GE53-01-04.pdf

27

Lloret, A., Villar, M. V., Sanchez, M., Gens, A., Pintado, X. & Alonso, E. E. (2003). Geotechnique 53, No. 1, 2740

Mechanical behaviour of heavily compacted bentonite under highsuction changes

A. LLORET, M. V. VILLAR,{ M. SA`NCHEZ, A. GENS, X. PINTADO and E. E. ALONSO

The paper reports the results of an experimental studycarried out on a bentonite compacted to a dry density ofup to 17 Mg=m3, a high value for this type of soil. Thesoil fabric has been studied using a variety of techniques,revealing a clear bimodal pore distribution that corre-sponds to two distinct structural levels: a microstructuralone and a macrostructural one. The main testing pro-gramme has been performed using oedometers especiallydesigned to apply a very large range of suctions. Byapplying the axis-translation technique (using nitrogen asthe gas fluid), it has been possible to reach suctions up to15 MPa. The higher suction range has been achieved byapplying a controlled atmosphere where the relativehumidity has been fixed by a solution of sulphuric acidor salts. In this way suctions up to 550 MPa could bereached. The maximum vertical stress that could beapplied in the apparatus was 10 MPa. Two types of testhave been carried out: (a) tests in which a combinationof loading paths at constant suction and drying/wettingpaths at constant load were applied; (b) swelling testsunder constant-volume conditions in order to determinethe swelling pressure and the stress path followed duringwetting. The results of the experimental programme areexamined, taking into account the role of the soil fabricin controlling observed mechanical behaviour. In addi-tion, the results of the laboratory tests are reproducedand interpreted using a generalised plasticity model thatconsiders explicitly the interaction between macrostruc-ture and microstructure. In this way, it is possible toachieve a more complete understanding of the mechan-isms that underlie observed behaviour, and in particularthe interplay between the two structural levels.

KEYWORDS: constitutive relations; expansive soils; fabric/structure of soils; laboratory tests; radioactive waste disposal;suction

Cet expose rapporte les resultats dune etude experimen-tale effectuee sur une bentonite compactee jusqua` unedensite se`che de 1,7 Mg=m3, valeur elevee pour ce typede sol. La structure du sol a ete etudiee en utilisant unevariete de techniques revelant une distribution de poresnettement bimodale qui correspond a` deux niveaux struc-turaux distincts: un niveau microstructural et un niveaumacrostructural. Le programme dessais principal a eterealise en utilisant des odome`tres specialement concuspour appliquer une tre`s vaste gamme de succions. Enappliquant une technique de translation daxe (utilisantde lazote comme gaz fluide), il a ete possible datteindredes succions allant jusqua` 15 Mpa. La plage de succionla plus elevee a ete obtenue en appliquant une atmo-sphe`re controlee ou` lhumidite relative avait ete fixee parune solution dacide sulfurique ou de sels. De cettemanie`re, on a pu atteindre des succions allant jusqua`550 Mpa. La contrainte verticale maximum qui a pu etreappliquee dans lappareil etait de 10 MPa. Deux typesdessais ont ete effectues :- Des essais dans lesquels une combinaison de cheminsdechargement a` succion constante et de chemins sechage/humectage a` charge constante ont ete appliques- Des essais de gonflement en conditions volumiquesconstantes afin de determiner la pression de gonflementet le chemin de contrainte suivi pendant lhumectage.

Nous examinons les resultats de ce programme experi-mental en etudiant en quoi la structure du sol affecte lecomportement mecanique observe. De plus, nous repro-duisons les resultats des tests en laboratoire et nous lesinterpretons en utilisant un mode`le de plasticite general-ise qui tient compte de manie`re explicite de linteractionentre macrostructure et microstructure. De la sorte, nousavons pu approfondir notre comprehension des meca-nismes qui regissent le comportement observe, notam-ment linteraction entre les deux niveaux structuraux.

INTRODUCTIONCompacted bentonite is often used as the basic material forconstructing isolating barriers in geoenvironmental applica-tions or as basal and cap liners for landfills/retention basins(Koch, 2002). High-density compacted bentonite in the formof blocks has also been proposed as sealing material inhigh-level radioactive waste repositories, as it provides verylow permeability, high exchange capacity, sufficient thermalconductivity, and adequate mechanical resistance. There is

therefore a need to advance understanding of the mechanicalbehaviour of this type of material.

Although bentonite (montmorillonite) pastes have beenactively studied in the past (e.g. Warkentin et al., 1957;Callaghan & Ottewill, 1974; Ormerod & Newman, 1983;Kraehenbuehl et al., 1987), knowledge concerning the mech-anical behaviour of compacted bentonite is still scarce. Themicrostructure of compacted expansive clays has been stud-ied by Pusch (1982), who observed a double structure madeup of clay aggregates and large macrostructural pores. Thepore space inside the aggregates was constituted of voids ofa much smaller size. The same double structure has beenidentified in several compacted clays intended for engineeredbarriers in waste disposal (e.g. Atabek et al. (1991) in FoCaclay, Romero et al. (1999) in Boom clay, Pusch & Moreno(2001) in MX80, and Cui et al. (2002a) in a mixture of70% Kunigel clay and 30% Hostun sand). As indicated inGens & Alonso (1992), Alonso (1998), Yong (1999) and Cuiet al. (2002b), the behaviour of this material is potentiallyvery complex, as it results from the interaction between the

Manuscript received 2 May 2002; revised manuscript accepted 2October 2002.Discussion on this paper closes 1 August 2003, for further detailssee p. ii. Department of Geotechnical Engineering and Geosciences,Universitat Polite`cnica de Catalunya (UPC), Barcelona, Spain.{ Department of Environmental Impact of Energy, Centro deInvestigaciones Energeticas, Mediambientales y Tecnologicas (CIE-MAT), Madrid, Spain.

volume change of aggregates made up of a highly expansiveclay mineral (microstructural level) and the rearrangement ofthe granular-like skeleton formed by the aggregates (macro-structural level). The microstructural level is the seat of thephysico-chemical phenomena occurring at clay particle level,and in the conceptual model introduced in Gens & Alonso(1992) it is assumed that the microstructural deformationbehaviour is fully reversible and not affected by the state ofthe macrostructure. Also, the microstructural level is likelyto be saturated, and therefore Terzaghis effective stressprinciple holds. In contrast, the macrostructure will desatu-rate when subjected to suction, and its behaviour may bedescribed by conventional frameworks for unsaturated soils.Although the behaviour of the microstructure is expected tobe independent of the macrostructure, there is an importantinfluence of the microstructure on the macrostructural level,where it can induce significant plastic strains. The magnitudeof the interaction depends on the current stress state and onthe density of the macrostructure.

