The Stability of Stage-constructed Embankments on Soft Clays

8/20/2019 The Stability of Stage-constructed Embankments on Soft Clays

1/23

The stability of stage constructedembankments on soft clays

F

TAVENAS

Depu r tmet~ t C ivi l Engineer ing , Ln\wl U t~ iw ui ry , uebec , P . Q. , Ca tlu tlo GI K 7P4

A N D

R .

BLANCHET

N D

R . GARNEAU

Terrurech L tke. ,

Montreal

P.Q . , Cunukr H4N IJI

A N D

S . LEROUEIL

Deprrrttnent ( Civil Engit~e ering. u\ d U r~i~v,:c . i ty ,~ ~ e b e c ,.Q . , C (~r~c lclnI K 7 P 4

Received June

6 ,

1977

Accepted December 14,

1977

The stage construction of seven high embankments on soft clays on the north shore of the St.

Lawrence Valley has provided an excellent opportunity to check the applicability of the concepts

of limit and critical stat es to the analysis of the behaviourof clay foundations.

The construction pore pressures have confirmed the development of

a

significant consolidation

in the initial period of construction.

The consequences of this consolidation on the behaviour of the clay, in particular in terms of

available strength, and on the method of stability analysis have been found entirely consistent

with the general model proposed by Leroueil, Tavenas, M ie ~~ ss en snd Peignaud.

It is suggested that the so-called

0

analysis based on the vane shear strength corrected

according to Bjerlum gives the minimum factor of safety and may be too conservative during

stage construction. Effective stress analyses are shown to be more representative of the true

stability conditions, and they have been successfully used to accelerate the construction of the

embankments.

La construction par etapes de sept grands remblais sur des argiles molles de la rive nord du

f le~~veaint-Laurent a fourni une excellente occasion cle verifier I'applicabilite des concep ts

d'i ta t limite et d'ktat critique I'analyse du comportement des fondations argileuses.

Les pressions interstitielles observees durant la construction ont confirme I'existence d'une

consolidation importante au debut de la construction. Les consCquences de cette consolidation

sur le comportement de I'argile, en particulier en ce qui concerne la resistance au cisaillement

disponible, et sur les methodes d'analyse de la stabilite ont ete trouvees entierement en accord

avec le rnodele general propose par Leroueil, Tavenas, Mieussens et Peignaud.

On suggere que la methode dite c

0

basee sur la resistance au cisaillement mesuree au

scissomktre et corligee selon Bjerrum donne la valeur minimum du facteurde securite et peut re

trop pessimiste pour une construction par etapes. I1 est montre que les analyses en contraintes

effectives sont plus representatives des conditions rCelles de stabilite, et qu'elles ont ete u tilisie s

avec succes pour accC1Crer la construc tion d es remblais.

Can Geotech J .

15,283-305 1978)

Introduction

T he evaluation of th e stability of e mba nkme nts

on soft clay deposits has always been a problem of

major interest to geotechnical

engineers

and re-

searchers. In recent years, much attention has been

paid to the evaluation of the stability of fills built

in a single construction phase. The use of a total

stress, or

0

method of analysis connectcd

with undrained shear strength values obtained from

in s i t ~ l ane tests has been widely accepted. How-

ever, based on the analysis of a number of ac-

cidental failures as well as on an impressive series

oE test fills built u p to failu re, th e shortco ming s of

this method have been put in cvidence particu-

larly by Bjerrum 19 72 ). As a result of the

continuing research effort on this problem, thc

necessity of empirical co rrecti ons of the results of

0

analysis, as proposed by Bjerrum, was at

first confirmed Pilo t 19 72 ; Dasca l et al. 1 9 7 2 ) ;

however, thc theoretical justification of such cor-

rections was questioned, and othcr approaches to

tbe

0

analysis were then proposed L a

Rochelle

et

al 19 74 ). Mo re recently, Schmertmann

1975) has even questioned the use of total


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284 C A N

G E O T E C H .

J . VOL.

15

1978

stress approaches to the analysis of stability prob-

lems in clays and he suggested that more con-

sideration be given to methods based on effective

stresses in spite of the practical difficulties noted

by Ladd (1975). Surprisingly, little attention has

been given to effective stress analyses in the

evaluation of accidental or test failures, so that the

relevance of this method has yet to be firmly

established. In spite of these p roblems, th e evalua-

tion of the stability of embankments built in a

single phase can presently be carried out with a fair

degree of reliability and economic designs are

possible.

The situation is quite different when stage con-

struction is necessary for high embankments on

soft clays. The method already discussed is ap-

plicable to determine the geometry and stability

conditions of the embankment at the end of the

first construction phase. However, for the design

of the further stages, two major difficulties arise in

the determination of the increased strength pro-

duced in the clay foundation during its consolida-

tion.

In preliminary design, the rate of increase of the

vertical effective stress P,. has to be evaluated

from the consolidation theory. Unfortunately, this

is done only with great uncertainties, owing to the

difficulties in measuring properly the coefficient of

consolidation C, of the clay and in defining the

true boundary conditions. Thus, in situ pore pres-

sure measurements appear as a necessary control

of the design assumptions during construction.

Th e gain in undrained strength C,, produced by

the consolidation under an increased effective ver-

tical stress P, may be evaluated by two equally

accepted methods. Having recognized that the

relationship between C,, and the consolidation pres-

sure is unique for a given material, one can deter-

mine the value of th e ratio C,,/P , for the site und er

consideration. This ratio, obtained at various

depths from

in situ

vane tests and from oedometer

test results, generally varies between 0.2 and 0.35.

Its local value may be used in conjunction with P,.

to obtain the new strength of the clay at any stage

of construction and consolidation. Another ap-

proach is by means of the so-called measured

in consolidated undrained triaxial tests. Knowing

the initial cohesion Cllo nd the corresponding con-

solidation pressure P C, the cohesion C,, produced

by a consolidatio n to P, is obta ined from

C,, C,,,,

(P,.

P C ) a n

c,,

,T h e new undrained strength profile , obtained in

this way at the end of e ach consolidation phase, is

used in a

4

0 analysis to determine the geomet

and stability condition for the next stage of con

struction of the embankment.

In general, this practice has been satisfacto

inasmuch as instability problems have seldom o

curred. However,

in situ

measurements have show

that undrained strength increases were not sy

tematic, and the possibility of a temporary d

crease of C,, during consolidation has even bee

suggested by Lefebvre

et 01

(1974) on the bas

of laborato ry tests. In th e framework of th e pre

ent methods of stability analysis such a decrea

in the undrained shear strength of the clay shou

result in a corresponding decrease in the safe

of the embankment so that delayed failure cou

be possible. Yet, no clear case of 'long term' failu

has been reported in the literature. This appare

discrepancy might be associated with a lack o

understanding of the true behaviour of the cla

and points out the necessity of

a

careful analys

of the phenomena that develop during constructio

in a clay foundation.

Schem atic Behaviou r of a Clay Foundation

Th e concepts of limit and critical states propose

by Roscoc et al. (1958) a nd the YL IGH T (Yie

Locus Influenced by G eological History and Tim e

model developed from these concepts by Taven

and Leroueil (1977) have proved helpful in th

analysis of the behaviour of clay foundations.

Natural clays occur in a more or less overcon

solidated state

itz situ.

