Aspects of the behaviour of compacted clayey soils on drying and wetting paths

Aspects of the behaviour of compacted clayeysoils on drying and wetting paths

Jean-Marie Fleureau, Jean-Claude Verbrugge, Pedro J. Huergo,António Gomes Correia, and Siba Kheirbek-Saoud

Abstract: A relatively large number of drying and wetting tests have been performed on clayey soils compacted at thestandard or modified Proctor optimum water content and maximum density and compared with tests on normally con-solidated or overconsolidated soils. The results show that drying and wetting paths on compacted soils are fairly linearand reversible in the void ratio or water content versus negative pore-water pressure planes. On the wet side of theoptimum, the wetting paths are independent of the compaction water content and can be approached by compactiontests with measurement of the negative pore-water pressure. Correlations have been established between the liquid limitof the soils and such properties as the optimum water content and negative pore-water pressure, the maximum drydensity, and the swelling or drying index. Although based on a limited number of tests, these correlations provide afairly good basis to model the drying–wetting paths when all the necessary data are not available.

Key words: compaction, unsaturated soils, clays, drying, wetting, Proctor conditions.

Résumé : Un nombre relativement important d’essais de drainage et d’humidification a été réalisé sur des sols argileuxcompactés à l’optimum Proctor normal ou modifié, et comparé avec ceux sur des échantillons normalement consolidésou surconsolidés. Les résultats obtenus montrent que les chemins de drainage et d’humidification des sols compactéssont sensiblement linéaires et réversibles dans les plans de l’indice des vides ou de la teneur en eau en fonction de lapression interstitielle négative. Du côté humide de l’optimum, les chemins d’humidification sont indépendants de lateneur en eau de compactage, et peuvent être approchés par des essais de compactage avec mesure de la pressioninterstitielle négative de l’eau. Des corrélations ont été établies entre la limite de liquidité des matériaux et lesprincipales caractéristiques de ces chemins, telles que la teneur en eau de compactage, la densité sèche et la pressioninterstitielle négative à l’optimum ou les indices de gonflement et de drainage. Bien qu’elles ne reposent que sur unnombre réduit d’essais, ces corrélations fournissent un point de départ intéressant pour modéliser les chemins dedrainage–humidification lorsque toutes les données nécessaires ne sont pas disponibles.

Mots clés : compactage, sols partiellement saturés, argile, drainage, humidification, conditions Proctor. Fleureau et al.1357

Introduction

Volume and water content changes in soils due to environ-mental factors such as drying–wetting cycles, and associatedchanges in the groundwater table level may result in settle-ment or heave of structures and, as such, should be accountedfor in the design of soil structures. The key driving parameterassociated with volume and water content changes is the neg-ative pore-water pressure, uc, also called capillary pressure orsuction, which is the difference between the pressures in theair, ua, and water, uw, phases

[1] uc = ua – uw

The soil-water characteristic curve (SWCC) describes thechanges in gravimetric water content, w, or volumetric watercontent, θ, or degree of saturation, S, versus negative pore-water pressure. It can be used as a tool in the determinationof degree of saturation or water content changes in the soilon drying or wetting paths. It is also of value to determinethe associated changes in void ratio versus uc simultaneouslyalong with the SWCC.

Several elastoplastic constitutive models presently avail-able can predict with reasonable accuracy the volumetric

Can. Geotech. J. 39: 1341–1357 (2002) DOI: 10.1139/T02-100 © 2002 NRC Canada

1341

Received 12 July 2001. Accepted 4 July 2002. Published on the NRC Research Press Web site at http://cgj.nrc.ca on 7 November2002.

J.-M. Fleureau.1 Ecole Centrale de Paris, CNRS UMR 8579, Laboratoire de Mécanique, Grande Voie des Vignes, 92295Châtenay-Malabry, France.J.-C. Verbrugge and P.J. Huergo. Université Libre de Bruxelles, Laboratoire Jacques Verdeyen, Avenue A. Buyl 87, B 1050Bruxelles, Belgium.A. Gomes Correia. Instituto Superior Técnico, Universidade Técnica de Lisboa, Centro de Geotecnia, Avenida Rovisco Pais 1,P 1049–001 Lisboa, Portugal.S. Kheirbek-Saoud. Tichreen University, Department of Transportation, Lattakia, Syria.

1Corresponding author (e-mail: [email protected]).

