namely, the required high-quality and specific data on the in-situconditions, physical properties and mechanical behaviour of soils, and quite high demands onthe user regarding the knowledge and understanding both of the phenomena considered and

particular features in the adopted numerical procedure. $rovided that the above requirements aremet, however, the effective stress analysis provides an e"cellent tool for assessment of theseismic performance of pile foundations in liquefiable soils.

%or preliminary assessment and design of piles, however, a simplified analysis may be more

appropriate provided that such analysis can satisfy the following requirements& i' the adoptedmodel must capture the kinematic mechanism associated with the spreading of liquefied soils(ii' The analysis should allow us to estimate the inelastic response and damage to piles, and iii'the method should allow for variations in key parameters and assessment of the uncertaintiesassociated with lateral spreading. Based on these premises, this paper addresses the use of the

pseudo-static analysis of piles in liquefying soils and focuses in particular on its application tothe analysis of piles subected to lateral spreading.

GROUND DISPLACEMENTS IN LI(UEFIED SOILS

) Senior *ecturer, +niversity of Canterbury, Christchurch, ew ealand $rofessor, Chuo +niversity, Tokyo, /apan0 Senior $rincipal, Coffey 1eotechnics $ty *td, Sydney, !ustralia



#hen analy2ing the behaviour of piles in liquefied soils, it is useful to distinguish between twodifferent phases in the soil-pile interaction& a cyclic phase in the course of the intense groundshaking and consequent development of liquefaction, and a lateral spreading phase followingthe liquefaction. 3uring the cyclic phase, the piles are subected to cyclic hori2ontal loads dueto ground displacements 4kinematic loads' and inertial loads from the superstructure, and thecombination of these oscillatory kinematic and inertial loads determines the critical load for theintegrity of the pile during the shaking. *ateral spreading, on the other hand, is primarily a

post-liquefaction phenomenon that is characteri2ed by very large unilateral grounddisplacements and relatively small inertial effects. Thus, the liquefaction characteristics andlateral loads on piles can be quite different between the cyclic phase and the subsequent lateralspreading phase.

C&)*i) G%o+n$ Dis*#)een.s5n order to illustrate some important features of ground displacements in liquefied soils,observations from well documented case histories in the )667 8obe earthquake are discussed inthe following. %igures )a and )b show computed hori2ontal ground displacements and e"cess

pore water pressures that developed in an )9 m thick fill deposit during the intense part of the

ground shaking in this quake. This response is representative of the cyclic phase of the freefield response of the fill deposits in areas that were not affected by lateral spreading.

Several features of the ground response shown in %igure ) are relevant to the behaviour andanalysis of piles in liquefied soils. %irst, the cyclic hori2ontal ground displacements in the

-:;

-0;

;

0;

:;

; 7 ); )7 ; < o r i 2 o n t a l d i

s p l a c e m e n t 4 c m '

Time 4seconds'

=elative displacements between 1*-;m and 1*-):m

4a'

;

7;

);;

)7;

;;

; 7 ); )7 ; > " c e s s p o r e w a t e r p r e s s u r e

4 k $ a '

Time 4seconds'

1*-)7.7 m

1*-7.7 m

σv?

σ

v

?

4b'

%igure ). 1round response of liquefied deposit in the )667 8obe earthquake& 4a' Cyclic grounddisplacement( 4b' >"cess pore water pressure

course of the strong shaking are very large with peak values of about 07-@; cm. Thesedisplacements correspond to an average peak shear strain of about 0-@ A throughout the );-)

m depth of the liquefied layer. e"t, it is important to note that at the time when the grounddisplacement reached a large value of about 0; cm for the first time since the start of the



shaking, i.e. at appro"imately 7.0 sec, the e"cess pore water pressure was well below theeffective overburden stress thus indicating that the soil has not fully liquefied yet, at this stage.These large displacements were accompanied with high ground accelerations of about ;.@ g atthe ground surface. This type of behaviour, where large ground displacements and highaccelerations concurrently occur ust before or at the time of development of full liquefaction,has been also observed in shake table e"periments, thus highlighting the need to carefullyconsider the combination of inertial loads from the superstructure and kinematic loads due toground displacements when analy2ing the behaviour of piles during the cyclic phase. Themagnitude of these loads depends on a number of factors including the e"cess pore water

pressure build-up, relative displacements between the soil and the pile, and relative predominant periods of the ground and superstructure, among others. Clear and simple rules forcombining the ground displacements 4kinematic loads' and inertial loads from thesuperstructure in the simplified pseudo-static analysis have not been established yet, thoughsome suggestions may be found in Tamura and Tokimatsu 4;;7' and *iyanapathirana and$oulos 4;;7'.