The experimental study of this type of material presentsvery significant challenges. One of the most important is theneed to cover large suction ranges, reaching on occasionvalues as high as 500 MPa. A good system of control ofhigh suctions is essential, as only the performance ofsuction-controlled tests will allow a sound interpretationof the results obtained. Moreover, the low permeability ofthe bentonite implies very long testing times that placesevere constraints on the extent of the testing programmes.Finally, there is also a need to apply or sustain high levelsof stresses; swelling pressures can be very high if thebentonite is heavily compacted. A range of new experimentaltechniques has been recently introduced for the study ofthese materials. Bernier et al. (1997), Al-Mukhtar et al.(1999), Villar (1999), Cuisinier & Masrouri (2001) andDueck & Borgesson (2001) have used suction-controlledoedometers based on various principles to test highly com-pacted clays, whereas Wiebe et al. (1997) and Tang et al.(2002) have studied the effect of suction and temperature onsand/bentonite mixtures using a triaxial apparatus that allowsthe application of a constant relative humidity value to thesample.

This paper reports the results of an experimental studycarried out on a bentonite heavily compacted to a drydensity of up to 17 Mg=m3. Given its importance, the soilfabric has been studied using a variety of techniques. Thetesting programme has been carried out using oedometersespecially designed to apply a very large range of suctions,up to 550 MPa, and vertical loads up to 10 MPa. In thepaper the results of constant load tests, constant suction testsand swelling pressure tests are examined and discussed,taking into account the role of the soil fabric.

The results of the experimental programme are interpretedand reproduced by means of a generalised plasticity modelthat considers explicitly the interaction between macrostruc-ture and microstructure. In this way, it is possible to viewthe results of the tests performed in a unified manner withina consistent theoretical framework.

PROPERTIES AND FABRIC OF THE MATERIALThe tests have been performed on a bentonite from the

Cortijo de Archidona deposit in Almera, south-easternSpain. The conditioning of the bentonite in the quarry, andlater in the factory, was strictly mechanical, consisting ofhomogenisation, removal of rock fragments, drying at 608Cand crumbling of clods. Finally, the clay was sieved using a5 mm square mesh to obtain the product to be used in thelaboratory.

The bentonite has a montmorillonite content higher than

90%. It also contains variable quantities of quartz and otherminerals such as plagioclase, cristobalite, potassium feldspar,calcite and trydimite. The cation exchange capacity is of111 9 meq=100 g, and the exchangeable cations are Ca(38%), Mg (28%), Na (23%) and K (2%). The liquid limitof the bentonite is 102 4%, the plastic limit is 53 3%and the hygroscopic water content of the clay at laboratoryconditions (relative humidity 50 10%, temperature21 38C) is about 13:7 1:3%. The value obtained for theexternal specific surface using the BET technique is32 3 m2=g, and the total specific surface obtained usingthe Keeling hygroscopicity method is about 725 m2=g. Therelatively low values of the Atterberg limits can be explainedby taking into account the significant quantity of silt-sizedaggregates that are pseudomorphs of the volcanic mineralstransformed into smectite (Villar, 2002).

As mentioned above, the fabric of the compacted soilplays an important role in the observed swelling behaviourof these materials. Mercury intrusion porosimetry (MIP)tests have been performed to examine the pore size distribu-tion of the statically compacted material used in the experi-mental programme. Fig. 1 shows the measured incrementalpore volume for two samples compacted to very differentvalues of dry density (rd), 15 Mg=m3 and 18 Mg=m3. Itcan be observed that the pore size distribution is clearlybimodal, which is very characteristic of this type of material(Alonso et al., 1987). The dominant values are 10 nm, whichwould correspond to the pores inside clay aggregates, and alarger pore size that depends on the compaction dry densityand ranges from 10 m (for rd 1:8 Mg=m3) to 40 m (forrd 1:5 Mg=m3). These larger voids would correspond tothe inter-aggregate pores. The boundary between the twopore size families can be seen to be around 130 nm, as poressmaller than this magnitude do not appear to be affected bythe magnitude of the compaction load. As Fig. 1 clearlyshows, compaction affects mainly the pore structure of thelarger inter-aggregate pores.

Figure 2 shows the relationship of pore diameter tointruded pore volume for two samples compacted at drydensities 155 Mg=m3 and 173 Mg=m3. Note that thereexists a significant pore volume into which the mercurycould not penetrate given by the difference between the voidratio of the soil and the void ratio intruded by the mercury.The magnitude of this volume (e 0:32) is practically thesame for the two specimens. Indeed, the intra-aggregate porevolumes are very similar for the two samples, with voidratio values around 046. The remaining of the pore space

0.2

0.16

0.12

0.08

0.04

01 10 100 1000 10000 100000

Pore diameter: nm

Incr

emen

tal p

ore

volu

me:

ml/g

Intra-aggregate Inter-aggregate

Dry density

1.8 Mg/m3 1.5 Mg/m3

Fig. 1. Distribution of incremental pore volume for twocompacted bentonite samples at different dry densities. Mercuryintrusion porosimeter test

28 LLORET, VILLAR, SANCHEZ, GENS, PINTADO AND ALONSO

therefore corresponds to inter-aggregate pores, accountingfor about 37% of the voids (e 0:28) in the sample with adry density of 155 Mg=m3 and about 20% (e 0:11) in thesample with a dry density of 172 Mg=m3. Fig. 3 shows amicrograph obtained using an environmental scanning elec-tron microscope where the presence of aggregates, the sizeof the inter-aggregate voids and the bimodal distribution ofpore sizes are readily apparent. Similar observations havebeen reported in Cui et al. (2002a).