At a given depth, the m

chanical behav iour of the clay might be complete

characterized by its preconsolidation pressure P

its void ratio e its compression index C and i

recomp ression index C,, a nd by its effective fri

tion angle 4' in the normally consolidated stat

In addition to being representative of the minimu

effective strength available in the clay, 4' has al

governed the stress conditions during the depos

tion and consolidation of the deposit since, as

well known, K O 0.9 (1 s in 4 ' ) . The void ra t

e

and the preconsolidation pressure P,

a re t

results of the combined effects of stress histor

aging, thixotropic hardening and possibly ceme

tation, the indices C, and C,, being also a resu

of the same phenomena. Roscoe

et

al. ( 1 9 5 8

have shown that the stress boundary between t

overconsolidated and normally consolidated stat

in a clay at a given void ratio, represented by P,

oedom eter test conditions, is a unique surface in th

stress space, called the limit state surface. The

also showed that the strength of this clay is uniqu


3/23

T A V E N A S ET A L . 285

FIG. 1. Limit s ta te sur faces in in tac t and no rmal l y con -

sol idated natural c lays .

for given values of e and + , thus defining the so-

called critical state.

Tavenas and Leroueil (1977) have shown that

these concepts could be applied to natural clays

provided that the phenomena occurring during

deposition and consolidation of these clays were

properly accounted for. In particular they showed

that the

KO

stress condition that prevails during

deposition a nd th e com bined effects of aging, thixo-

tropic hardening and cementation result in a limit

s tate surface with a shape s imilar to Yo (Fig. la) .

The critical state for such clays with a strain soft-

ening behaviour is within the limit state at a point

such as Co, on the 'large strain' effective strength

envelope of the clay.

Th e initial s tudy by Tavenas and L eroueil (1977)

had indicated th at the limit state surface Y o was

time dependent. In addition, a recent investigation

by Brucy (19 77 ) showed that the shape of Y o is

partly due to structural effects in the clay: when

consolidating the intact clay just beyond P these

effects are destroyed in the now normally con-

solidated clay; the new limit state surface assumes

a shape such as Y,, (Fig. l b ) , i .e. coinciding in i ts

upper portion with the effective strength envelope

at large strains; on the other hand, the critical state

Co

remains practically unchanged owing to the

minimal variation in void ratio during this process.

Leroueil

et

al. (1978) have used these concepts

to interpret the variations of p ore pressures ob-

served during embankment constructions in clay

foundations. Based on the analysis of 30 case his-

tories, they established that a significant consolida-

tion develops during the early stages of construc-

tion of an em bankm ent, owing to the initial rigidity

and the associated high C in the overconsolidated

clay. As a result of this consolidation the effective

stress path under the center line of the embank-

ment is such as OP (F ig . I b ) . At P, the clay be-

comes normally consolidated; its rigidity and thus

its coefficient of consolidation C, are considerably

reduced so that further loading occurs in undrained

condition. The limit state theory implies that, dur-

ing undrained loading, pore pressures are gen-

erated at such a rate that the effective stress con-

dition remains on the limit state surface of the

clay. In the present case this surface is the new Y,,

corresponding to the normally consolidated state

of the clay and the effective stresses will thus fol-

low Y,, between

P

and o where complete failure

of the fou ndation will occur. Leroueil

et

al. (1978 )

have shown that the variations of the observed

pore pressure with the applied embankment loads

were entirely consistent with these principles for

all considered case histories. As a result they have

suggested some consequences of these effective

stress variations during construction on the meth-

ods of stability analysis and on the strength to use

in these methods to arrive at a proper design of

cmbankments .

Th e design and construction of the Berthier-

ville-Yamachiche section of the A.4 0 Highway on

the north shore of the St. Lawrence River (F ig.

2

provided a n interesting case for the application and

verification of these principles and suggestions.

Seven overpass structures with 8.5-9 m high access

embankments had to be built in stages on soft clay

deposits similar to those encountered under the

Berthierville embankments described by Samson

and Garneau (1973). Four of the seven overpass

structures, identified as B,

C,

D

and

E

on Fig.

2

are considered here and it is the purpose of this

paper to discuss the behaviour of the foundation

clays during the stage construction in terms of the

generation of pore pressures, the variations in un-

drained shear strength and the stability conditions.

As a result, the validity of the concepts developed

by Tavenas and Leroueil (1977) and Leroueil

e al. (1978) will be demonstrated, enabling the


4/23

286 C A N . G E O T E C H . J .

VOL..

IS, 1978

FIG . Location of investigated sites

formulation of a general procedure for the eval-

uation of the stability of stage constructed embank-

mcnts.

escription of the Sites

Between Bcrthierville and Yamachiche (Fig. 2 ) ,

the new section of the A.40 Highway runs through

lowlands along the north shore of t he St. Lawren ce

River and Lake St. Peter. This region is char-

acterized from a geological point of view by very

thick deposits of soft clays formcd in the Cham-

plain Sea, during the period extending between

12 00 0 years and 8500 years before present. Fol-

lowing this period, the region was first covered

by the post-glacial lake Lampsilis (Prest 1970),

and later progressively assumed its present deltaic

configuration. During these stages stratified de-

posits of clay silt and sand were formcd in soft or

brackish water.

The geotechnical investigation at the sites of

the various structures between Berthierville an

Yamachiche has confirmed the existence of th

two types of deposits under the ground surfac

which is nearly horizontal at elevation 6 m . A

all sites, the deep marine Champlain clay extend

over a thickness of 53-73 m above the bedrock o

a thin layer of bottom till, which are encountere

at elevations varying from -60 to -80 m. Th

upper limit of the marine clay is remarkably ho

izontal at elevation

-6

m. This deposit is ver

uniform in its properties: the natural water con

tent varies arou nd an average of 6 0 , the liqu

limit is of the ord er of 75 and the plasticity inde

is abo ut 50 over the full extent of the depos

at all sites. The undrained shear strength measure

by field vanc tests increases linearly with dep

with average values of 40 kP a at elevation -10

and 80 kPa at elevation -30 m ; the sensitivity a

mea sured by field van c is of th e ord er of 10-15

Consolidation tests have shown that this clay


5/23

sl ight ly overconsol idated, the measured values of

P,. Po varying between 0 and 70 kP a wi th an

average of 30 kPa. The C J P , , rat io is thus of the

order of 0.29.

Above this homogeneous marine clay formation,

the lacustrine and fluvioglacial deposi ts are some-

what variable from si te to s i te. In the western part

of the region, at Rang St-Georges and Rang du

Fleuve (F ig . 3 ) , the upper par t of the depos i t con-

sists of a thin layer of weathered silt , clay and sand

down to elevat ion +5.5 m, a very soft s i l ty clay

layer extending to an elevat ion varying from 2 to

0.5 m, and a stratified deposit of silty clay and

clayey silt with local seams or discontinuous layers

of silty sand. The soft silty clay between elevation

+5.5 m and 0.5 m has a water content of the

ordcr of 80%, a l iquid l imit of 70% and a plast ic-

i ty index of 5 0 % ; i ts undrained shear s t rength is

about cons tan t and equal to 12 kPa throughout the

layer. The strat i f ied deposi t encountcred between

elevat ion +2 and

-6

m at Rang S t -Georges and

between elevat ion 0.5 m an d -6 m a t R a n g d u

Fleuve is well identified in terms of its natural

water content, liquid limit and plasticity index, but

i ts undrained shear s t rength exhibi ts a cont inuous

profi le with the s t rengths of the over- and under-

lying clay formations. These surficial deposi ts ap-

pear pract ical ly normally consol idated with an

overconsol idat ion P,. Po of the order of 10 kPa.