L#.e%#* S%e#$in/ Dis*#)een.s

5n the )667 8obe earthquake, the ground distortion was particularly e"cessive in the waterfrontarea where many quay walls moved several meters towards the sea and lateral spreadingoccurred in the backfills that progressed inland as far as ;; m from the revetment line. 5shiharaet al. 4)66' investigated the features of movements of the quay walls and ground distortion inthe backfills by the method of ground surveying and summari2ed the measured displacementsin plots depicting the permanent ground displacement as a function of the distance inland fromthe waterfront, as shown in %igure . <ere, the shaded area shows the range of measured

;

;.7

)

).7

; 7; );; )7; ;;

+o2akihama $ier ))

ikagehama Tank );)

<-Building

%ukaehama Building

ikagehama Tank T!-

<igashi-ada Building

* a t e r a l g r o u n d d i s p l a c e m e n t ,

+

1

4 m '

3istance from the waterfront, * 4m'

5shihara et al. 4)66'

easured 4-S'

+C*-ma"

D 0;-@; cm

Sea *ateral spreading

+1

%igure . $ermanent lateral ground displacements due to spreading of liquefied soils indisplacements along -S sections of $ort 5sland, and the solid line is an appro"imation for theaverage displacement. Superimposed in %igure are the cyclic ground displacements in the freefield showing that in the 2one within a distance of appro"imately 7; m from the quay walls, the

permanent ground displacements due to lateral spreading were significantly greater than the

cyclic ground displacements. The permanent ground displacements reached about ) - @ m at the



quay walls. Since the lateral spreading is basically a post-liquefaction phenomenon, it isassociated with higher e"cess pore pressures and hence lower stiffness of the liquefied soils, ascompared to its preceding cyclic phase. This feature, together with the unilateral down-slope orseaward ground movement, results in very large permanent spreading displacements. Clearlythe magnitude and spatial distribution of ground displacements, as well as the stiffness of thesoils undergoing large lateral movements, are quite different between the cyclic phase andlateral spreading phase, and these differences have to be accounted for in the simplified analysisof piles.

TYPICAL DAMAGE TO PILES

! large number of pile foundations of buildings, storage tanks and bridge piers located in thewaterfront area of 8obe were damaged in the )667 8obe earthquake 45shihara and Cubrinovski,)669( ;;@( /1S, )669( Tokimatsu and !saka, )669'. 3etailed field investigations wereconducted on selected piles using a borehole video camera and inclinometers for inspecting thecrack distribution and deformation of the pile respectively throughout the depth of the deposit,as well as by a visual inspection of the damage to the pile head. By and large, the damage to the

piles can be summari2ed as follows&

). ost of the piles suffered largest damage at the pile top and in the 2one of the

interface between the liquefied layer and the underlying non-liquefied layer 4%igure0'.

. $iles in the 2one of large lateral spreading displacements were consistently damaged

at depths corresponding to the interface between the liquefied layer and theunderlying

0

10

20

30

40

3 e p t h 4 m '

* i q u e f i e d s o i l

Pile

Pile 10Pile 4

C%#)ks

S S S SNN

45.7 m

4.18 m

o t - l i q u e f i e d

%igure 0. Typical damage to piles observed in the )667 8obe earthquake

4+o2akihama bridge pier $))'

non-liquefied layer. Since this interface was at large depths where inertial effects fromthe superstructure are known to be less significant, this damage can be attributed to thelateral loads arising from the e"cessive ground movement due to spreading.



0. 3amage at the pile head was encountered both for piles in the free field and piles

located within the lateral spreading 2one, near the quay walls. Both inertial loadsfrom the superstructure and kinematic loads due to lateral ground displacementscontributed to the damage at the pile head.