Additional information on the soil fabric can be inferredfrom an examination of retention curves obtained under freeswelling and constant-volume conditions (Fig. 4). It can beobserved that the water retained at suctions higher thanabout 15 MPa is independent of the total void ratio (or drydensity). It seems plausible that the water retained at thosehigh suctions belongs to the intra-aggregate pores where thetotal porosity plays no relevant role (Romero et al., 1999).The fact that the water content corresponding to the intra-aggregate pores appears to be somewhat higher than theirpore volume (as measured in the MIP tests) may be due to

the fact that the density of the water tightly bound to theclay particles is higher than that of free water (Martin, 1962,Villar, 2002). It is also likely that part of the water isretained on the surface of the aggregates. Summarising, allthe observational data concerning the fabric of compactedsoils indicate a clear presence of two structural levels in thematerial: a microstructure inside the aggregates and amacrostructure consisting of the ensemble of aggregates andinter-aggregate pores. Microstructure features appear to belargely independent of compaction effort.

The saturated permeability of the bentonite compacted toa dry density of 17 Mg=m3 is of the order of 1014 m=s(Villar & Lloret, 2001). As a consequence, testing times ofstages requiring water content changes and hydraulic equili-bration are very long, usually taking several weeks. Thisplaces important time constraints on the laboratory testingprogramme.

LABORATORY EQUIPMENT AND EXPERIMENTALPROCEDURES

Oedometer tests with suction control have been carriedout at CIEMAT and UPC laboratories. The main limitationof oedometer testing is that it does not provide full informa-tion on the stress state of the material, as the horizontalconfining stress is not usually measured. As a result, diffi-culties are sometimes encountered in the interpretation ofthe results and in the comparison with model predictions. Inaddition, in the case of unsaturated soils, there is alsouncertainty regarding the degree of saturation of the soil,which is always unknown, although the retention curvesobtained at constant volume may provide an approximatewater content for every suction value. However, most signifi-cant trends and features of soil behaviour may be obtainedusing this type of test (Gens & Alonso, 1992), the perform-ance of which is far less complex than that of triaxial tests.Moreover, testing times are shorter in oedometer tests ascompared with triaxial tests, a paramount considerationwhen testing materials with such a low coefficient of per-meability. To reduce the duration of the tests further, theinitial thickness of the specimens was selected to be 12 mmapproximately, smaller than that of conventional oedometersamples. Friction effects, always a concern in oedometertests, were thereby also reduced.

The main objectives of performing the series of testsreported here have been to identify the main patternsof compacted bentonite behaviour and to increase theexperimental data available on the strain behaviour of very

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0.01000 100 10 1 0.1 0.01 0.001

Pore diameter: m

Cum

ulat

ive

void

rat

io

Total void ratio(e 0.57

d 1.73 Mg/m2)

Total void ratio(e 0.74

d 1.55 Mg/m2) Intra-aggregatevoid ratio(e 0.47)

Apparent delimitingzone (200 nm)

Intra-aggregatevoid ratio(e 0.46)

Fig. 2. Relationship between pore diameter and intruded porevolume for two samples compacted at different dry densities.Mercury intrusion porosimeter test

Fig. 3. Micrograph of a compacted bentonite sample (drydensity 1:72 Mg=m3) obtained using an environmental scan-ning electron microscope

1000

100

10

1

0.110 15 20 25 30 35 40 45

Water content: %

Suc

tion:

MP

a

Dry density (Mg/m3)

1.601.70

1.651.670.97

Intra-aggregategoverning suction

Inter-aggregategoverning suction

Fig. 4. Retention curves for compacted bentonite obtainedunder free swelling and constant-volume conditions

HEAVILY COMPACTED BENTONITE UNDER HIGH SUCTION CHANGES 29

expansive compacted materials under conditions of highsuction. Two types of test were carried out:

(a) tests in which a combination of loading paths atconstant suction and wetting and drying paths atconstant load were applied

(b) swelling tests under constant-volume conditions in orderto determine the swelling pressure and the stress pathfollowed during wetting.

The first type of test was carried out at the CIEMATlaboratory. Apart from the initial and final equilibrationstages, the actions applied on the samples have been loadincrements at constant suction and changes of suction atconstant load. Samples were compacted statically to a drydensity of 1:70 0:02 Mg=m3 with the material at hygro-scopic water content.

Because of the high suction values that have to be reachedin the study of this type of material, the procedure forapplying prescribed values of suction requires special con-sideration. For suction values less than 14 MPa, the axis-translation technique has been used in a cell of the typedescribed in Escario & Saez (1973). The base of the cellhas an embedded porous stone, below which there are twoinlet and outlet holes, connected to a burette with water atatmospheric pressure. A cellulose membrane is placed overthis stone, with the sample resting directly on it. Thismembrane allows water and ions to pass, but not gas. Aperistaltic pump, installed between the burette and the cellinlets, facilitates the removal of the gas that could diffusethrough the membrane. Nitrogen is used to apply the gaspressure in excess of the water pressure on top of thesample.

Higher suction values have been applied by prescribing avalue of the relative humidity of the atmosphere in contactwith the sample (Esteban 1990; Villar, 2002). Suction andrelative humidity are related by the psychrometric (Kelvins)law. In turn, relative humidity is controlled by the use ofsolutions of sulphuric acid at various concentrations(Esteban, 1990) or of saturated solutions of various salts(Delage et al., 1998). For the higher suction ranges, the useof sulphuric acid is required. Temperature must be strictlycontrolled, as the activity of the solution is very sensitive tothermal fluctuations. With this technique, it was possible toapply total suctions in the range between 3 MPa and500 MPa. Suctions less than 3 MPa are not reliable becauseit was difficult to ensure the necessary level of temperaturestability. The oedometer cell used in this case is depicted in

Fig. 5, where the solution reservoirs placed above the speci-men can be seen.