In the cen t ra l part o f the reg ion , a t Ran g de l a

Concession (Fig. 4a) a 2 m thick layer of loose to

medium dense s i l ty sand is encountered at ground

surface. Und er this layer, at elevat ion 4 m, the

stratified silty clay and clayey silt deposit contains

very l i t t le sand and becomes more clayey between

elevat ions 0 and -6 m; i t presents a water content

and a l iquid l imit of the order of 50%, a plasticity

index averag ing 25% and an undrained shear

strength with a minimum value of 2 0 kP a a t eleva-

t ion 0, increasing with depth under this level . Fi-

nal ly, at the eastern end of the region, at Rang du

Bri lC (Fig. 4b), the s t rat igraphy is s imilar to that

encountcred in the west but the s i lty clay layer be-

tween elevat ion +5 a nd 0.5 m is less plast ic, and

i t has a smaller water content of 60 % and a higher

undrained sh ear s t rength of the order of 2 0 kP a a t

elevat ion +3 m. Thes e deposi ts have probably been

submit ted to a preconsol idat ion somewhat greater

than in the western area, P,.

P

being of the or dcr

of 20-30 kPa. The C , , / P , ratio in these surficial

deposi ts will b e discussed later.

T A V E N A S T

A L .

287

Design and onstruction of the Fills

The lowlands on which Highway A.40 i s bu i l t

are frequently flooded in the spring a nd i t was nec-

essary to establish the finished grade of the high-

way 2 m above the original ground surface. As a

resul t , al l overpass s t ructures had to be raised and

access emb ankm ents with heights of 8.5-9 m were

necessary. Considering the low undrained shear

strength of the foundat ion clays, such high em-

bankments could not be bu i lt in one sequence and

the s tage cons t ruct ion method had to be used .

Design Method

Based on the usual assumption of undrained be-

haviour of the clay foundat ion during each short

period of fill placement, the stability of embank-

ments during construct ion is commonly evaluated

by means of a 0 method of analysis . Fo r the

initial construction, the maximum height of fill

compatible with an acceptable factor of safety,

taken equal to 1.3 here, was determined by such

a

0 method using the undrained shear s t rength

of the clay measured b y

it2 situ

vane tests. Th e com-

puted factors of safety were corrected as proposed

by Bjerrum (1972) to account for the d i f ferences

in the shear s t rength, measured by vane test and

actual ly avai lable under embankments .

For the analysis of the stability during subse-

quen t construct ion stages, the use of the 0

method was cont inued. The new increased un-

drained shear s t rength produced by the part ial

consol idat ion of t he clay foundat ion was est imated

on the basis of local values of the C , , / P , rat io. In

the present case, a n av erage value of C,, /P , . 0 . 2 7

was considered representative of the different clay

deposi ts a nd was used for al l designs.

In ord er to evaluate the undrained shear s t rength

avai lable during each construct ion stage, the ver-

tical effective stress

P,

had to be know n. Al though

an initial estimate of the variations in P, , C,, and

the resul t ing permissible construct ion schedule was

mad e on the basis of a one-dimensional consol ida-

t ion analysis , i t was considered necessary to check

the actual progress of the consol idat ion by means

of direct pore pressure observat ions. A minimum

of 3 or

4

pore pressure cells were installed in all

foundat ions as shown in F igs. 3a and 4 b . Fur ther-

more , for the embankm ents located in the wes tern

area of the region, multiple construction stages

could be an t ic ipated and i t appeared des i rab le to

check the actual magnitude of the increase in un-

dra ined shear s t rength under these e mbankme nts in

an effort to opt imize the construct ion; in situ v an e

tests were thus carried out under the fi l ls during

construct ion.

Geometry and Constr~~ct iotzf t he E m bank m e nt s

T he requirements of the Ministry of Trans port

of Quebec cal led for the beginning of c onstruct ion


6/23

CAN GEOTECH J. V O L . 15, 1978

Gloetzl piezometer

LP plast ic l imit

( lo)

LLL

L

l iqu id l imit ( Io)

2 0 4 0 6 0 8 0 0 2 0 4 0 6 0 8 0

na tur ol water c ontent ( Io)

W.LL.LP,(~/O) C,.kPa

-RANG ST-GEORGES

b-R AN G DU FLEUVE

FIG Geon ~et ry oundat ion condi t ions and ins trunlenta t ion ang St -Georges and Rang du Fleuve .


7/23

T A V E N A S

ET A 1

- ror ia i no l around sur face

Lwater table

W P Z - 1 5 3

v

W P Z - 1 5 2

silt and cloy,soft,sensitive

@

marine si l ty c lay,sof t to f ir rqsensit ive

1 I I \ l

Legend

Hydraulic piezometer

Gloetzl piezometer

Geonor piezometer

P L L W L P =plastic limit (O/O)

2 0

40

60

8 0

0

2 0

40

6

8

LL

=l iqu id l im i t

O/O)

W notural woter content ( )

0 RANG DE L A CONCESSION.

15

10

orioinol

5

0

- 5

E-

10

..

I5

-20

25

Gloetzl piezometer

30

L P plas t ic l imit (O/O)

35

W,LL,LP,(O/O) Cu,

kPa

b-RANG

DU BRULE

iJopsoil

@ silty clay and sond

@

si l ty c loy ,sof t ,sensit ive

@ stratif ied silty cloy and clayey

silt ,soft,sensitive

@

morine si l ty c la y,sof t to f irm sc

FIG . Geometry foundation conditions and instrumentation ang de la Concession and Rang du Briil6.


8/23

C A N .

G E O T E C H . J. VOL. 15 1978

2

4

7

9

1

h lght

of i l l

m

FIG

Variation of the factor of safety with the height of fil l p laced in the first construction stage.

of the most critical fills at Rang St-Georges and

Rang du Fleuve in 19 72 , in view of a comp letion

around 1977. Othcr embankments were to b e built

later, starting between 1 97 3 and 19 75 .

The stability analysis had first indicated the

necessity of providing all embankments with lateral

and frontal berms. Th e height and width of such

berms had initially been selected at 2.4 and 15 m,

respectively, for all embankments. However, the

height was later increased to 3.0 at Rang du BrQlC

and Rang de la Concession to take advantage of

Ihc higher strcngth of the foundation at these loca-

tions.

The stability analysis of the initial construction

sequence was carried out for various heights of fill,

with the actual berm geometry and with a unique

slope inclination of 2 : l . The modified Bishop

method was used for all computations. The

strength parameters were taken equal to 35 for

the friction angle of the compacted fill material, to

30 for thc loose silty sand forming the upper

foundation layer at Rang de la Concession and to

the vane shear strength profile obtained by n

s t ~

tests at each site prior to construction, and were

corrected according to Bjerrum 19 72 ).

Th e variations of the comp uted facto rs of safety

with the height of fill are shown on F ig. 5. T he low

initial shear strength at Rang St-Georges and Rang

du Fleuve resu lts in low factors of safcty, which

become rapidly less than the permissible minimum

value of 1 .3. O n the other hand the higher strengt

at Ran g du BrQlC and the uppcr silty sand layer a

Rang de la Concession have an obvious beneficia

effect, the co mp uted fact ors of safety being nearl

sufficient to consider a construction in one stag

As can be secn on Fig. 5 the maximum heights a

the end of the first construction stage, compatib

with a factor of safety of 1.3 after correction ac

cording to Bjerrum (1972) , a re 4 .7 m a t Ran

St-Georgcs, 3.8 m at Rang du Fleuve, 7.1 m a

Rang de la Concession and 8.2 m at Rang d

Brfild. However, the actual construction schedul

was affected by non-technical considerations s

that the heights of fill actually placed during th

initial and subsequent construction stages hav

varied, as shown on Fig.