@. The variation of lateral spreading displacements with the distance from thewaterfront shown in %igure may result in different lateral loads being applied toindividual piles, depending on their position within the pile-group. This in turn maylead to significant cross-interaction effects and consequent bending deformation anddamage to piles in accordance with these interaction loads from the pile-cap-pilesystem. 5n some cases where these pile-group effects were significant, the piles failedwithin the liquefied layer or at least several meters below the pile head.

5n addition to the typical damage patterns described above, other less significant damage wasconsistently found at various depths for many of the inspected piles thus reflecting the comple"dynamic nature of loads and behaviour of piles in liquefying soils.

PSEUDO-STATIC APPROAC! FOR SIMPLIFIED ANALYSIS

The most frequently encountered soil profile for piles in liquefied deposits consists of threedistinct layers, as illustrated in %igure @ where the liquefied layer is sandwiched between a non-liquefied crust layer at the ground surface and non-liquefied base layer. *iquefaction duringstrong ground shaking results in almost a complete loss of strength and stiffness of the liquefiedsoil, and consequent large lateral ground displacements. !s demonstrated in the previoussection, particularly large and damaging for piles are post-liquefaction displacements due tolateral spreading. 3uring spreading, the non-liquefied surface layer is carried along with theunderlying spreading soil, and when driven against embedded piles, the crust layer isenvisioned to e"ert large lateral loads on the piles. Thus, the e"cessive lateral movement of theliquefied soil, lateral loads from the surface layer and significant stiffness reduction in theliquefied layer, are key features that need to be considered when evaluating the pile response to

lateral spreading.

5n light of the liquefaction characteristics and kinematic mechanism as described above, a three-layer soil model was adopted for a simplified analysis of piles based on the pseudo-staticapproach, in a previous study 4Cubrinovski and 5shihara, ;;@'. !s indicated in %igure @, in thismodel the pile is represented by a continuous beam while the interaction between the liquefied

soil and the pile 4 p-δ relationship' is specified by an equivalent linear spring 4β 2k 2'. <ere, k 2 is

the subgrade reaction coefficient while β 2 is a scaling factor representing the degradation of

stiffness due to liquefaction and nonlinear behaviour. 5n the analysis, the spreading is

represented by a hori2ontal free-field displacement of the liquefied soil while effects of the

surface layer are modelled by an earth pressure and lateral force at the pile head, as illustrated in

%igure @. +sing an iterative procedure based on the equivalent linear approach, a closed-formsolution was developed for evaluating the pile response to lateral spreading. The analysis

permits estimation of the inelastic response and damage to piles, yet it is based on a simple

model that requires a small number of conventional engineering parameters as input 4%ig. 7'.

eedless to say, one may use an %> beam-spring model instead of the above closed-form

solution and conduct even more rigorous analysis, as compared to the three-layer model,

because it will permit consideration of a multi-layer deposit with different load-deformation

properties. 5n principle, however, the following discussion applies to the pseudo-static analysis

of piles, in general.



$ile

%ree fielddisplacementof liquefied soil

*iquefiedlayer

on-liquefied base layer

on-liquefiedsurface layer

*ateralforce

UG

,1-#0

p

δ

k β

>arth pressure

2 2

%igure @. Simplified kinematic mechanism of lateral spreading

5nput parameters of the computational model and adopted load-deformation relationships for

the soil and the pile are shown in %igure 7. The equivalent linear p-δ relationship for the

liquefied layer was adopted in order to simplify the modelling of the highly nonlinear behaviourof liquefied soils undergoing spreading and to allow parametric evaluation of the effects of this

parameter. 5n the analysis of a given pile, it is envisioned that β will serve as a parameter that

will be varied over a relevant range of values, thus permitting evaluation of the pile response by

assuming different properties of the liquefied soil. En the other hand, bilinear p-δ relationships

and a tri-linear moment-curvature relationship 4 M-φ ) were adopted for modelling the nonlinear behaviour of the non-liquefied soil layers and the pile respectively. ote that p1-max defines the