The swelling tests under constant volume have beenperformed in the UPC laboratory. These tests aim to repro-duce the conditions during hydration of an engineeredbarrier in underground waste repositories where the com-pacted bentonite is rigidly constrained by the waste canisterand the host rock (Fig. 6). Samples were compacted at aninitial dry density of 1:63 0:01 Mg=m3 at hygroscopicwater content (13:7 1:3%). This density is somewhat lowerthan in the specimens reported previously because there issome capacity for the barrier to expand slightly initiallyowing to the presence of gaps between the compacted blocksof bentonite. Four swelling pressure tests have been per-formed.

A schematic layout of the oedometer used in this type oftest is presented in Fig. 7. To prevent the swelling of the soilin the constant-volume stages, the lever arm is fixed(Bjerrum et al., 1964); a load cell measures the developmentof the swelling pressure. To permit the application of highloads, the lever arm has a loading ratio of up to 1 : 20.Suction is again applied by means of an atmosphere withcontrolled relative humidity. In this oedometer cell the air iscirculated, by means of a peristaltic pump, through theporous plates located at the two ends of the sample.

TEST RESULTSTable 1 presents the initial conditions and the stress paths

of five tests in which a combination of loading paths atconstant suction and suction change paths at constant load

Fig. 5. Schematic layout of the high-suction oedometer used in the CIEMATlaboratory (Esteban, 1990)

Rock

Nuclear waste

Container

Compactedbentonite

Fig. 6. Proposed engineered barrier made up of compactedbentonite clay for a radioactive waste repository


have been applied. The various load or suction changes wereapplied in stages, as shown in Fig. 8. In order to use alogarithmic scale a constant value of 01 MPa has beenadded to all suction values plotted in this figure. The sameconvention has been used in the rest of the paper. All tests

start at an applied vertical stress of 01 MPa and at thecompaction suction of about 125 MPa. Afterwards a varia-tion of suction is applied under constant load, except for onespecimen in which suction was kept unchanged. Afterwardsthe load was increased also under constant suction, whichwas then followed by wetting to saturation (zero suction).Maximum vertical stresses under saturated conditions wereeither 5 MPa or 9 MPa, depending on the test.

Figure 9 shows the variation of void ratio during theinitial stage of suction modifications and subsequent loading.The starting points for the loading stages are very differentbecause of the large dependence of volumetric strains onsuction applied at low loads. On loading, the stiffness of thebentonite (that is, the slope of the void ratio against thevertical stress line plotted on a semi-logarithmic scale)reduces slightly as the suction applied during loading in-creases. However, the most noticeable effect of suction isthe shifting of the point at which there is a change in theslope of these lines, indicated by a vertical arrow in Fig. 9.In the framework of elasto-plasticity this change is inter-preted as the crossing of a yield surface, and the load atwhich it takes place can be considered as an apparentpreconsolidation pressure. Fig. 10 shows the dependence ofthis preconsolidation pressure on suction. Large preconsoli-dation pressure reductions are apparent at low suction

Pump

Salt solution

Load cell

Porous stones

Oedometric cell

Loading ram

Soil

Fig. 7. Schematic layout of the high-suction oedometer used in the UPC laboratory

Table 1. Stress paths of the tests, in which a combination of loading paths at constant suction and suction change paths at constantload were applied. Tests performed at CIEMAT laboratory

Test Initial conditions Path

rd: w: % I II III IV VMg=m3

v:MPa

s:MPa

v:MPa

s:MPa

v:MPa

s:MPa

v:MPa

s:MPa

v:MPa

s:MPa

S1 172 130 01 138 01 550 51 460 51 0 01 0S2 168 137 01 126 91 127 91 0 01 0S3 169 142 01 121 01 14 90 14 90 0 01 0S4 171 129 01 119 01 37 84 41 84 0 01 0S5 172 132 01 138 01 520 01 0 50 0 001 0

1000

100

10

1

0.10.01 0.1 1 10

Vertical stress: MPa

Suc

tion:

MP

a

Vapourequilibrium

Axistranslation

S1

S2

S3

S4

S5

Fig. 8. Generalised stress paths followed by tests S1S5


values. According to the conceptual model of Gens &Alonso (1992), the reduction of the yield point is due tothe irreversible macrostructural strains induced bymicrostructural deformations that have occurred during theswelling to low suction values. Yielding was not reached intests S1 and S2, and in the plot of Fig. 10 it is assumed thatthe preconsolidation stress corresponds approximately to thevertical stress value reached during static compaction, about18 MPa.

The variations of void ratio during the stages at which thevertical stress is maintained constant and suction is variedare presented in Fig. 11. Large void ratio changes areobserved in the specimens where wetting takes place at01 MPa, whereas in the samples where wetting takes placeat 51 MPa and, especially, at 9 MPa load the volumechanges are quite small. Fig. 12 summarises this informationin terms of the slope of the swelling line (in a semi-logarithmic plot) with the applied vertical stress duringwetting. The effects of applied load are very noticeable.

One of the samples (S5) was subjected to drying to500 MPa suction before being wetted to a suction of01 MPa. The measured void ratio changes are plotted inFig. 13. It can be seen that the drying produces very smallvolumetric strains. However, when wetting is continued

towards smaller suction values a large increase of swellingstrains is observed, signalling a significant change of behav-iour. A comparison of the results of this test with those oftest S4, which did not undergo this initial drying/wetting

1.1

1.0

0.9

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0.50.0 0.1 1.0 10.0


Voi

d ra

tioApplied suctionduring loading

500 MPa

127 MPa

14 MPa

4 MPa

0 MPa

S5

S4

S3

S2

S1

p0

Initial state

Fig. 9. Variation of void ratio during the initial stage of suctionvariation and subsequent loading. Tests S1S5

1000

100

10

1

0.10.1 1 10 100

Apparent preconsolidation stress: MPa

Suc

tion:

MP

a

S5

S4

S3

S2

Fig. 10. Relationship between apparent preconsolidation stressand applied suction identified in loading stages of tests S3, S4and S5. The preconsolidation stresses of samples S1 and S2 areassumed to be approximately equal to the static compactionstress

1.1

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Voi

d ra

tio

0.1 1.0 10.0 100.0 1000.0Suction: MPa

Vertical stress (MPa)

0.1

5.1

9.0

0.1

9.1

Axis translationtechnique

S2

S3

S1

S4

Initial state

S5

Fig. 11. Variation of void ratio during the stages at whichvertical stress is maintained constant and suction varied. TestsS1S5

0.08

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0.020.01 0.1 1 10


e/

[In(s

p

atm

)]

S1

S4

S2

S5

S3

Fig. 12. Influence of applied vertical stress on slope of swellingline measured on wetting. Tests S1S5

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0.5010 100 1000

Suction: MPa

Voi

d ra

tio

Vertical stress: 0.1 MPa

S4

S5

Fig. 13. Variation of void ratio with suction of test S5 during aninitial drying/swelling cycle and subsequent wetting. The resultsof test S4 are included for comparison


cycle (Fig. 13), suggests that such a suction cycle does nothave any noticeable effects on subsequent behaviour.