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deformations of unsaturated soils subjected to changes instresses and negative pore-water pressure (Alonso et al.1990; Kohgo et al. 1993; Modaressi et al. 1996; Wheelerand Karube 1996). However, they are unable, in their pres-ent state, to directly calculate the corresponding changes inthe degree of saturation. In these models, the experimentaldata of the SWCC must be introduced as a parameter. Theknowledge of the SWCC is also required in semi-empiricalmodels, such as those proposed by Lloret and Alonso (1985),Ho et al. (1992), and Nanda et al. (1993). The SWCC isequally important in the prediction of other engineeringproperties of unsaturated soils, such as the coefficient of per-meability (Brooks and Corey 1964; van Genuchten 1980;Mualem 1986; Fredlund et al. 1994) or the shear strength(Biarez et al. 1994; Fredlund et al. 1996).

Compacted soils are commonly used in the constructionof soil structures such as roads, railroad embankments, andearth dams. Several investigators have highlighted the im-portance of the hydromechanical stress history on thedrying–wetting paths of compacted soils (Guillot et al.2001). A large number of factors, which are either not mea-sured or difficult to control, influence the engineering behav-iour of compacted soils. A major concern is related to theheterogeneity of the specimens. Even if the material is ini-tially homogenous and if the applied stresses are the same,many factors will influence the final properties of the speci-mens, for instance, the size of the mould, and the diameterof the piston, which can result either in oedometric ortriaxial conditions (depending on the relative diameter of thepiston and the mould). In the case of a dynamic compaction,several additional factors such as the metal of the mould orthe material on which it is placed during compaction, willaffect the propagation of the shock waves and lead to differ-ent results.

The following two standardized conditions of compactionhave been considered in this paper to avoid these difficultieswhen comparing the results of different soils:

(1) Proctor conditions, according to ASTM standard D698–91 (ASTM 1995a): specimens are compacted in a mould11.6 cm high and 10.2 cm in diameter, in 3 layers, eachlayer receiving 25 blows from a 2.490 kg rammer droppingfrom a height of 30.5 cm. The corresponding specific energyis 0.6 MJ/m3.

(2) Modified Proctor conditions, according to ASTM stan-dard D1557–91 (ASTM 1995b): specimens are compacted ina mould 15.2 cm high and 15.2 cm in diameter, in 5 layers,each layer receiving 55 blows from a 4.535 kg rammer drop-ping from a height of 45.7 cm. The corresponding specificenergy is 2.7 MJ/m3.

A significant dispersion is observed, even in carefullycontrolled tests: the variations may reach ±1% of the dryunit weight, ±2% of the water content, and ±4% of the de-gree of saturation (Fig. 1).

The primary objective of this paper is to examine the engi-neering behaviour of a compacted soil on drying and wettingpaths, based on a detailed study of Jossigny silt prepared un-der two different conditions: as a slurry, consolidated in anoedometer under various stresses or compacted at the Proc-tor maximum dry density. The secondary objective is to gen-eralize this study to a large database of different clayey soils,

both artificial and natural, with liquid limits ranging from 20to 170%. The following points are examined:

(1) the values of the initial state parameters (dry density,water content, and negative pore-water pressure) of the soilscompacted to Proctor optimum water content and maximumdry density (SPO) and modified Proctor optimum water con-tent and maximum dry density (MPO), and their relation tothe liquid limit;

(2) the drying and wetting paths of compacted specimensand their relationship with those of slurry specimens of thesame soils;

(3) the influence of the initial water content on the dryingand wetting paths;

(4) the analogy between the wetting path of a soil com-pacted at the Proctor optimum water content and the com-paction path of specimens compacted at different watercontents; and

(5) the modelling of the wetting paths starting from theoptimum water content and the correlation of the parametersof the model with the liquid limit of the soil.

As drying–wetting tests are time consuming, the latterpoint is important for the practising engineer who needs a

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1342 Can. Geotech. J. Vol. 39, 2002

Fig. 1. Vertical heterogeneity of the properties of a compactedsoil in a Proctor mould (after Bouabdallah 1998).

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first estimation of the parameters very quickly, or at leasttheir order of magnitude, to start simulations or parametriccalculations. As usual, correlations will not replace a carefulexperimental determination of the properties, but they pro-vide a means of comparison and evaluation of the results.