*iquefied

layer

Base layer

U G2

M

φ

p

δ

%ooting

$ i

l e

C

F+

Surface layer

k 3 p3-maxo

H 3

H 2

H 1

H f

D

22β k

δ

p

1-max1 pk

δ

p

%igure 7. Characteri2ation of nonlinear behaviour and input parameters of the model



ultimate lateral pressure that can be applied from the crust layer on the pile. 5n cases when therelative displacements between the soil and the pile are very large, it would be necessary tolimit the ma"imum pressure that the liquefied soil can apply to the pile. The undrained strengthor steady state strength would be one obvious choice in the definition of the ultimate pressureform the liquefied soil. This modification of the original model is indicated with a dashed line

in the p-δ relationship for the liquefied soil in %igure 7.

KEY PARAMETERS AND UNCERTAINTIES INVOLVED

The analysis of piles in liquefying soils is burdened by unknowns and uncertainties associatedwith liquefaction and lateral spreading in particular. Thus, it is very difficult to estimate thestrength and stiffness properties of liquefied soils or predict the magnitude and spatialdistribution of lateral spreading displacements. Ene of the key aspects of the simplified analysisis therefore to properly address these uncertainties.

L#.e%#* G%o+n$ Dis*#)een.sThe lateral displacement of the spreading soil 4U G2' can be evaluated using empiricalcorrelations for ground displacements of lateral spreads 4Tokimatsu and !saka, )669( <amadaet al., ;;)( Foud et al., ;;'. 5t is important to recogni2e, however, that in most cases itwould be very difficult to make a reliable prediction for the spreading displacements. Thisdifficulty is well illustrated in %igure where a large scatter in the ground displacements isseen, even for a single earthquake event and generally similar ground conditions. 5n thisconte"t, Foud et al. 4;;' suggested the use of a factor of for the displacements predictedwith their empirical model, in order to cover the e"pected range of variation in the spreadingdisplacements.

Cyclic ground displacements can be estimated more accurately by means of an effective stressanalysis, but this combination of an advanced analysis being used for the definition of the inputin a simplified analysis is not consistent or practical. %or this reason, it seems more appropriateto estimate the peak cyclic displacements for the simplified analysis by using simplified chartscorrelating the ma"imum cyclic shear strain that will develop in the liquefied layer with thecyclic stress ration and S$T blow count, as suggested by Tokimatsu and !saka 4)669', fore"ample. The hori2ontal cyclic displacement profile can be then easily obtained by integratingthe shear strains throughout the depth of the liquefied layer. 5n both cases of cyclicdisplacements and spreading displacements, the lateral ground displacement that is used as aninput in the simplified analysis of piles is a free field ground displacement which is unaffected

by the pile foundation. C%+s. L#&e%

The lateral load from the unliquefied crust layer may often be the critical load for the integrity

of the pile because of its large magnitude and unfavourable position as a Gtop-heavyH loadacting above a laterally unsupported portion of the pile in the liquefied soil. %or the adopted

bilinear p-δ relationship for the crust layer shown in %igure 7, the key input parameter is the

ultimate lateral pressure, p1-max.

The ultimate soil pressure from the surface layer per unit width of the pile can be estimated

using a simplified e"pression such as, p1-max = α u p p, where p p(z 1 ) is the =ankine passive

pressure while α u is a scaling factor to account for the difference in the lateral pressure between

a single pile and an equivalent wall. %igure : shows the variation of α u with the relative

displacement observed in a benchmark lateral spreading e"periment on full-si2e piles

4Cubrinovski et al., ;;:' with the ma"imum lateral pressure on the single pile being about @.7



;

)

0

@

7

:

; ; @; :; 9;

Steel pile

$<C pile C e a s u r e d

r e s u l t a n t p r e s s u r

α

=

= a n k i n e

p a s s i v e p r e s s u r e

$ $ p

=

=elative displacement between soil and pile,

δ D +g42' - +

p42' 4cm'