6. As of Novembcr 197

the em bankm ent at R ang d u BrQlC was practicall

completed and within 0.5 m of its final height

Rang de la Concession; on the other hand, owin

to the accumulated settlements about 2 m of fi

remained to be placed to bring the embankmen

to their final grade at Rang St-Georges and Ran

du Fleuvc.

onstruction Pore Pressures

The systematic control of pore pressures durin

construction was considered an essential part o

the design of the embankments. The foundation

of all fills were thus instrum ented b y mcan s of 3 o

4 Gloetzl-type pneumatic pore pressure cells fo


9/23

T A V E N A S T A L

time, days

m oy 72 nov 72 m 73 n 73 m 74 n 74 m 75 n 75 m ay 76 nov 76

time, years

FIG.

.

Schedu le o f const ruct ion o f the four embankments .

lowing patterns such as those presented in Figs. 3a

and 4b. However, in order to develop a clear

understand ing of the pore pressure distribution, the

foundations at Rang du Fleuve and Rang de la

Concession were equipped with complementary

instru men tation consisting respectively of 1 2 and

14 Gloetzl (pneu matic) and Geon or (vibrating

wire) pore pressure cells, as shown on Figs. 3 b and

4a. Systematic pore pressure observations before,

during and after each construction phase have

allowed the analysis of the setup of construction

pore pressures.

Pore Pressures under the Center Lines of the Fills

Figure 7 shows the variations as a function of

th e em bankm en t lo ad A ~ Hf the excess pore pres-

sure A U at various depths under the center line of

the fills at Rang du Fleuve and Rang de la Con-

cession.

T h e variations of Au ar e in agreement with the

general model of pore pressure generation pro-

posed by Leroueil et al. (1978), the principle of

which is presented in Fig. 8. At the beginning of

construction, the clay behaves as an overconsol-

idated material with

a

high C, and a significant

consolidation develops to produce values of the

pore pressur e coefficients r,, o r of the order of

0.4-0.6. A s a result the effective vertical stress P,

increases and eventually becomes equ al to P, when

AU,

ACT^,,,.^, P,

Po1

At this point the clay

becomes normally consolidated and any further

construction at constant effective vertical stress has

a resulting value of r or equal to 1 O. A s indi-

cated on Fig. 8, the horizontal distance between th e

actual hu

f

( ay H ) curve and the Au AyH line,

which is equal to the effective vertical strcss,

s ho ul d, f o r l o ad s i n ex cess of A w ~ , ( , ~ ~ ,epresent the

overconsolidation pressure in the clay at the level

of thc pore pressure measurement. Analysing Fig.

7 on e finds a preconso lidation difference P, P,,

of the order of 10-15 kPa at 9 m depth at Rang

du Fleuve and of the order of 25-35 kPa at 9 m

depth at Rang de la Concession; these values are

in very close agreement with the results of o edom -

eter tests.

Th e model of pore pressure generation proposed

by Leroueil et al. (1978) implies that the rate of

pore pressure buildu p during the co nstruction stages

subsequent to a first consolidation period must

be such that r, , or is equal to 1.0 . Such a pore

pressure behaviour may be observed on Fig. 7b for

embankment loads up to 10 0 kPa ; it was also

observed fo r embankment loads up to 12 0 kPa

at Rang St-Georges and for all construction stages

at Ran g d u BrQlC and Ra ng dc la Concession thus

confirming the validity of the model of Leroucil

t

al. However, during the final loading stages at

Rang St-Georges and Rang du Fleuve (Fig. 7b),

the pore pressure increase was less and corre-

spon ded to r,, of the ord er of 0.5-0.7. Thes e re-

duced values of r, , may be associated with the

narrowing of the fill crest, and mainly with the


10/23

CAN G E O T E C H . J . VOL. 15 1978

piezometer depth

A PZ-8 9 . 2 m

A

PZ 102

9 . 2 m

Z - 9

3 . 2 m

a RANG DE

LA

CONCESSION

RANG DU FLEUVE

FIG.

.

Variation of the observed pore pressure with the applied embankment load.

change

in total vertical

stress

Am

FIG.

Principle of the pore pressure generation model

proposed by ~eioueil

l

d

1978) .

development of non-uniform stress distribution

in theffill when it is deformed by the settlements

of the foundation. This effect of the settlement of

the foundation on the stress distribution at the

base of the embankrncnt has been evidenced by

Parry 19 72 ). Indeed the f il ls where the reduced r

were observed had been in place for 3-4 years

and had settled 1.4-1.6 m when the phenome no

developed. Because of this arching the total stres

increase induced by construction would be reduce

under the center of the embankment but addition

stresses would be thrown onto the toes of th

fill. The pore pressures observed at different loca

tions Fig. 9) under the Ran g St-Georges cmbank

ment would tend to confirm the validity of th

assumption: although the r distribution is norm

for the load increments up to 6.5 m of fill, fo

the last construction stage, rcduced r develo

under the center line whereas increased r occu

under the slope and the berm of the embankrncn

This observation, which has an important influenc

on t he analy sis of stability in effective stresse

\vould need to be confirmed by more detaile

investigation of the pore pressure behaviour und

other embankments .

Distributior7s of Po re Pressur es ar7d r Fa cto rs

Under the embankments at Rang du Fleuv

and Rang de la Concession, the instrumentatio

for pore pressure measurements was sufficient

allow for the determination of the complete por

pressure regime. Figure 1 0a shows the lines of e qu

pore pressure isobars) under Rang de la Conce

sion on July 6, 197 6.

A

similar result was o btaine

at all stages of construction undcr the two wel

instrumented fills; by analogy and considering th


11/23

T A V E N A S E T

A L.

lo r

I

I

FIG. . Distribution of the

r.

coefficient a t various stages of construction of the Rang St-Georges fill.

10 and fill 2

v

yt 18.4kN m3

roriginol ground

s u r f a c a

v Gloetzl ptezometer

- 2 0

P

r Geonar p~ezomeler

3 sobar in metre of eoter

-1.0- iso-

r

AY

.FIG.

0.

Distribution of the pore pressures and

r.

coefficient under the Rang de la Concession f i l l

local stratigraphy and the limited number of p ore

pressure observations isobar networks were also

established a t various stag es of constru ction of all

fills. On the basis of such networks the stability of

the embankments could be evaluated by means of

effective stress analysis at any time.

In order to predict the stability of an embank-

ment befo re increasing its height it is necessary

to predict the pore pressure after construction.

Although the method proposed by Leroueil

t al.

1978)

applies below the center line of the em-

bankm ent the only reliable metho d applicable to

the entire foundation consists in determining the

distribution of the

r

au/a. H factors during

each construction phase and using this result for

predicting thc pore pressure setup during the next

construction stage. Figure lob presents the iso-i

observed under the Ran g d e la Concession fill . Th e

high r values under the berm far away from the

zone of load applica tion should be note d. A similar

pattern was observed under all emb ankm ents in-

dicating total stress variations different from those

predicted by the thcory of elasticity or

a

significant

effect of lateral spreading of the pore pressure by

consolidation-like phenom ena. I n view of the strong

influence of such pore pressure patterns on the

effectivc stress stability analysis this phenomenon

deserves further consideration in future.

It is beyond the scope of this paper to analyse

the dissipation of pore pressures between con-

struction periods. It should only be noted that pore

pressure dissipation did occur at all times and loca-

tions leading to increa ses in cffective stresses as

shown on Fig. 7b.


12/23

294 C A N . G E O T E C H

bserved Variations of the Vane Shear Strength

In order to check the validity of the design

,~ssum ptions oncerning the gain in undrained shear

strength resulting from the consolidation under the

embankm ent loading, it situ vane shear tests were

performed by means of a Nilcon vane under the

centcr line and the berms of the different embank-

ments. In addition, a reference vane profile was

determined in an unloadcd area near each s ite.