Materials and methods

Tests were carried out on natural and artificial soils, aswell as on mixtures of clay and sand in various proportions.A few natural soils used in the study were obtained fromsites at the Chambon, La Verne, and Vieuxpré dams in France.On the other hand, Jossigny silt, Sterrebeek silt, and FoCaclay are reference soils extensively used as research materi-als in France and Belgium. Additional data come from theliterature in cases where the procedure followed was consis-tent with the approach used in the test program. The mainproperties of all of these soils are presented in Table 1. Thepreparation of the test specimens was carried out in threesteps as follows:

(1) In the first step, natural soils were air-dried, and theaggregates were carefully crushed with a rubber mallet toavoid breaking the grains. The crushed soil was passedthrough a 2 mm sieve to eliminate the biggest particles. Inthe case of sand-clay mixtures, dry clay and sand powderswere mixed up before adding water.

(2) In the second step, the required quantity of water wasadded to the soil and both were carefully mixed by hand.The soil-water mixture was kept in a sealed plastic bag for atleast 24 h to achieve uniform moisture conditions.

(3) In the third step, the materials were dynamically com-pacted to the corresponding dry density of the standard ormodified Proctor optimum (i.e., using the required standardenergy in a Proctor or California bearing ratio (CBR) mould).

For comparison purposes, specimens of two materials(Vieuxpré and FoCa clays) were statically compacted in asmaller mould (40 mm in diameter, 40 mm in height) usingidentical soil–water mixtures. These specimens were pre-pared by progressively loading them in a loading frame, atstrain rates lower than 3% per minute, to values of 1 or5 MPa. Several studies have shown that to be equivalent tostandard and modified Proctor conditions, respectively(Subbarao 1972; Fry 1977).

A water content equal to one and a half times the liquidlimit value of the soil was added to prepare slurry speci-mens. Mixing was carried out by means of a mechanicalmixer of the type used for the preparation of bentonites forgrouting.

For the tests on consolidated specimens, consolidation wasperformed directly on the slurry in an oedometer (40 mm indiameter and 60 mm in initial height), in the usual way, i.e.,by doubling the load at each step, starting from a small ver-tical stress (≈ 4 kPa) up to the maximum chosen stress.

After compaction or consolidation, the specimens werekept in their mould and put again in a sealed plastic bag forat least one week. When a saturated consolidated specimenis unloaded, the change in external confining stress results ina decrease in its pore pressure that may become negativeand, depending on the plasticity of the soil and the value ofthe consolidation stress, may lead to its desaturation. Com-pacted specimens are partially saturated and their pore-water

pressure is obviously negative. In most cases, a calibratedfilter paper (Whatman 42) was used to obtain the initial neg-ative pore-water pressure of the soil at the end of the rest pe-riod. This value is derived from the water content of the filterpaper, measured with a very precise force transducer (10–6 N),by reference to a calibration curve (ASTM standard D5298–94, ASTM 1995c). The calibrated filter papers were placedin the specimens during compaction, protected on both sidesby ordinary filter papers, between two soil layers. It must benoted that the filter papers were directly in contact with thesoil, thus measuring only the matrix negative pore-waterpressure. In the case of Sterrebeek silt, the negative pore-water pressure measurements were made with a thermocou-ple psychrometer (Verbrugge 1974, 1978, 1979).

Immediately before the drying or wetting tests, the largecompacted or consolidated specimens were cut into smallerspecimens (2–3 cm3), which were put in the different de-vices used for these tests. In the tests, the specimens pass inone step from their initial state to the final applied negativepore-water pressure. This procedure was shown to be equiv-alent to the technique where the specimen is subjected toseveral successive steps of negative pore-water pressure. Thedrawback of this method is that each point corresponds to adifferent specimen, which gives an increase in the scatter ofdata, but offers the advantage of shorter test duration.

Several techniques were used to control the negative pore-water pressure in the soil specimens (i) tensiometric plateswere used to achieve low soil suction values, between 0.1and 30 kPa; the specimens were placed on sintered glass fil-ters, and a negative pressure was applied to the water, the airpressure being atmospheric; (ii) osmosis was used to achieveintermediate soil suctions, between 0.1 and 1.5 MPa. In theosmotic technique, Visking dialysis membranes from UnionCarbide, with very small pores (50 nm), were placed be-tween the specimens and a solution of polyethylene glycol20000 (PEG) (from Merck Laboratories) to prevent the pas-sage of macromolecules. As the macromolecules tend to hy-drate and attract water from the soil, the specimen wassubjected to negative pore-water pressure, which dependedon the PEG concentration in the solution; (iii) to achievehigh suctions, between 2 and 400 MPa, the relative humidityin the soil specimens was controlled by salt solutions, usingcopper and zinc sulphate, potassium, sodium and calciumchloride, and sulphuric acid as salts. In the two former meth-ods, a good contact between the specimen and the solution isnecessary, as liquid water is transferred from one to theother. In the case of compacted specimens, whose surfacemay be rough, a thin layer of kaolinite slurry is placed be-tween the specimens and the membranes to ensure this con-dition. Details of the experimental techniques are given, forinstance, in Biarez et al. (1988), Verbrugge (1975), andVerbrugge and Fleureau (2002).