αu

%igure :. =atio of lateral pressure from the crust layer on a single pile and =ankine passive pressure, measured in full-si2e test using large-scale shake table

times the =ankine passive pressure. 3ata from other e"perimental studies, shown in %igure ,

also indicate quite large values for the parameter α u, clearly indicating that very large lateral

loads can be applied by the crust layer to the pile. <ere, it is important to distinguish between

two types of loading conditions, namely, active pile loading and passive pile loading. 5n the

case of active pile loading, the hori2ontal force at the pile is the causative load for the pile

deformation, as shown in %igure 9a( in this case, the mobili2ed earth pressure provides the

resisting force. 5n the case of passive pile loading, on the other hand, the mobili2ed pressure

from the crust layer provides the driving force for the pile deformation, as illustrated in %igure

9b. ote that the two sets of e"perimental data on passive piles yield a value of α u D @.7. The

test data used by Broms 4)6:@' yielded mostly values of α u D 0 I :, and Broms adopted thelower-bound value of α u D 0 as a conservative estimate for active piles. This value has been

adopted in many design codes for active loading on piles, but may be unconservative for

passive piles.

5t is important to note in %igure : that a large relative displacement of nearly ; cm was needed

to mobili2e the ultimate lateral pressure from the crust layer. This relative displacement du at

which pu is mobili2ed depends on the relative density of the sand, as illustrated by the

)

0

@

7

:

9

0; 07 @; @7 7;

eyerhof 4)69)'

$rasad 4)666'

Broms 4)6:@'

!ngle of internal friction, 4degree'φ

C e a s u r e d u l t i m a t e p

r e s s

α

=

= a n k i n e p a s s i v e p r e s s u r

$ u $ p =

u

$assive piles

!ctive piles

$assive piles

Cubrinovski et al. 4;;:'

$oulos 4)667'

Single piles in sand



%igure . =atio of ultimate pressure from the crust layer on a single pile and =ankine passive pressure, obtained in e"perimental studies 4Cubrinovski et al., ;;7'

..

. ... .. .

..

4b' $assive pile loading

ovement

of soil

ovement

of soil

1round

. . .. . .

.

. ...

..

. .....

..

. ... .. .. ..

. .

. ....

..

..

.

.. . . ...... .... .

..

$assive

earth pressure$assive

earth pressure

. .

..

.

*ateral

load

4a' !ctive pile loading

. .

displacement

%igure 9. Schematic illustration of lateral loading of piles& 4a' !ctive-pile-loading( 4b' $assive- pile-loading

e"perimental data summari2ed in %igure 6. <ere < denotes the height of the model wall or pile

cap used in the test. 5t is evident that for dense sands with 3r D ; A to 9; A, the ultimate

pressure was mobili2ed at a relative displacement of about δu D ;.;< to ;.;9< and that larger

movement was needed to mobili2e the passive pressure in loose sand. =ollins 4;;' suggested

that the presence of a low strength layer below the surface layer may increase the required

deflection to mobili2e the passive pressure, and this appears to be a relevant observation for a

crust layer overlying liquefied soils.

Li+e2ie$ L#&e%

The factor β 2, which specifies the reduction of stiffness due to liquefaction and nonlinear

behaviour 4β 2k 2', is affected by a number of factors including the density of sand, e"cess pore

pressures, magnitude and rate of ground displacements, and drainage conditions. Typically, β 2

takes values in the range between )J7; and )J); for cyclic liquefaction and between )J);;; and

)J7; in the case of lateral spreading. The values of β 2 back-calculated from full-si2e tests on

piles 4Cubrinovski et al., ;;:' are shown in %igure ); as a function of lateral ground

displacement, illustrating that β 2 is not a constant, but rather it varies in the course of lateral

spreading.

The equivalent linear p-δ relationship for the pile, defined by the degraded stiffness β 2k 2, can be

easily e"tended to a bilinear p-δ relationship by using the undrained or steady state strength of

the soil in the definition of the ultimate lateral pressure from the liquefied soils. The empirical

correlation between the undrained strength and S$T blow count proposed by Seed and <arder

4)66)' can be used for appro"imating the undrained strength in these calculations.



;

;.)

;.

0; @; 7; :; B; 9; 6;

Summary of e"perimentsfrom 7 studies 4=ollins, ;;'

%ang 4;;'

Cubrinovski et al. 4;;:'

u J

<

=elative density, 3r

4A'

δ

δ u4min'

δ u4ma"'

%igure 6. =elative displacement required to fully mobili2e the passive pressure as a function ofthe relative density of sand& summary of data from e"perimental studies

;.;;)

;.;)

;.)