The periods at which these tests were carried out

at the four sites discussed in this paper are in-

dicated on Fig. 6.

At Rang St-Georges, vane tests were performed

in Jun e 1 973 , 40 0 days after the beginning of the

construction, whcn the fill was 6.0 m high, and

again in September 1975, 1130 days after the be-

ginning of construction, when the fill was 7.4 m

high. The average initial vane shear strength pro-

file and the shear strengths measured in June 1973

and September 1975 are shown on F ig . 11 . The

increase in strcngth is evidcnt in the up per 12 m

of the foundation. The shape of the C, , profiles

suggests that the stratified silt and sand layer

between elevation 0.5 m and 2 m had a draining

effect in the foundation. The effects of increased

times and embankment loads on the shear s trcngth

are obvious, confirming that thc development of

consolidation in the clay does result in an increased

shear strcngth. The magnitude of the increase in C, ,

could be compared to the change in vertical effec-

tive stress at elcvation 4.2 an d -3.0 m, whe re

pore pressures were measured. Before construction

C, , /P , were computed from the results of con-

solidation tests; after construction the vertical effec-

tive stress was obtained from the computed vertical

total stresses and the obscrved pore pressures. The

resulting

C, , /P ,

and

C,,/P,

are shown in Table 1.

At Rang du Fleuve, vane tests performed under

the 5.8 m high f il l and the 2.4 m high berm 830

days after the beginning of construction also evi-

denced the gain in undrained shear strength caused

by consolidation under the embankment loads, as

shown in Fig. 12: the vane shear strength profiles

assume an isochrone-like shape, influenced by the

draining effect of a stratified sandy layer between

elevations +0.5 and -1.5 m ; the s trength increases

are mo re significant und er the high load of the m ain

embankment than under the lower berm; no

strength increase is observed below elcvation -7.0

m where little settlement has developed. The varia-

tions in the C,,/P, ratio could be estimated at

elevation 3.5 m unde r the center line of the fill

and at elevation 3.7 m under thc berm, where

pore pressure observations were available. The

J.

V O L . 15,

1978

C . undr m e d s hwr S t r ena lh . k Pa

sand before and after selllement

rntttal vane shear strength

overage of

4 profiles

.

vane shear strenqlh under 6. 0

metres of f111 4CC days afte r

beqinnmg of construction

vane shear strength under 7 4

metres of ftl1 1130 days after

FIG 1

Varia t ions

of

the vane shear s trength under th

center line of the embankment at Rang St-Georges.

values of C, , /P , before construction and C, , /P

at the time of the vane shear tcsts are given

Table

1.

As shown in Table 1, in all cases the consolid

tion under the embankment load has produced

reduction in the C,,/P, ratio. In order to check

such reduction was systematic, the

C,,/P,

rati

were computed at all possible locations under t

seven fills. In each case, the in itialvane shear streng

and thc P,, measured in oedometer tests were use

to compute C,,/P,. before construction; the s imu

taneous observations of

C, ,

and the pore pressur

during construction were used to obtain C,,/P

corresponding to thp, prevailing embankment loa

The results are presented in Fig. 13. The initi

C,,/P, .decreases from a maximum of 0.4 at 1.5

depth to an average of 0.26 at 6.0

m

in the upp

plastic and probably slightly weathered clay; in t

intermediate low plastic silty clay ( I P 2 5 %

down to a depth of 13 m,

C, , /P, .

varies betwe

0.23 and 0.29; in thc lower plastic marine cl

C,,/P,. increases slightly with depth to an avera

of the order of 0.29 at 24 m depth. After co

struction, the com puted C,,/P,. are generally low


13/23

T A V E N A S ET

A L . 29

5

u undroined shear strenqth kPo

2

4

6

8 100

FIG.12. Variat ions of the vane shear s t reng th under the

cen ter l ine and the berm of the embankment a t Rang clu

Fleuve.

TABLE.

Variations of the C,,/P,. ratio

C, , /PVf

Location

C,,/Pc

2 3

R m g Saint-Georges

I +4 .2 m 0.35 0.26 0.26

El . -3 .0m

0.26 0.23 0.22

R m g i FIt~ioe

1 . +3 .5

m

0.34

0.25

Berm El. +3.7111

0.33

0.24

NOTES: 1. Obscrved 400 days after

s tart

of construction. 2 Ob-

served

113 days

after s tart

of construction. 3.

Observed 830

d y

af t e r start

of

construction.

than the initial

C,,/P,.

and they vary very little with

depth, between 0.21 and 0.26 around an average

of 0.24.

From thcse observations, it may be concluded

that the consolidation of the clay to vertical effec-

tive stresses in excess of the preconsolidation pres-

sure produces both an increase in the vane shear

strength and a decrease in the C,,/P,. ratio. From

Fig. 13, it appears that the reduction of C J P ,

is strong in plastic clays with a high initial

C , , /P , , but is limited in the low plastic clays; the

final C J P , stems to vary little with the plasticity

of the clay. Furthermore, from the observations

at Rang St-Georges, the reduction of

C , , / P ,

ap-

pears to occur rapidly during the initial stages of

construction and probably as soon as P, exceeds

P , ,

the ratio C,,/P, remaining constant during

i'urther consolidation.

An exact interpretation of the strength measured

by it

s tu

vane tests is difficult because the effec-

tivc stress conditions at failure are not known.

However, published evidence, for example by

Bjerrum (1972), and the common observation of

a strain softening bchaviour during the test sug-

gest that the vane strength is representative of the

intact clay structure. The corresponding effective

stress condition should thus be located on the

initial limit state surface, at a point that remains

to be defined. Th e shape of the limit state surface

being imposed by the geological history, thc C,,/P,.

ratio representative of this shape is also imposed

by the history of the clay deposit as noted by

Bjer rum (1972) .

It has been demonstrated by

F.

Brucy (1977)

that the limit state surface is strongly deform ed when

the clay becomes normally consolidated; as shown

in Fig. lb, this deformation results in a reduction

of the strength and thus of the C , , /P , ratio. The

pore pressure observations have confirmed that

the clay in the foundations of the emb ankme nts

become normally consolidated during the early

stages of construction. The observed variations

in

C,,/P, (Fig. 13) could thus be explained by the

above-described phenomenon

i

the limit state sur-

face of the intact clay presents a shape similar to

that of Fig. la.

The limit state surface of the intact clay has

been determined on undisturbed samplcs, obtained

at

4.5

m depth at Rang du Fleuve by means of a

20 cm diameter overcoring thin wall sampler

developcd at Lava1 University. The method used

is the same as that describcd by Tavenas and

Lcroueil 19 77 ) it consists of ( 1 ) a series of con-

solidated, undrained triaxial tests with pore pres-

sure measurements (CIU) to obtain the 'strength

portion' of the limit state surf ace as we l as the

peak strength envelope of the nornlally consol-

idated clay and the 'large strain' strength envelope,

(2 ) a series of v , / ~ T : , = C t triaxial consolidation

tests, as well as (3) oedometer tests to obtain

the 'volumetric portion' of the limit state surface

below the Mohr-Coulomb envelope. Figure 1 4

presents the results of these tests. They are typical

of natural clays in eastern Canada, the shape of


14/23

296

C A N . G E O T E C H .

J.

VOL. IS,

1978

FIG 13. Variation of the CJP,. at io under the embankments .

040

\ ?

before constructton

0

36

f t e r

c o n s t r u c t m i

p;

0 32

>

P

028

,p

0

24

c

0

20

P a V e

z 4 5 X - a P a V e

~ 2 5

IPaVe

=

50

0

I6

FIG 14

Effective stress path and limit state surf ace fro m triaxial tests at Rang

d u

Fleuve.