An important feature of behaviour concerns the possiblestress path dependence on or independence from the materialunder generalised stress including suction. In unsaturatednon-expansive soils it is often found that the behaviour isstress-path independent if the trajectories include only load-ing and wetting (suction reduction) stages (Alonso et al.,1987). However, Gens & Alonso (1992) argued that, inexpansive soils, there would be irreversible macrostructuralrearrangements caused by the swelling of the microstructurewhen wetting the samples. This interaction would be higherwhen applied stresses were low. As a consequence, thismacrostructural change could result in stress-path depen-dence of volume changes even in the cases where onlywetting paths are involved. This issue has been examined inthis experimental programme.

Figure 14(a) shows void ratio variation for two specimensthat share the initial and final stress points but have followeddifferent stress paths. Note that the final state is different, sothere is a measure of stress path dependence. As observed inother cases (Brackley, 1973; Justo et al., 1984), the finalvoid ratio of the sample wetted under low stresses is higher.The disruption of the macrostructure would be more severein this case because of the larger swelling strains developedduring wetting, inducing large macrostructural volumechanges that cannot be recovered upon subsequent loading.

Fig. 14(b) shows a similar plot for three other specimenswith the same initial and final points where, again, the finalvoid ratio values do not coincide. As before, the sample thatwas wetted at the lower applied vertical stress (4 MPa)reaches the highest final void ratio.

The main features of the swelling pressure tests performedin the UPC laboratory are presented in Table 2. Initially asuction change is applied to three of the samples in order toachieve a range of initial conditions for the next testingstage. During the swelling pressure phase a condition of novolume change is prescribed, and suction is reduced instages under controlled conditions. In this way it is possibleto follow the stress path, in terms of suction against verticalstress, throughout the tests, as shown in Fig. 15.

Examination of these stress paths provides importantinformation on the stressstrain characteristics of the bento-nite in this type of test (which corresponds to a number ofrelevant field situations) but, also, on the underlying causesof the observed behaviour. Three zones can be distinguished(Fig. 15). The first corresponds to the stage of high suctionand low applied loads. In this region the vertical stressremains below the preconsolidation stress (in elasto-plasticterms, the stress state has not reached the yield locus) andthe stress path is defined by the increase of load required tocompensate for the small swelling strains due to suctionreduction.

Once the preconsolidation pressure (yield locus) isreached, the second zone is entered and a drastic change ofslope ensues. If the load is sufficiently high, collapse of themacrostructure occurs, and the value of vertical stress tendsto reduce to compensate for the collapse compressive strains.Naturally, the microstructural swelling strains also contributeto the compensation for collapse deformation, but at thisstage they may not be large enough to offset the reductionof vertical stress.

The points indicating the preconsolidation stress identifiedin the loading tests under constant suction have been addedto Fig. 15 for reference. A close coincidence of yield pointsfrom the two types of test should not be expected becauseof the different stress paths followed, which lead to differentmagnitudes of irreversible strains and therefore to somewhatdifferent yield surfaces. Generally, in the constant load teststhe larger swelling strains should lead to smaller sizes ofyield locus.

Finally, zone III corresponds to the region of low suction,where microstructural swelling strains exhibit their largestmagnitude. Now microstructural strains overcome any possi-ble collapse strains, and the vertical stress must increaseagain to compensate for the large swelling strains. It isinteresting to note that even the rather complex behaviourdisplayed by the compacted bentonite during the swellingpressure tests can be readily explained in the context of aconceptual framework that considers the interaction betweenmicrostructure and macrostructure.

The dependence of swelling pressure on compacted drydensity has been examined by a large number of testsperformed in conventional oedometers without suction con-trol, where the wetting is applied in a single step. Theresults of the swelling pressure tests performed in suction-controlled equipment have been added to Fig. 16, where itcan be observed that they agree closely with the indepen-dently determined trend. It is reassuring that no spuriouseffects due to the equipment appear to affect the results ofthe swelling pressure tests.

MODEL RESULTSIn the previous section the test results have been qualita-

tively interpreted in terms of a conceptual framework in

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Applied suctionduring loading

500 MPa

0

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S1Initial state

Vertical stress: MPa(a)

Voi

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tio

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0.6

0.50.0 0.1 1.0 10.0

Vertical stress: MPa(b)

Voi

d ra

tio

Applied suctionduring loading

4 MPa

14 MPa

127 MPa

Initial state

S4

S3

S2

Fig. 14. Observed variation of void ratio with the same initialand final stress states: (a) tests S1 and S5; (b) tests S2, S3 andS4


which two structural levels (macro and micro) interact.Although this interaction has a dominant effect when exam-ining the behaviour of compacted expansive clay, it may alsobe relevant to some aspects of the behaviour of non-expan-sive clays (see for instance the results of tests on kaolinreported by Sivakumar & Doran, 2000).