; ; @; :; 9;

β .

1round displacement at top of liquefied layer 4cm'

;.;7

S t i f f n e s s d e g r a d a t i o n f a c t o

r ,

Steel pile

Best fit

valueβ2

β2values

=ange of

%igure );. 3egradation of stiffness in the liquefied layer as a function of lateral grounddisplacement observed in full-si2e test on piles 4Cubrinovski et al., ;;:'

Pi*e-G%o+ E22e).s$ile groups may generally affect the behaviour of piles in liquefying soils in two ways, firstthrough the cross interaction among the piles within the group, and second, by influencing thekey parameters controlling the pile response such as the stiffness of the liquefied soils, and themagnitude and spatial distribution of spreading displacements. Both effects are brieflydiscussed in the following.

$iles in a group are almost invariably rigidly connected at the pile head, and therefore, whensubected to lateral loads, all piles will share nearly identical hori2ontal displacements at the

pile head. 3uring lateral spreading of liquefied soils in a waterfront area, each of the piles will be subected to a different lateral load from the surrounding soils, depending upon its particularlocation within the group and the spatial distribution of the spreading displacements 4%igure))'. Consequently, both the interaction force at the pile head and the lateral soil pressure alongthe length of the pile will be different for each pile, thus leading to a development of distinct



Crust layer

*iquefied

layer

Base layer

UG314

UG3n4U

G3k4

%igure ))& $iles in a group subected to lateral ground displacements due to spreading

%igure )& 5llustration of cross-interaction effects on end piles subected to different grounddisplacements 4small ground displacement acting on $ile )( large grounddisplacement acting on $ile 7'( dashed lines indicate response of individual pileswithout cross Iinteraction effects( solid lines indicate response of individual pilesincluding pile-group effects( 4a' $ile displacements, and 4b' Bending moments

patterns of deformation and stresses along the length of individual piles in the group 4%igure))b'. This response feature, in which the piles share identical displacements at the pile head buthave different deformations throughout the depth, is considered to be a significant feature of thedeformational behavior of pile groups subected to lateral spreading. These pile-group effectscan be easily captured by a simplified method of analysis using a single pile model4Cubrinovski and 5shihara, ;;7'.

The second influence of the pile-group regarding its effects on the magnitude and distributionof ground displacements, stiffness characteristics of spreading soils and ultimate soil pressure,is more difficult to quantify. >"perimental data on these effects for piles in liquefiable soils isscarce and not conclusive. %igure )0 illustrates a clear tendency for reduction in the ultimate

lateral soil pressure with increasing number of piles within the group, as compared to that of an

;

7

);

)7

;

-;; -);; ; );; ;;

Bending moment, C 4k-m'

3 e p t h

4 m '

354;

7

);

)7

;

; ; @; :; 9; );;

$ile displacement, +$ 4cm'

3 e p t h

4 m '

113 kN 78 kN

3#4$ile )

$ile 7

Single pile

analyses&

$ile group

analysis&



;

;.7

)

).7

.7

; 7 ); )7

eyerhof 4)69'cKay 4)667'Brown 4)699'=uesta )66'$assive piles$an 4;;'$oulos 4)667'

p u 4 p i l e i n g r o u p '

umber of piles

p u 4 s

i n g l e p i l e '

@"@

group0"0

!ctive pilesd D .73

o or 03

o

%igure )0& =eduction of lateral soil pressure due to pile-group effectsindividual pile. These data are for pile spacing of .7-0 diameters, and include both active and

passive piles, though the trend is basically derived from active piles. %urther evidence for the pile-group effects on key parameters controlling the pile response in liquefying soils such as

U G2, β 2 and p1-max discussed herein is urgently needed.