While the tensiometric plate and osmotic devices controlmatrix negative pore-water pressure only, the salt solutionstechnique controls the total negative pore-water pressure.Since the osmotic component of the negative pore-waterpressure is weakly dependent on water content, while thematrix component increases rapidly when the water contentdecreases, the difference between the total and the matrixnegative pore-water pressure should become smaller andsmaller as the water content decreases (Vanapalli et al. 1999).

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Fleureau et al. 1343

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Once the capillary equilibrium was reached (generally af-ter one week for negative pore-water pressures lower than1.5 MPa and two months for larger values of uc) the finalproperties of the specimens were measured. The specimenswere weighed then immersed in a nonwetting oil of knowndensity (commercial kerdane, from BP); their external vol-ume was derived from the difference between the initialweight in air and the apparent weight when immersed inkerdane. Finally, their dry weight was measured after theevaporation of both water and oil in an oven at 105°C for24 h and used to calculate the water content, void ratio, anddegree of saturation.

Description of the drying and wetting pathsof compacted soils

The simultaneous changes in void ratio, water content,and degree of saturation versus negative pore-water pressureand water content are represented in five corresponding dia-grams, to highlight the correspondence between the curvesand the different phases in the behaviour of the soil. The de-scription of the drying and wetting paths on normally con-solidated (NC) or overconsolidated (OC) soils has been

developed in previous papers for a large number of clayeysoils (Biarez et al. 1988; Fleureau et al. 1993, 1999). In thispaper, the drying and wetting paths of slurry specimens areused as references to analyse the behaviour of compactedspecimens, and their main properties will be briefly summa-rized below. Figures 2 and 3 show the drying and wettingpaths of a slurry of Jossigny silt (wL = 37%) and a slurry ofFoCa smectite (wL = 90%), respectively, both prepared at aninitial water content wi = 1.5 wL.

In the [log(uc), e] diagram (Fig. 2b), representing thecompressibility behaviour of the soil under the effect of thenegative pore-water pressure, the drying path shows two dis-tinct phases: (i) a first phase, similar to that of a saturatedsoil, as evidenced by the parallelism between the drying pathand the oedometric or isotropic compression paths on satu-rated specimens, with large plastic strains; and (ii) a secondphase where the soil becomes quasi-rigid and behaves elasti-cally. The transition negative pore-water pressure betweenthe two domains, ucSL (1500 kPa for Jossigny silt), is termedthe “shrinkage limit negative pore-water pressure”. Thispressure plays an important part in modelling the behaviourof the soil as it corresponds to a drastic change in its proper-ties (Modaressi et al. 1996; Kohgo 2002). At the micro-

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MaterialwL

(%)wP

(%)IP

(%) Gs

< 2 µm(%)

wSL

(%)Conditions ofcompaction

wopt

(%)γdmax

(kN/m3)ucopt

(kPa)

Talybont silta na na 4 dyn. SPO 9.5 3030% Jos. + 70% sand 17 10 7 2.68 5 dyn. SPO 9.8 19.497% sand + 3% kaolin 18 3.5 14.5 2.65 7 dyn. MPO 14 16.950% Jos. + 50% sand 19 10 9 2.69 9 11 dyn. SPO 10.4 18.6 3170% Jos. + 30% sand 20 11 9 2.71 12 12.2 dyn. SPO 11.5 18.4 6330% London clay + 70% sandb 24 18 6 na dyn. MPO 14 18.2 166Chambon till 26 19 7 2.70 4 17 dyn. SPO 12.6 18.4 150Sterrebeek loam 27 20 7 2.70 13 22 dyn. MPO 9.9 18.8 35390% Jos. + 10% sand 29 14 15 2.73 17 12.5 dyn. SPO 15 17.2 100Vieuxpré clay 32 19 13 2.67 36 18.5 stat. SPO