Some references have also been made to general conceptsof elasto-plasticity. However, a more satisfactory interpreta-tion requires the use of a properly formulated constitutivelaw so that quantitative comparisons are possible and, moreimportantly, to ensure that the conceptual model used isinternally consistent and has no arbitrary ad hoc rules. Inthis section some of the most characteristic results presented

earlier are examined using the double-structure elasto-plasticmodel described in Sanchez et al. (2001). This model isbased on the conceptual approach contained in Gens &Alonso (1992) and on the mathematical formulation pre-sented in Alonso et al. (1999). It is graphically summarisedin Fig. 17. The behaviour of the macrostructure is defined interms of the Barcelona Basic Model (BBM; Alonso et al.,1990), represented in the figure by the LC yield curve. Thebehaviour of the microstructure is considered always rever-sible and logarithmically dependent on the classical effectivestress p s, where p is the mean net stress and s is thesuction (total suction in this case). When the stress statecrosses either the SI (suction increase) or SD (suction de-crease) yield locus, the microstructural strains induce irre-versible plastic strains in the macrostructure that result inthe movement of the LC yield surface. Important compo-nents of the model are the interaction functions that controlthe magnitude of the macrostructural plastic strain caused bymicrostructural strains. Fig. 18 shows generic interactionfunctions indicating the physical phenomena underlying eachof the branches. The shape of the fD interaction curveimplies that, when the stress point is far from the LC (thatis, values of p=p0 much less than 1), the interaction duringwetting is very strong. This corresponds to the fact that, inthat situation, the macrostructure is in a dense state, far fromthe collapsible behaviour implied by the LC curve. Thereforethe expansion of the microstructure causes large disruptionsand significant irrecoverable strains. As the state of the soilmoves towards the LC, the specimen becomes less dense(relative to the applied stress), and the strength of the inter-

Table 2. Main features of the swelling pressure tests performed at the UPC laboratory

Test Initial conditions Path before swelling pressure test Dry density beforeswelling pressure

rd: w: % I II test, rd: Mg=m3

Mg=m3

v: MPa s: MPa v: MPa s: MPa

SP1 162 146 011 128 011 424 165SP2 163 132 015 146 163SP3 162 146 016 128 016 70 157SP4 163 142 015 128 015 39 150

1000

100

10

1

0.1

Suc

tion:

MP

a

2.0 4.0 6.0 8.0 10.00.0Vertical stress: MPa

Yield points inswellingstress tests

Yield points in suctionreduction stressincrease paths

Zone I

Zone II

Zone III

SP1

SP2

SP3

SP4

d 1.50 Mg/m3

d 1.57 Mg/m3

d 1.63 Mg/m3

d 1.65 Mg/m3

Fig. 15. Generalised stress paths observed in the swellingpressure tests

20

15

12

8

4

0

Sw

ellin

g pr

essu

re: M

Pa

1.20 1.30 1.40 1.50 1.60 1.70 1.80Dry density: Mg/m3

Soaking tests

SP1

SP2

SP3

SP4

Fig. 16. Dependence of swelling pressure on compaction drydensity as determined in conventional oedometers. The swellingpressures measured in the suction-controlled oedometers agreewell with the general trend

Current stress state CS

Mic

rost

ruct

ure

SI

NL

SD

Mac

rost

ruct

ure

LC

Elasticdomain

Macrostructural

void ratio plastic

increase

Microstructural swelling

Microstructural shrinkage

Macrostructural void ratio

plastic decrease

p

Fig. 17. Graphical summary of the double-structure elasto-plastic model for expansive soils


action reduces accordingly. Indeed, the interaction maychange sign if the expansion of the aggregates leads to aninvasion of the macropores with a consequent reduction ofthe macrostructural void ratio. Opposite considerations applyto the f I interaction curve. Additional information on themodel is given elsewhere (e.g. Gens & Alonso, 1992;Alonso et al., 1999; Sanchez et al., 2001). The relevantmodel equations are presented in Appendix 1.

An important advantage of this model is that it makes itpossible to take into account the two levels of structurereally existing in the material tested, so that the variablesassociated with each level can be followed throughout thetest and a more detailed examination of the patterns ofbehaviour can be made. It must be emphasised that the aimof this modelling exercise is not to test the predictivecapability of the model. In fact, the parameters have beenchosen specifically to give a good representation of theexperimental observations. The objective is to use the con-stitutive model as a consistent tool to gain a better under-standing of the behaviour of the soil and of the mechanismsthat underlie it.

The model is used one-dimensionally, substituting appliedvertical stress for mean net stress. Therefore p now denotesnet vertical stress. SI and SD surfaces coincide, so that thereis always some degree of interaction between micro andmacro levels. The parameters used in the modelling areshown in Table 3. The results of tests S1 and S5 have beenused to determine them. Elastic macrostructural parametershave been obtained from the sections of the tests that areinside the LC yield curve. The value of the slope of thesaturated consolidation line, (0), has been based on theloading stage of test S5. The parameters of the interactioncurves have been adjusted to yield the correct amount of

stress path dependence, and the parameters for the micro-structural behaviour have been derived from the swellingstages of tests S1 and S5. Finally, the parameters controllingthe shape of the LC yield surface have been evaluated fromthe experimental results plotted in Fig. 10, which shows thevariation of the apparent preconsolidation stress with suc-tion.

The initial values of the microstructural and macrostruc-tural void ratio are 045 and 011 respectively, in accordancewith the observations on the soil fabric. The static compac-tion stress (18 MPa) gives the initial position of the LC yieldsurface. With the expression used to define the LC curve(equation (1)), the hardening parameter p0 has an initialvalue of 12 MPa.

The model is first applied to the analysis of tests S1 andS5. They share the same initial and final generalised stressstates but their trajectories are very different (Fig. 19(a)). Intest S1 the specimen is loaded under a high (550 MPa)suction up to a 51 MPa vertical load and is then wetted,reducing the suction to zero in stages. In contrast, test S5 isfirst wetted at a low applied vertical stress value of 01 MPaand then the sample, already saturated, is loaded to a verticalstress of 50 MPa. These two tests therefore provide theopportunity to examine the behaviour over a wide range ofstress paths. Fig. 19(a) shows the computed variation of voidratio over the stress paths followed by the two tests, togetherwith the experimental results. Major features of behaviourare correctly reproduced, including the following:

(a) There are large swelling strains when the material iswetted at low stresses (path BD, test S5).

(b) There are smaller, but still significant, swelling strainswhen the soil is wetted under a 5 MPa vertical stress(path CE, test S1).

(c) The slope of the compression line changes duringloading, indicating yield, in test S5 (path DE). Noyield is apparent during the loading at high suction ofspecimen S1 (path BC).