I

the limit statc surface being identical to that shown

in Fig. la. Since this surface is well above the

Mohr-Coulom b envelope at large strains, the ef-

fects of consolidation shown o n Fig. l b will also

apply to the limit state surface of this clay. Con-

sequently the observed variations in C,,and C,,/P,

can be related to the modification of the shape

of the limit state surface of the clay when it bc-

comes normally consolidated.

It might be interesting to note h ere that a similar

bellaviour has been observed under embankments

built on

a

variety

of clays. Analysing the test

embankments on

a

normally consolidated plastic

clay at Sk:k Edeby in Sweden, Holtz and B roms

1972) report a systematic reduction in the ratio

of

C, , /P , :

the initial values of C,,/P,, the Sk5

0

2

4 6

8 10

1A- r

ep t h , m2 14 16

18

20 22 24

Edeby clay is rcportedly no r~n ally consolidate

varied from 0.22 to 0.28 around an average

0.2 6; after 14 years of consolidation u nder t

embankment loads, the observed vane she

strength

C

and effective vertical stresses

P,

sulted in C,,/P, varying from 0.21 to 0.25 arou

an average of 0.22. However, as was the case

the A.40 embankments, this reduction in

C,,/

was associated with an increase in

C .

Under t

Saint-Alban test section B Tav enas

t

a l . 197

a

slight increase in C was also observed 2.5 yea

after construction in the upper

3

m of the found

tion, but the C /P initially equal to 0.26 w

reduced to

C,, /P,

of 0.21. On the other hand,

a

site near Ottawa, Can ada, Eden and Pooroosha

1968n,b) haw noted no var ia t ion in the va


15/23

T A V E N A S

T

A L . 297

shear strength 3.5 ycars af ter the construction of

an em bank men t: a t that si te the or[ginal values of

CJP,

in a clay of medium to low plasticity were

probably of the order of 0.28 (based on the scat-

tered

P,.

measurements values of

C,,/P,

f r om 0 .23

to 0 .33 ma y be c om pute d) ; ba se d on t he da t a

presented in the paper , thc

C,,/P,

af ter 3.5 ycars

vary betwcen 0.16 and 0.20. A similar reduction

in

C,,/P,

without significant vane strength increase

has been obta ined under a 12 year o ld embank ment

on the Eas te rn Township Highway southeas t of

Montreal: in the clay with a plasticity index of

4096, the ini t ia l

C,,/P,.

of 0.32 were reduced to

C,,/P,

of 0.24 but the vanc shear strcngth was

essentially unchanged.

Thus i t may be concluded that the consolidation

of a clay foundation under an cmbankment wil l

result in most cases in a modif icat ion of th e shap e

of the limit state surface of the clay and in a cor-

responding decrease of the

C,,/P,

rat io f rom init ial

values of

C,,/P,.

of the ord er of 0.25-0.40 an d

variable with the plasticity, the geological history

and other featurcs of the intact clay structure, to

values of the

C,,/P,

rat io varying much less with

the clay propert ics and general ly of the order of

0.1 8-0.25. This decrease occurs in the ear ly phases

of the consolidation process, probably as soon as

I- ,

exceeds

P,.

Together with these reductions in

C,,/P1,

he vane shear strengths havc becn o bserved

to be larger or in some cascs equal to the ini t ia l

strengths measured in the intact clay; however, it is

possible that a reduction in

C,,

develops momen-

tar i ly when the

C,,/P1

atio is reduced in the ear ly

phases of consolidation.

Evaluation of the E mbankm ent Stability

During onstruction

As already noted, the ini t ia l dcsign was based

on total strcss analyses, taking into accou nt the

increases of th e undrained shear strength produced

by the consolidation of the foundation clay. How-

ever , by the end of 197 5 it appeared necessary to

accelerate the construction process, and i t was

decided to check the stabil i ty of the cmbankment

by m eans of effective strcss analysis. T o this end

thc effective strength parameters of the clays were

determined in the laboratory and a complementary

instrumentat ion was instal led at Rang du Fleuve

(Fig . 3b) a nd Rang de la Concession (F ig . 4a) .

As a result , a t any given stage of construction the

stabil i ty of the embankrncnts could be determined

by means of

a

total stress method based on the

vane shear s t r ength measured under the embank-

ment and of an effect ive stress method using the

actual pore pressures.

Total Stress nalyses

Th c tota l stress analyses of stability hav e been

carr icd out by means of a computer program using

the modif ied Bishop meth od, an d accepting ver tical

undrained shear strcngth profiles of different shape

and magni tude depending on the i r loca t ion under

the embankments. In al l cases the strength of the

fill, represente d by a friction angle of 3 5 , w as

assumed to be ful ly mobil ized.

Th e var iat ions of th e factors of safcty with the

height of fill during th e first constru ction s tage have

a l ready been discussed. They were based on the

vane sh ear strength of th e intact clay and corrected

to account for Bjerrum's vane strength correction.

Durin g construction the new factors of safety were

est imated on the basis of thc new shear strength

measurements made under the f i l ls;

as discussed

later thc vane strength correction proposed by

Bjer rum (1972) was cons idered not appl icable

in these cases.

At Rang St -Georges , vane s t rength measure-

ments were made in Jun c 19 73 when the f il l he ight

was 6.1 m and in September 1975 when the f i l l

height was 7.4 m. The factor of safety could thus

be estimated for the diffcrent heights of fill using

the strength prevail ing before and af ter the con-

struction. Th e different factors of safety arc sh own

as a function of the cmb ankm ent height in Fig. 15.

The increase in the factor of safety caused by

the gain in vane shear strength is very important

be twcen the in i t ia l condit ion and Ju ne 197 3 but

i t is much less be tween Jun e 197 3 and September

1975 . This may b c explained not b y a var iable

magnitude of the gains in the vane shear strength,

but rather by the location of these gains in the

foundation. Ind eed, a t the beginning of construc-

t ion, addi t iona l loads a re imposed on the ent i r e

arca of the embankment and i ts berms. After con-

solidation und er these loads, increases in

P,

a nd

C,,

thus deve lop throughout the cnt i r e founda t ion; the

available shear strength is increased along near ly

the ful l extent of the potential fai lure surface. On

' the contrary, in subsequent construction stages only

the main embankment i s bui l t up, so tha t the in-

creases in vertical effective stress and thus in un-

drained shear strength arc l imited to thc central

area of the foundation, i .e . to a small port ion of th e

potential fai lure surface (see Fig. 1 6 ) . Conse-

quently, to take ful l advantage of the stage con-

struction process i t would be desirable to also

increase the height of the berms af ter thc f irst


16/23

298 C A N . G E O T E C H .

J.

VOL.

15.

978

o

+ O

a na l y r r , u xn~1~al;correcleaaccording 10 Bjerrum(19721

+

= onalyr8r.

Cu

umol unrorrecfed

I

+

=

ono l y l l ,

u A C u

meorurea

@

in

une

973

September 1975

FIG 15 Compu ted factors of safety ang St-Georges

fill.

period of consolidation so as to produce increased

strengths along the maximum possible length of

potential failure surface. As can be seen, the over-

all increase of the factor of safety prod uced by the

gain in van e shear strength over a period of 3

years is of th e ord er of 0 .6 at a fill height of 7.5 m ,

i.e. very significant.

As of December 197 6, the factors of safety cor-

responding to the prevailing geometries and vane

shear strengths are of th e order of 1.25 at Rang

St-Georges and Rang du Flewe, 1 4 at Rang de la

Concession and 1.55 at Rang du BrGlC. The small

factors at Rang du Fleuve and Rang St-Georges,

less tha n the originally selected minimu m of 1.3 ,

are considered acceptable in view of the observed

settlement and pore pressure behaviour as well as

of the results of the effective stress analysis.