stat. MPOSelset claya 33 16 17 na 20 dyn. SPO 11.2 350LaVerne clay 35 19 16 2.71 2 28.5 dyn. SPO 16.5 17.9 48Jossigny loam 37 16 21 2.74 35 15 dyn. SPO 18 17.4 200Mangla shalea 38 21 17 na 21 dyn. SPO 16.2 270P300 kaolin 40 20 20 2.65 60 22 stat. SPO 24 15.7 166

dyn. MPO 17.2 17 50150% London clay + sandb 40 17 23 na dyn. MPO 14 18.1 1106Mirgenbach clayc na na dyn. SPO 2280% kaolin + 20% sand 52 24 28 2.65 68 dyn. MPO 18.9 16.6 125970% London clay + sandb 54 19 35 na dyn. MPO 17 17.5 1371White kaolinite 61 30 31 2.65 85 32 stat. SPO 28 14.2 17090% London clay + sandb 69 24 45 na dyn. MPO 21 16.5 1745London clayb 77 29 48 2.67 dyn. MPO 23.5 15.7 180480% bentonite + sand 82 31 51 2.73 60 dyn. MPO 21 16.3 2512FoCa interstratified clay 90 35 55 2.68 dyn. SPO 32.5 13.0 2000

stat. 30 MPa 10 18.8 5×105

FVO montmorillonited 164 64 100 na stat. SPO 28 14.6 3524Ca-montmorillonite 170 60 110 2.74 40 25 stat. SPO 40 11 3000

Note: na, not available; Jos., Jossigny silt; stat., static; dyn., dynamic.aData from Bishop et al. (1964).bData from Marinho and Chandler (1993).cData from Nanda et al. (1993).dData from Guiras-Skandaji (1996).

Table 1. Main properties of the soils used in the study.

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scopic level, the change from plastic to elastic behaviourreveals a decrease in the obliquity of the intergranular forceswith respect to the normal to the contact planes between thegrains. It may be assumed that this change is associated witha pendular state of water, which becomes mainly discontinu-ous, and is located in the menisci around the contact points.

In the [log(uc), S] diagram (Fig. 2d), representing thechanges in degree of saturation versus suction, there is aclear change from the saturated to the unsaturated domainwhen uc exceeds the “desaturation negative pore-water pres-sure” ucd (800 kPa for Jossigny silt), otherwise termed theair entry value. From this diagram, we can also derive thevalue of the “residual degree of saturation”, Sr, correspond-ing to the degree of saturation at which the liquid phase be-comes discontinuous, i.e., for uc = ucSL. The direct derivationof Sr is often difficult in slurried soils and expansive clays(Vanapalli et al. 1999), while it appears quite straightforward

using the [uc, e] diagram. This value is important to obtainthe fitting parameters to model the SWCC (van Genuchten1980; Fredlund and Xing 1994).

The third [w, e] diagram (Fig. 2a) represents the shrink-age behaviour of the soil, with void ratio as a measure ofvolume change. The shrinkage limit wSL corresponds to theintersection between the saturation line e = (γ s/γ w)w andthe horizontal asymptote of the curve when w tends to-wards 0. The corresponding value of the negative pore-water pressure is ucSL.

In the last two diagrams, in the [w, S] (Fig. 2c) and[log(uc), w] (Fig. 2e) planes, both compressibility and satu-ration parameters are combined in water content changes.

The same general behaviour is observed in Fig. 3 in thecase of a highly plastic clay. The parameters ucSL and ucd areboth approximately equal to 30 MPa. Compared to Jossignysilt, the plastic and elastic strains are much more important

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Fig. 2. Drying and wetting paths on a saturated slurry of Jossigny silt.

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(Fig. 3b), the saturated domain is very large (Fig. 3d), and thetransition to the unsaturated domain is not so obvious. Thereason for these differences lies in the deformability of theclay sheets and the fabric of the smectite, which look ratherlike a “mille feuilles,” where the simple picture of a mediummade of grains is no longer appropriate (Van Damme 2002).

As a consequence of the analogy between compressibilityand drying paths when the soil is saturated or quasisaturated,it is possible to obtain the drying path using the referencelines derived from correlations with the liquid limit proposedby Biarez and Favre (1975)

w = wL or e = (γ s/γ w)wL for uc = 7 kPa

w = wP or e = (γ s/γ w)wP for uc = 1000 kPa

Figures 2b and 3b show that these lines, plotted in the[log(uc), e] and [log(uc), w] coordinate systems are in good

agreement with the drying path of the slurry, which cantherefore be called the “NC drying path”.