SUMMARI6ED PROCEDURE FOR PSEUDO-STATIC ANALYSIS

%or analysis of the cyclic phase of the pile response, key requirement is to concurrentlyconsider the effects of ground displacements and inertial loads from the superstructure, and to

properly consider the characteristics of liquefaction and subsequent ground displacementsduring the cyclic phase of loading. Cyclic ground displacements can be evaluated either bymeans of an effective stress analysis or by estimating the ma"imum cyclic shear strain in theliquefied soil based on empirical correlation. The inertial load from the superstructure can besimply appro"imated as peak ground acceleration times supported vertical load by the pile.

The proposed practical procedure for preliminary assessment and design of piles subected tolateral spreading can be summari2ed in the following steps&

). ! simplified three-layer model is developed for the soil deposit, where the liquefiablelayer is sandwiched between a non-liquefiable crust layer at the ground surface and anon-liquefiable base layer. The water table may be used in defining the thickness of thesurface layer. $roperties of the base layer within @-: pile diameters below the interface

with the liquefied layer generally control the p-δ relationship of the base layer. ! single

pile with a nonlinear moment-curvature relationship is adopted in this model.

. The magnitude of lateral spreading displacement can be estimated using empiricalcorrelations for ground surface displacements of lateral spreads. 5n view of the

uncertainties involved in the assessment of these displacements, a range of values needsto be considered. 5t is practical to assume a cosine distribution for the grounddisplacement within the liquefied soil and that the surface layer will move together withthe top of the liquefied soil.

0. 5nitial stiffness in all p-δ relationships can be defined based on empirical correlations

between the subgrade reaction coefficient and S$T blow count or elastic moduli. Thisstiffness should then be degraded in order to account for the effects of nonlinearity andlarge relative displacements that are required to fully mobili2e the lateral soil pressure.

@. Stiffness degradation of liquefied soils is generally in the range between )J7; and )J);for cyclic liquefaction and )J);;; to )J7; for lateral spreading.

7. +ltimate lateral pressure from the crust layer can be appro"imated as being @.7 times the

=ankine passive pressure. The undrained or steady state strength can be used, on theother hand, for the ultimate lateral pressure from the liquefied soil.



:. ! static analysis in which the pile is subected to ground displacements defined in step, and adopted stiffness degradation in step @, is performed and pile displacements and

bending moments are obtained. The analysis should be repeated while parametricallyvarying the magnitude of applied ground displacement and stiffness degradation in theliquefied soil.

. $ile group effects should be eventually considered including cross interaction among the piles within the group through the pile-cap-pile system, and effects on key parameterscontrolling the pile response such as the stiffness of the liquefied soils, and themagnitude and spatial distribution of spreading displacements. The latter effects may

potentially reduce the severity of the ground movement influence.

CONCLUSIONS

*ateral ground displacements of liquefied soils can be quite large during the intense shaking orcyclic phase of loading and especially during the post-liquefaction lateral spreading phase.Since the properties of liquefied soils and loads on piles can be remarkably different during thecyclic phase and subsequent spreading phase, it is necessary to separately consider these two

phases in the simplified analysis of piles. #hen evaluating the pile response during the cyclic phase it is important to consider a relevant combination of kinematic loads due to cyclic grounddisplacements and inertial loads from the superstructure. 5n the case of lateral spreading, theuncertainties associated with the spreading of liquefied soils, and in particular, the magnitudeand the spatial distribution of spreading displacements, and stiffness degradation of liquefiedsoils need to be carefully considered. The lateral load from a non-liquefiable crust layer at theground surface may often be the critical load for the integrity of piles subected to lateralspreading, and therefore, special attention needs to be given to the modelling of the surfacelayer and its effects on the pile response. Cross-interaction effects may be significant for pilefoundations near the waterfront area, where individual piles within the group are subected tovariable ground displacements. >ffects of group interaction on key parameters controlling the

pile response need to be considered in perhaps reducing the severity of the ground movement

effects. Based on these premises, a practical procedure for preliminary assessment and design of piles subected to lateral spreading has been proposed.

REFERENCES

Broms B. 4)6:@'. *ateral resistance of piles in cohesionless soils. J Soil Mech and ound !n"n"# $S%! ( 6;4S0'&)0-)7:.

Cubrinovski, . and 5shihara, 8. 4;;@'. Simplified method for analysis of piles undergoinglateral spreading in liquefied soils, Soil& and ounda'ion&, Kol. @@, o. 7& ))6-)00.