Efe ct iv e S tress Analyses

Peak strengths corresponding to a Mohr-Cou-

lomb envelope defined by c = and

+

= 28

have been given by CIU tests on the normally

consolidated clay (Fig. 1 4 ). At large strains the

strengths measured in C IU tests on b oth normally

consolidated and overconsolidated samples cor-

respond to the same residual strength envelope,

which may be defined here by c = 4 kPa a nd

I

+ = O A N A L Y S I S

I N I T I A L

Cu

FIG 16 Potent ial fai lure surfaces from +

=

a n d

+ analyses ang St-Georges fill.

+ = 28 . To obtain a conservative estimate of th

factors of safety, the stability analyses were ma

using

c

= and +

=

28 .

Stability analyses in effective stresses require th

knowledge of th e pore pressures in the foun dation

As already noted, two types of instrumentation f

pore pressure observations were available. Fro

the observation in the well instrumented found

tion at Rang d u Fleuve and Ran g d e la Concessio

typical pore pressure distribution patterns we

defined (Fig. l o ) , which were then used to extr

polate the limited number of observations und

the other embankments. In this way complete po

pressure distributions could be defined under a

embankments at all stages of construction, base

on the corresponding observations.

The results of the stability analysis in effecti

stresses at various stages of con struction of th

four embankments are shown on Figs. 15 and 1

At R ang St-Georges (F ig. 1 5) the factors of safe

corresponding to the June 1973, September 19

and November 197 6 situations vary between 1

and 2.1 and are much higher than those obtaine

from the total stress analysis. The con dition is qui


17/23


18/23

C A N . GE OT E C H. J .

V O L.

15 978

l l

FIG.17b

Computed factors of safety ang de la Concession.

those obtained from the 0 analysis in spite of

the rapid construction sequence, the better drainage

conditions at Rang de la Concession resulting in

higher differences.

iscussion

The concepts of limit and critical state, which

have just been applied successfully to tlic analysis

of the variations of the pore pressures and the vane

shear strength under the A .40 embankments, might

servc as a basis for the development of a rational

approach to the design of stage-constructed ern-

bankm ents. T o this end it is first necessary to

establish a clcar picture of thc variations in effective

stresses and strength in the clay foundations.

Effective Stresses and Strength Variations under

a Stage constr~tctedEm bankm en t

Consider the case of an embankment built in

threc stages on a slightly overconsolidated clay.

Figure

18

shows the corresponding total and effec-

tive stress paths as well as the relevant strength

characteristics of the clay.

During the first construction phase, the tota1

stress path is such as OA. As shown by Leroueil

i

a / . 1978 ,

and as confirmed here, a significant

consolidation develops at the beginning of con-

struction, so that the effective stresses increase

rapidly along O P and the clay becomes normally

consolidated at PI. During a further increase in

total stresses up to point A, pore pressures are


19/23

T A V E N A S

T

AL

+ =

o n o l y r ~ r ,

,

lnlllol cnrecte d occordmg to Bp rrum 197 2 1

o n o l y r ~ r , u nlllol uncorrrcted

onolyris , C, bC u meorurea in Nowember 1976

C

+ Onolyur , 3 )numbor d doyr otter lorl loodfng

FIG.

17c Computed factors

of

safety ang du BrGlC.

generated so a s to maintain t he effective stress path

on the limit state surface along P A ; this norm ally

occurs at constant effective vertical stress g,. PCt

with a pore pressure coeficicnt

r 1 O

This type

of bchaviour was observed under all embankments

of the Berthierville-Yam achiche section of High-

way

A.40.

As shown by F. Brucy 19 77 ), the initial limit

state surface is deformed when the clay becomes

normally consolidated at P , and the new limit state

surface is such as P1F,, Ro (see also Fig. l b ) . Th e

strength of the intact clay, as obtained from UU or

CIU tests in the laboratory or from vane tests

in

situ

is thus reduced to values corresponding to the

large strain Mohr-Coulomb effective strength en-

velope. In this process, C,,will rcmain less than th

initial strength as long as the effect of consolidation

on th e effectivc stresses is small, an d C,,/P,. wil

be permanently reduced to a value representative

of the new shape of the limit state surface of th

normally consolidated clay.

If the first phase of construction werc contin ued

beyond the height corresponding to the stress con

ditions at points A a nd A , failure would be initi

atcd at Fo on the Mohr-Coulomb envelope of th

normally consolidated clay. Further load applica

tion associated with the strain-softening behaviou

of the clay would result in an effective stress path

FolR o following the new limit state surface of the

normally consolidated clay down to point

Rot

cor


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302 C A N . G E O T E C H .

J .

VOL.

15

1978

FIG.18. Tota l and e f fec t ive s t ress pa ths under an embankment bu i l t up in s tages .

responding to the residual strength and probably

the critical state of the clay;

C :

would be the

strength mobilized at complete failure. The magni-

tude of this strength at failure has been analysed

by Mesri (1 97 5) . Associating the correlations pro-

posed by Bjerrum (1972) between the plasticity

index and the ratios C,,/P,,' and P,./P,,' as well as

the empirical vane strength correction fa ctor, Mesri

has shown tha t the strength pC,, C,,, :,,, available

at failure under an embankment was nearly in-

dependent of the plasticity index and of such effects

as aging and could be related to P,. by pC,,/P,. equa l

to 0.18-0.23. These values are rema rkably similar

to the ratio C,,/P,' of 0.18-0.25 measured by vane

tests under embankments. It might thus be hypo-

thesized that the strength correction factor

p

pro-

posed by Bjerrum (1972) accounts essentially for

the modification of the limit state surface when the

clay becomes normally consolidated.

The actual construction sequence corresponds

to the interruption of construction at A, A' for a

period of consolidation, which results in an effec-

tive stress variation A'B'. During the next loading

stage BC, th e effective stresses will stay on th e new

limit state surfacc produced by consolidation under

the effective stress condition at B' and they will

follow B'C'. T he second consolidation period w ill

produce a stress path C'D' and a corresponding

new limit state surface D'Fs'Rsl as well as a new

critical s tate at Rsl. If the embankment were then

to be built up to failure from the s ituation D, D' ,

the stress paths followed up to the initiation of

failure would be such as DF, in total stresses and

D'F,' in effective stresses, with a corr espo ndin g

pore pressure factor r 1.0. Failure would

be initiated at F2'; the development of strain

softening under imposed total vertical stresses

would necessarily result in an increase of the mean

total s tresses along F 2R 2 and in

a

very marked

increase in pore pressures, with I. much in cxcess

of 1.0, to prod uce an effective stress path such as

F2'R2'. T he principles of limit and critical state

imply that the limit state surfaces A'Fo'Ro'and

DIF,'R,' are homothetic and entirely defined by

the respective values of the vertical effective stress

and the effective strength param eters of the clay in

the normally consolidated and residual states. As a

consequence the C,,/P,.' ratio will remain constant

and equal to the reduced ratio representative of the

unstructured, normally consolidated clay during all

stages subsequent to the first consolidation period.

Simultaneously the vane shear strength will in-

crease continuously from its momentarily reduced

value, obtained when the clay becomes normally

consolidated. Therefore, depending on the magni-

tude of the consolidation allowed before vane tests

are carried out under an embankment, it will be

possible to measure vane strengths smaller, equal

to o r greater than th e initial values of C ,,. Both the

abo ve dcscribed variatio ns of C,, an d C,,/P,' are

consistent with the field observations. It should be

noted though that the strength really available at

failure, and thus significant in design, will con-

tinuously increase with ongoing consolidation from

its initial value C,,(,:,,,.