When the soil is consolidated, the drying paths are similarto overconsolidated compression paths. Figure 4 representsthe drying paths of specimens of Jossigny silt consolidatedin an oedometer under stresses of 0.1, 1, and 3 MPa; thepath on the compacted specimens has also been reported inthis figure, while the drying–wetting cycle on the slurryspecimens is represented as a dashed line. The drying testswere carried out on specimens first saturated to a 0.1 kPanegative pore-water pressure, and the wetting tests, on speci-mens dried to a 400 MPa negative pore-water pressure. Forthe smallest consolidation stress, the consolidated path inter-sects the NC path when it is still saturated and the de-saturation negative pore-water pressures and shrinkage limitsare the same for both paths. On the contrary, for the largestconsolidation stress, the consolidated path does not meet the

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Fig. 3. Drying and wetting paths on a saturated slurry of FoCa clay.

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NC drying path and presents a higher desaturation negativepore-water pressure and shrinkage limit. The path corre-sponding to the 1 MPa consolidation stress lies between thetwo others, as its intersection with the NC path occurs just atthe beginning of the shrinkage plateau. The wetting path ofthe specimens consolidated to 1 MPa, starting from a com-pletely dry specimen, which was plotted in that case, showsthat the drying and wetting paths are nearly superimposed.The compacted specimens were prepared at the Proctor opti-mum water content and maximum density, corresponding toa negative pore-water pressure of approximately 200 kPa:the points corresponding to larger values of uc belong to thedrying path and those corresponding to smaller values of ucbelong to the wetting path.

In the case of the smectite (Fig. 5), the negative pore-water pressure at the Proctor optimum water content is

2 MPa and a second drying–wetting cycle was carried out onthe material. The wetting path of the specimens isotropicallycompacted to 30 MPa is also plotted in the figure.

Comparing the paths on slurry and consolidated or com-pacted specimens leads to the following observations (Figs. 4and 5):

(1) While the paths on the slurries present large ir-reversibilities, both in terms of volumetric variations and satu-ration, the paths on compacted soils are fairly reversible. Thesecond drying–wetting cycles on the specimens of Jossignysilt consolidated to 1 MPa (Fig. 4) and on compacted speci-mens of FoCa clay (Fig. 5) confirm this observation.

(2) The wetting paths of the compacted soils starting fromthe Proctor optimum water content and maximum densityare linear, both in the [log(uc), e] and the [log(uc), w] coordi-nate systems. This is generally true for most clayey soils (re-

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Fig. 4. Comparison between drying and wetting paths on a slurry of Jossigny silt and on specimens compacted to standard Proctor optimum.

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Fig. 5. Comparison between drying and wetting paths on a slurry of FoCa smectite and on specimens compacted to standard Proctoroptimum and high stresses.

Fig. 6. Correlation between the liquid limit and the optimumwater content and maximum dry unit weight of soils compactedat the standard or modified Proctor optimum (open symbols referto data from the literature).

Fig. 7. Correlation between the liquid limit or the optimumwater content and the negative pore pressure of soils compactedat the standard or modified Proctor optimum (open symbols referto data from the literature).

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fer to the section titled “Description and modelling of thewetting paths of different soils, starting from the SPO orMPO” and Figs. 8 and 9).

(3) In the case of Jossigny silt (Fig. 4) and FoCa clay(Fig. 5), the wetting paths of the specimens compacted tothe Proctor optimum water content and maximum densityare close to the wetting paths of the slurries and, in thesecond case, are nearly superimposed on the wetting pathof the specimens compacted under high stresses. In fact,these paths depend on many parameters, among which theequivalent consolidation stress induced by drying or com-paction and the compaction parameters (water content,density, negative pore-water pressure, etc.) play an impor-tant part.

Finally, the broad similarity between the behaviour ofcompacted and consolidated soils must be considered care-fully, as the observations of several investigators show thatthe texture of the soil is very different in both cases (Cui1993; Alonso et al. 1987; Schreiner 1991; Guillot et al.2001). Grain sizes are more uniform in the case of speci-mens prepared by consolidation of a slurry, while the parti-cles tend to gather in aggregates in compacted materials.This difference is more visible in the [log(uc), S] plane,where the fabric of the soil plays a greater role.

The liquid limit is generally considered as a good classi-fication parameter for clayey soils (wL > 20%) (Casagrande1948; Terzaghi and Peck 1967). It is therefore this parame-ter that will be used in this study to express the interrela-tionships between the different variables.