(d) The final void ratio (point E) is different in the twosamples. There is a measure of stress path dependence,at least in relation to volumetric strains

Good reproduction of behaviour is also achieved whenconsidering the experimental results in terms of void ratioagainst suction variation (Fig. 19(b)), although some depar-tures are observed at intermediate stages of the swelling oftest S5.

To summarise, the model is capable of offering a goodsimulation of the observed results, and, with the set ofparameters adopted, even the quantitative agreement is quiteclose. Accepting then that the constitutive law is a reason-able representation of the real behaviour, it is interesting toexplore the further information that can be obtained from

fD

fI1

p/p0

Equilibrium statefor a large numberof cycles

Microporesinvademacropores

Suction decrease

Suction

increase

Compressionaccumulatesupon cycles

Expansionaccumulatesupon cycles

Macroporositydevelops as aresult of drying

dM

/d m

0

Fig. 18. Generic interaction functions that indicate the intensityof the interaction between the two structural levels

Table 3. Parameters used to define the elasto-plastic constitutive law used inmodelling

Parameters defining the Barcelona Basic Model (BBM) for macrostructural behaviour

k 0005 ks 0001(0) 0080 pc 050 MPar 090 10

Parameters defining the law for microstructural behaviour

m 2:1 3 102 MPa1 m 2:3 3 103 MPa1

Interaction functions

f I 1 0:9 tanh[20( p=p0) 0:25] f D 0:8 1:1 tanh[20( p=p0) 0:25]


the model so that the behaviour mechanisms underlying themechanical behaviour of the soil can be better understood.

Figure 20 shows the evolution of the microstructural andmacrostructural void ratio computed for test S5. During theswelling stage (path BD), the microstructural strains arerelatively large, and they cause even larger irreversiblestrains in the macrostructure because of the large couplingbetween the two structural levels that exist at low stresses.During the subsequent loading (path DE) under saturatedconditions the deformation of the macrostructure is signifi-cant, but it is not due to microstructural strains, which arenow quite small. This part of the test is basically controlledby the behaviour of the macrostructure. Fig. 21 shows theparts of the interaction functions involved in the variousstages of the test.

Some of the positions of the LC yield curve during theperformance of test S5 are shown in Fig. 22. It can be seenthat during swelling (path BD) the LC curve moves to theleft in response to the irreversible swelling strains takingplace in the macrostructure. Subsequent loading (path DE)takes the LC again to the right to the final load value of50 MPa. Indeed the yield point observed and computed(Fig. 19(a)) corresponds to the crossing of the LC duringthis loading stage. Continuous information on the evolutionof the LC yield curve can be obtained by plotting theevolution of the hardening parameter, p0 (Fig, 23). Thereduction of p0 during swelling and subsequent increaseupon loading are readily apparent.

The behaviour of sample S1 is quite different (Figs 24and 25). During the first stage of drying (path AB) andsubsequent loading (path BC), the microstructural volu-metric strains are very small. The macrostructural strains are

1000

100

10

1

0.1

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0 1 2 3 4 5

S1

S5

B

A

D


Suc

tion:

MP

a

Voi

d ra

tio

0.1 1.0 10.0

Vertical stress: MPa(a)

S1 test

S1 model

S5 test

S5 model

Initial state A

B C

E

E

D

1000

100

10

1

0.10 1 2 3 4 5


1.1

1.0

0.9

0.8

0.7

0.6

0.5

Voi

d ra

tio

0.1 1.0 10.0 100.0 1000.0Suction: MPa

(b)

Suc

tion:

MP

a S1

S5E

C B

A

D

C

E

S1 test

S1 model

S5 test

S5 model

Initial state

AB

C

E

E

D

Fig. 19. Computed variation of void ratio for tests S1 and S5: (a) variation over stress paths; (b) constant load, variedsuction. Experimental results are provided for comparison.

0.6

0.5

0.7

0.4

0.3

0.2

0.1

Voi

d ra

tio

0.1 1.0 10.0 100.0 1000.0Suction: MPa

Void ratio micro

Void ratio macro

1000

100

10

1

0.10 1 2 3 4 5


Suc

tion:

MP

a

B

A

D ES5

D

E

D

E

Initial stateA B

Initial state

A B

Fig. 20. Evolution of computed microstructural and macro-structural void ratio. Test S5


also small even during loading because of the high stiffnessimparted to the sample by the large (500 MPa) suction.During the swelling stage (path CE) under a 51 MPa load,the microstructural strains are significant although smaller

than for test S5 because of the higher load applied. Themost significant difference is, however, that the macrostruc-tural strains that are induced are quite small because nowthe stress state is much closer to the LC (Fig. 26): that is,

1000

100

10

1

0.1

2.0

1.5

1.0

0.5

0.0

0.5

f

0.0 0.2 0.4 0.6 0.8 1.0

0 1 2 3 4 5Vertical stress: MPa

p/p0

fIfDS5

fI

fD

D

B

A

B E

S5ED

B

A

Suc

tion:

MP

a

Fig. 21. Interaction functions involved in the various stages oftests S5

1000

100

10

1

0.1

1000.0

100.0

10.0

1.0

0.1

Suc

tion:

MP

a

Suc

tion:

MP

a

0 1 2 3 4 5

0.0 4.0 8.0 12.0 16.0 20.0p, p0: MPa


Initial state

LC in loadingyield point Final LC Initial LC

pp0

B

A

D E

S5

B

A

D E

Fig. 22. Stress path and successive LC yield surfaces for test S5

1000

100

10

1

0.1

1000.0

100.0

10.0

1.0

0.1


S5

S1A

B

D

C

E

Suc

tion:

MP

a

Suc

tion:

MP

a

0 2 4 6 8 10 12 14p0*: MPa

S5

Initial state

B C

A

EED

S1

Fig, 23. Evolution of hardening parameter p0 for tests S1 andS5

1000

100

10

1

0.1

0.60

0.50

0.40

0.30

0.20

0.10

Voi

d ra

tio

Suc

tion:

MP

a


0.1 1.0 10.0 100.0 1000.0Suction: MPa

Initial state

Initial state

Void ratio microVoid ratio macro

E

E C

BA

A B

C

S1

B

A

C

E

Fig. 24. Evolution of computed microstructural and microstruc-tural void ratio. Test S1


the sample is in a comparatively loose state and the potentialfor macrostructural disruption is much lower. The loadingstage (path BC) takes place inside the LC yield surface, sono yield is expected and none was observed. The evolutionof the hardening parameter for test S1 has also been plotted

in Fig. 23. It can be observed that the changes are veryslight (indicating a LC curve that is practically stationary)and that the final value of p0 is higher than for test S5.