Evnluatiorz of

the

M eth od s of Stability naly sis

The results of the stability analysis in total and

in effective stresses may be evaluated by consider-

ing the stress paths followed up to failure in the

foundation clay (Fig. 18).

At any given point in the foundations, failure

is initiated when the effective stress condition satis-

fies the Mohr-Coulomb criterion of the normally


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T A V E N A S ET A L

consolidated clay. Subsequent strain sof tening de-

velops unti l the residual shear strength

C O:

is

reached, the strength reduction from F to R being

associated with a strong increase in the pore pres-

sures. Two factors of safety may thu s be def ined at

any stage of con struction pr ior to complete fai lure:

1. The effective stress analysis using the large

strains effect ive strength parameters, or the con-

servative c 0 , + values for the sake of safety,

and the actual pore pressures prevail ing in the

foundation wil l produce a tru e image of the stabil-

ity condition.

2. Th e minimum strength available in the foun-

dation being

C :

stabil i ty analysis based on

this resistance will yield the minimum possible

value of the factor of safety. This residual strength,

probably corresponding to the cr i t ical state of the

clay, is the extremity of an effective stress path.

However , i t appears to b e a unique function of the

effective stress acting on the clay prior to the be-

ginning of the construction up to failure, i.e. of P,.

in a single stage construction or of

P,

in multiple

stage construction. This effect ive strength thus has

all the usual at tr ibutes of an undrained shear

s t rength and m ay be used as such in a + 0 anal-

ysis. This residual strength may be est imated from

vane test results in the intact clay corrected ac-

cording to Bje r rum 19 72 ) f rom uncor rec ted vane

test results in the normally conso lidated clay unde r

the embankment dur ing const ruc t ion, f rom the

correlat ion to P,. o r P, proposed by Mesr i 19 75 )

o r f r o m CIU tests on the overconsolidated clay

carr ied out to large strains.

The strength available in the foundation being

higher than the residual strength at any t ime but

at complete fai lure, the factor of safety computed

from th e effective stress analysis will be necessarily

higher than tha t obta ined f rom the

+

0 type

analysis using the residual shear strength; this is

consistent with the results reported above. HOW

ever, at complete failure the two analyses will yield

the same value of F S 1.0 if the local pore pres-

sures are correctly measured. Figure 19 shows in

principle the variations of the two factors of safety:

before local failure is reached at any point in the

foundation, t he two factors of safcty decrease at a

similar rate with the increasing height of fill, the

difference between the two factors being a function

of the differences between the reference strengths

at points F and R ; using the m ore representat ive

cffective stress analysis will lead to a significant

increase of the permissible height of fill as shown

on Fig . 19. When some points of the founda t ion

reach local fai lure at F, increased pore pressures

FIG. 19. ariations o f the factors o f safety with the

height of fill.

are generated and the available strength decreases

along F R ; as a result , the factor of safety will dc-

crease rapidly towards 1.0 at com plete fai lure when

the entire foundation is in the residual state . I t

would be obviously dangerous to build u p embank-

ments to such height that the factor of safety would

be in the F R zone Fig . 19 ) s ince the ac tua l mar -

gin of safety in term s of height of fill would be very

limited. T he l imited exp erience presently available

indicates that the discontinuity in the variation of

the factor of safety occurs at a value less than 1.3

An addit ional control is provided by the observa-

t ion of the pore pressure parameter I. o r B either

of which assumes values muc h in excess of 1.0

when local failure and strain sof tening develop.

onclusions

Th e observations of construction pore pressures,

vane shear strength var iat ions and stabil i ty con-

dit ions in the clay foundations of four embank-

ments buil t in stages, a long with the use of the

YLIGHT

model of clay behaviour proposed by

Tavenas and Leroue i l

(1977),

have led to some

important conclusions.


22/23

304

C A N .

CEOTECH. J . VOL. I S 1978

1 Owing to an initial consolidation the clay be-

comes normally consol idated during the early

stages of con struct ion of an emban kment .

2. T h e passage from t he ini tial overconsol idated

state to the normally consol idated s tate produces

impo rtant ~nod ificatio ns of th e characteristics of

the clay. In part icular: ( i ) T he s t ructure of the

clay resulting from the combined effect of aging,

thixotropic hardening and eventual cementat ion is

obl i terated. ( i i ) Th e shape of th e l imit s tate surface

of the clay, representative of its structure, is modi-

fied, the s t rength of the clay being reduced. ( i i i )

The ini t ial rat io of vane shear s t rength to precon-

solidation pressure, C , , / P , , ssociated with the shap e

of the limit state surfac e of th e intact clay is redu ced

to a new Cl , /P , ratio, which seems to vary litt le

with th e type of clay. Th e vane s t rength correc t ion

factor

p

proposed by Bjer rum (1972) p robably

represents the magnitude of the deformation of th e

limit state surface of the clay when it becomes

normally consolidated.

3.

With the development of consol idat ion the

l imit s tate surface of the clay keeps i ts new shape

and is homothet ical ly displaced with the increase

in effective stresses. As a result, the vane strength

will increase, the ratio Cll/P,. remaining constant

and e qual to i ts new low value. I t is suggested th at

Bjerrum s correct ion factor should not be appl ied

to the s t rength obtained from vane tests carried

out und er an embankm ent dur ing conso lidat ion .

4. The s t rength avai lable at fai lure under an

em b an k m en t m ay b e o b ta i ned f ro m C IU t es ts o n

the overconsol idated clay carried to large s t rains ,

from vane tests in the intact clay corrected accord-

ing to Bjer rum (1 97 2) , f rom vane t est s in the nor-

mally conso l idated c lay unde r the embankm ent o r

from the correlat ion with P,. proposed by Mesri

(1 97 5) . When used in a 0 ty pe a nalysis, i t will

lead to the minimum possible factor of safety of

the embankment at any s tage of construct ion.

5.

T he only way of obtaining an est imate of th e

true s tabil i ty condit ion at an y s tage of con struct ion

is by means of an cffectivc stress an2lysis using the

actual pore pressures observed in the foundat ion

and the large s t rain effect ive s t rength parameters

c ,

+'.

However, for increased safety, the param-

eters

c

0 , +', obtained from the peak s t rength

measured in C IU tes ts o n normal ly conso l idated

samples may be used . Thi s approach wi l l l ead to

increased permissible heights of fill during stage

cons t ruct ion . However , care should be t aken no t

to ini t iate local fai lure and s t rain softening in the

foundat ion clay; this may be achieved by keeping

the fa ctor of safety in effective stresses in excess of

1.3 or preferab ly 1.4 and by observ ing that th

pore pressure parameter r or does no t exceed 1 .0

The use of this approach on the Berthiervi l le-

Yamachiche sect ion of Highway A.40 has resul te

in a significant accelcration of the constructio

schedule of seven important embankments .

cknowledgements

Th e present s tudy has been m ade possible by th

kind cooperat ion of the Minist ry of Transport o

the Province of Quebec, and of Ham cl Rue

Consul t ing Engineers for the project .

Th e au thors a re indebted to the ir co l leagues L

Samson and B. Trak for the i r comments , and t

the edi tors and reviewers of th e Cana dian Geotech

nical Journal for their valuablc suggestions.

Parts of the work have been supported by th

research fund of Terratech LimitCe and by Gran

number A .7724 of the Nat ional Research Counc

of Canada.

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Department of Civil Engineering, Laval University, Quebe

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p. 248.

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The Stability of Stage-constructed Embankments on Soft Clays

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