Correlation between the properties ofclayey soils at standard and modifiedProctor optimum and the liquid limit

Figure 6 shows correlations between the optimum watercontent and the maximum dry unit weight of the soils, com-pacted in the standard or modified Proctor conditions (SPOand MPO, respectively), and their liquid limit. There is ageneral agreement between these correlations and those pre-viously proposed by other authors (Ring et al. 1962; Biarezand Favre 1975; Soriano and Sanchez 1995, for soils com-pacted at the SPO, and Kheirbek-Saoud 1994, for soils com-pacted at the MPO). The changes in the negative pore-waterpressure at SPO and MPO (on a log scale) versus the liquidlimit or the optimum water content have been plotted inFig. 7.

As a first approximation, the data were fitted using straightlines or parabolas. In the case of the negative pressures, abetter correlation was obtained using power functions. Thecoefficients of regression r2 indicated below are generallylarger than 0.86, with the exception of the dry density ofsoils at the modified Proctor optimum (for which r2 = 0.77).The reason for this is probably the small number of data un-der those conditions.

The equations of the regression lines are (with the liquidlimit in percent) as follows:

At the SPO

[2] wopt = 1.99 + 0.46 wL – 0.0012wL2 r2 = 0.94

© 2002 NRC Canada


Fig. 8. Wetting paths of different soils compacted at the Proctor optimum.

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[3] ucopt = 0.118 (wL)1.98 r2 = 0.88

[4] γ dmax = 21.00 – 0.113 wL + 0.00024wL2

r2 = 0.86

At the MPO

[5] wopt = 4.55 + 0.32 wL – 0.0013wL2

r2 = 0.88

[6] ucopt = 1.72 (wL)1.64 r2 = 0.88

[7] γ dmax = 20.56 – 0.086 wL + 0.00037wL2

r2 = 0.77

Description and modelling of the wettingpaths of different soils, starting from theSPO or MPO

As mentioned before, the wetting paths present an excel-lent linearity in the [log(uc), e] and [log(uc), w] coordinatesystems in the range of negative pressures investigated. Thewetting path of a soil can be defined by its initial point (theSPO or the MPO) and the slope of the line, i.e., five parame-ters: the coordinates of SPO (or MPO) in the [uc; e; w] spaceand the slopes of the lines, also called swelling indices, withrespect to void ratio, Cms, and water content, Dms. The wet-ting paths were plotted for 14 soils compacted at the SPO(Fig. 8) and 5 compacted at the MPO (Fig. 9) On the twographs, there is a clear classification of the lines with the liq-uid limits of the soils when considering both the slope andthe position of the lines.

The values of Cms and Dms versus their liquid limits forspecimens compacted either at the SPO or MPO were plot-ted in Fig. 10. Linear or quadratic fits have again been cho-sen for simplicity, with the following equations (with wL inpercent):

For wetting paths starting from the SPO

[8] Ceu

w wmsc

L L20.0018= − = + − + × −∆

∆[log( )].0029 5 10 6

r2 = 0.97

© 2002 NRC Canada


Fig. 9. Wetting paths of different soils compacted at the modi-fied Proctor optimum.

Fig. 10. Swelling indices with respect to void ratio and watercontent for different soils compacted either at the standard ormodified Proctor optimum.

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[9] Dwu

w wmsc

L L20.54 0.030= − = − − + × −∆

∆[log( )].33 10 6

r2 = 0.85

For wetting paths starting from the MPO

[10] Ceu

wmsc

L0.0040 0.0019= − = + −∆∆[log )]

r2 = 0.74

[11] Dwu

wmsc

L1.46 0.051= − = − −∆∆[log( )]

r2 = 0.40

The last value of r2 indicates the absence of correlation,again due to the scarcity of data.

Using the coefficients derived from these correlations fora given value of the liquid limit, it is possible to normalizethe void ratio or the water content of the soil. As an exam-ple, the normalized water contents w/Dms versus negativepore-water pressure for the paths starting from the SPO andMPO have been plotted in Fig. 11. The graphs show thevalidity of the liquid limit as a primary classification param-

eter, without excluding the use of another secondaryparameter that would reduce the scatter of data.