Cubrinovski, ., and 5shihara, 8. 4;;7'. !ssessment of pile group response to lateralspreading by single pile analysis. $S%! Geo'echnical &pecial pulica'ion# 1*& @-7@.

Cubrinovski, ., 8okusho, T. and 5shihara, 8. 4;;:'. 5nterpretation from large-scale shaketable tests on piles undergoing lateral spreading in liquefied soils, Soil D+namic& and

!a,'huake !n"inee,in" , Kol. :& 7-9:.

%eng F-S, <o F-C, Chen T-/. 4;;' $assive earth pressure with critical state concept. JGeo'ech Geoen. !n"n"# $S%! ( )949'& :7)-:76.

<amada, ., #akamatsu, 8., Shimamura, 8. and ire, T. 4;;)'. ! study on the evaluation ofhori2ontal displacement of liquefied ground, /,oc0 2'h JS%! !a,'h0 !n","0 S+mp0& :@6-:7 4in /apanese'.

5shihara, 8, Foshida, 8. and 8ato, . 4)66'. Characteristics of lateral spreading in liquefieddeposits during the )667 <anshin-!wai earthquake, Jou,nal of !a,'huake !n"inee,in" ,Kol. ), o. )& 0-77.



5shihara, 8. and Cubrinovski, . 4)669'. $erformance of large-diameter piles subected tolateral spreading of liquefied soils, e+no'e ec'u,e# /,oc0 13'h Sou'hea&' $&ianGeo'echnical %onf0, Taipei& )-)@.

5shihara, 8. and Cubrinovski, . 4;;@'. Case studies of pile foundations undergoing lateral

spreading in liquefied deposits, State of the art paper, if'h 4n'0 %onf0 on %a&e Hi&'o,ie& inGeo'echnical !n"inee,in" , ew Fork, C3-=E.

/apanese 1eotechnical Society 4)669'. Special 5ssue on 1eotechnical !spects of the /anuary ))667 <yogoken-ambu >arthquake # Soil& and ounda'ion&, September )669.

*iyanapathirana, 3.S. and $oulos, <.1. 4;;7'. $seudostatic approach for seismic analysis of piles in liquefying soil. !SC> Jou,nal of Geo'echnical and Geoen.i,onmen'al !n"inee,in" , Kol. )0), o. )& )@9;-)@9.

eyerhof 11, athur S8, Kalsangkar !/. 4)69)'. *ateral resistance and deflection of rigidwalls and piles in layered soils. %an Geo'ech J ( )9&)76-);.

$rasad FKS, Chari T=. 4)666'. *ateral capacity of model rigid piles in cohesionless soils.

Soil& and ounda'ion&, 064'& )-6.$oulos <1, Chen *T, <ull TS. 4)667'. odel tests on single piles subected to lateral soil

movement. Soil& and ounda'ion&( 074@'& 97-6.

=ollins 8, Sparks !. 4;;'. *ateral resistance of full-scale pile cap with gravel backfill. JGeo'ech Geoen. !n"n"# $S%! ( )946'& ))-0.

Seed, =.B. and <arder, *.%. 4)66)'. S$T-based analysis of cyclic pore pressure generation andundrained residual strength, H0 5ol'on Seed Memo,ial S+mpo&ium /,oc0, Kol. & 07)-0:.

Tamura, S. and Tokimatsu, 8. 4;;7'. Seismic earth pressure acting on embedded footing basedon large-scale shaking table tests, $S%! Geo'echnical Special /ulica'ion 1*& 90-6:.

Tokimatsu, 8. and !saka, F. 4)669'. >ffects of liquefaction-induced ground displacements on

pile performance in the )667 <yogoken-ambu earthquake, Special 4&&ue of Soil& and ounda'ion&, September )669& ):0-).

Foud, T.*., <ansen, .C. and Bartlett, %.S. 4;;'. =evised multilinear regression equationsfor prediction of lateral spread displacement, $S%! J0 of Geo'ech0 and Geoen.i,onmen'al

!n","0, Kol. )9, o. )& );;-);).

pseudo static analysis of piles subjected to late

Documents