Now the basic reason for the stress path dependence ofvolumetric strains can be readily identified. The basic differ-ence is that, in test S5, the large swelling strains take placeat low stress values, and consequently the interaction withthe macrostructure is very strong and results in large plasticstrains that are not fully recovered upon subsequent loading.In test S1, development of plastic strains in the macrostruc-ture is quite reduced because, when the swelling of themicrostructure takes place, the interaction between the twostructural levels is small. The different states of the samplesat the end of the test are reflected in the different finalpositions of the LC curves (and p0 values). Exactly thesame analysis could be made regarding the results of testsS2, S3 and S4.

As a final test of the adequacy of the model to representthe behaviour of compacted swelling materials, it has beenapplied to the modelling of the swelling pressure tests usingthe same set of parameters without any attempt at furthercalibration. Fig. 27 shows the results of the model calcula-tions together with the experimental results. Although differ-ences between computed and experimental results can beobserved, the model is capable of producing swelling stresspaths very similar to the observed ones comprising the threephases of behaviour identified previously.

CONCLUSIONSThe mechanical behaviour of compacted bentonite has

been investigated by the performance of a testing pro-gramme carried out in suction-controlled oedometers capableof applying high suctions and large vertical loads. Two typesof test have been performed: (a) tests with simultaneouscontrol of vertical stress and suction, and (b) suction-con-trolled swelling pressure tests. Examination of the resultshas revealed significant features of behaviour, such as yieldphenomena, dependence of swelling strains on applied stres-ses, stress path dependence of strains, and compound stresspaths in swelling pressure tests. In spite of the complexity ofthe behaviour, a good understanding of the behaviour ispossible by considering explicitly the fabric of the soil,which has been independently studied, revealing a dualarrangement with microstructural and macrostructural com-ponents.

1000

100

10

1

0.1

2.0

1.5

1.0

0.5

0.0

0.5

fS

uctio

n: M

Pa

0 1 2 3 4 5

0.0 0.2 0.4 0.6 0.8 1.0p/p0

fIfDS1

fI

fDE

C

B

A

S1


B

A

C

E

Fig. 25. Interaction involved in the various stages of tests S1

1000

100

10

1

1.0

Suc

tion:

MP

a

1000.0

100.0

10.0

1.0

0.1

Suc

tion:

MP

a

0 1 2 3 4 5

0.1 1.0 10.0 100.0p: MPa

Initial state

InitialLC

FinalLC

A

B C

E

S1A

B C

E


Fig. 26. Stress path and LC yield surfaces for test S5

1000.0

100.0

10.0

1.0

0.1

Suc

tion:

MP

a

0 2 4 6 8Vertical stress: MPa

Model predictions TestsSP1

SP2

SP3

SP4

Fig. 27. Computed stress paths for swelling pressure tests SP1SP4. Experimental results provided for comparison


A deeper insight into the behaviour of the compactedbentonite, and of the basic mechanisms controlling it, hasbeen achieved using an elasto-plastic framework that incor-porates the interplay between microstructural and macro-structural fabric levels in a simplified manner. Most of themain features of behaviour are correctly reproduced by themodel, allowing a more detailed examination of the role thatthe different variables play on the explanation of the overallbehaviour of the soil.

ACKNOWLEDGEMENTSThis work has been supported by ENRESA and the

European Commission through contract FI4W-CT95006(FEBEX I). The authors also wish to acknowledge thesupport of the Ministerio de Ciencia y Tecnologa throughresearch grant BTE20012227.

APPENDIX 1The relevant equations for the elasto-plastic model are as follows.

LC yield surface:

p0 pc p0

pc

! 0 k s k

and (s) (0)[r (1 r)es] (1)

Hardening law:

d p0p0

(1 eM)dv (0) k (2)

Elastic macrostructural behaviour (reversible):

devM d p

K t ds

Kswith K t (1 eM) pk and

Ks (1 eM)(s patm)ks (3)

Microstructural behaviour (reversible):

devm d( p s)

Kmwith Km e

m( ps)

m(4)

Interaction micromacro:

dpvM fD devmwhen SD is activated (5)

dpvM f I devmwhen SI is activated (6)

NOTATIONe void ratio

eM macrostructural level void ratiof I, f D interaction function between micro and

macrostructural levels for suction increase anddecrease respectively

Km microstructural bulk modulus for changes insuction plus mean stress

Ks macrostructural bulk modulus for changes insuction

Kt macrostructural bulk modulus for changes in meanstress

p net mean stress (excess of mean stress over gaspressure)

pc reference stressp0 net mean yield stress at current suctionp90 net mean yield stress for saturated conditions

patm atmospheric pressure (01 MPa)r parameter defining the maximum macrostructural

soil stiffnesss suction

w gravimetric water content

m parameter controlling the rate of increase ofmicrostructural soil stiffness with mean stress

parameter controlling the rate of increase ofmacrostructural soil stiffness with suction

m parameter controlling the microstructural soilstiffness

v total volumetric strainsevM,

pvM elastic and plastic volumetric strains at

macrostructural levelevm elastic volumetric strain at microstructural levelk macrostructural elastic stiffness parameter for

changes in net mean stressks macrostructural elastic stiffness parameter for

changes in suction(s) macrostructural stiffness parameter for changes in

net mean stress for virgin states of soil at suction srd dry densityv net vertical stress (excess of vertical stress over gas

pressure)

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INTRODUCTIONPROPERTIES AND FABRIC OF THE MATERIALLABORATORY EQUIPMENT AND EXPERIMENTAL PROCEDURESTEST RESULTSMODEL RESULTSCONCLUSIONSACKNOWLEDGEMENTSAPPENDIX 1NOTATIONREFERENCES

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