Influence of the compaction water contenton wetting paths

As compaction is often carried out in practice at watercontents different from the optimum, e.g., in dams or roadembankments, the influence of this parameter on the dryingand wetting paths has been examined. Compaction at differ-ent water contents results in different structures of the soil(Cui 1993; Gens et al. 1995; Vanapalli et al. 1999). Mitchell(1976) considered two levels of particle associations: themicrostructure level, consisting of the association of elemen-tary particles, and the macrostructure level, representing theassociation of aggregates. Cui (1993), after observing with ascanning electron microscope (SEM) the arrangement ofgrains within specimens compacted to the same dry density,but at different water contents, concluded that the macro-structure level was predominant in soils compacted dry ofoptimum, while the microstructure was more important insoils compacted at the optimum or at a higher water content.Vanapalli et al. (1999) carried out drying tests on specimenscompacted under Proctor conditions at three different watercontents and the corresponding densities of the Proctor curve.

© 2002 NRC Canada


Fig. 11. Normalized water contents for the wetting paths of soils compacted at the Proctor or modified Proctor optimum.

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The results show that the desaturation curve of the specimencompacted dry of optimum is noticeably different from thatof the specimen compacted at the optimum or wet of opti-mum, with little difference between the latter. At the samenegative pore-water pressure, the degree of saturation of thespecimen compacted dry of optimum is somewhat lowerthan that of the two others, which is in agreement with themore open structure revealed by the SEM observations. Asimilar behaviour should be observed during wetting.

To examine the role of the compaction water content,drying–wetting tests were performed on La Verne dam mate-rial compacted under standard Proctor conditions, at three dif-

ferent water contents, all on the wet side of optimum: wopt,wopt + 3% (conditions of laying in the dam), and wopt + 5%.For the drying paths, the compacted specimens were firstwetted to a 0.1 kPa negative pore-water pressure, while thewetting tests were carried out on specimens first dried to a400 MPa negative pore-water pressure. The results areshown in Fig. 12, where the data obtained on drying paths areindicated by full symbols and the data on wetting paths, byempty symbols. The reversibility of the paths is again con-firmed. There is some scatter in the data but the points gener-ally appear close to a mean line, independently of the initialcompaction water content, especially in the [log(uc), e] and

© 2002 NRC Canada


Fig. 12. Influence of compaction water content on the drying and wetting paths of specimens of LaVerne material (full symbols are forthe drying paths; open symbols are for the wetting paths).

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[log(uc), w] planes. In the [log(uc), S] plane, however, aslight difference appears between the paths; the path corre-sponding to the specimens compacted at the optimum being

located below that of the specimens at wopt + 3%, itself be-low that of the specimens at wopt + 5%. For water contentslarger than the optimum, the difference between the three

© 2002 NRC Canada


Fig. 13. Compaction curves and changes in negative pore-water pressure with water content for (a) two soils compacted in standardProctor conditions and (b) two soils compacted in modified Proctor conditions.

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paths remains very limited and it can therefore be con-cluded, as a first approximation, that this parameter plays nosignificant part in the behaviour of drying–wetting paths.

The same conclusion was drawn from tests on FoCasmectite compacted under 60 MPa isotropic stresse at twowater contents: 8 and 13%, whose wetting paths were alsovery close (Fleureau et al. 1998).

Comparison between wetting andcompaction paths

Measurements of the negative pore-water pressure, usingthe filter-paper method (Chandler and Gutierrez 1986), weremade on specimens compacted under Proctor or modifiedProctor conditions, at the optimum and also for other water

© 2002 NRC Canada


wL (%) wopt (%) γdmax (kN/m3) eopt ucopt (kPa) Cms Dms

White kaolin 61 25.6 15.0 0.79 404 –0.062 –2.358Jossigny silt – Hostun sand mixture 20 10.7 18.8 0.43 45 –0.005 –1.139

Table 2. Parameters of the model for the two materials.

Fig. 14. Comparison between the results of the model based on correlations with the liquid limit and experimental data for two soilscompacted to the Proctor optimum and wetted.

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contents. The corresponding values, represented in Fig. 13 inthe [w, uc] coordinate system, exhibit a linearity of the com-paction path in a large domain on both sides of the optimum,especially on the wet side. In most tests, the slope of the lineon the dry side is smaller than the slope on the wet side,which reflects the decrease in density below the optimum.

The comparison between the compaction and the wettingpaths starting from the SPO or the MPO shows that, for thesoils presented here, the data are very similar. Therefore, itis possible to derive the wetting path of a soil from measure-ments of its water negative pressure after compaction at dif-ferent water contents larger than the optimum.

As evidenced by Vanapalli et al. (1999), this conclusioncannot be extended to the dry side of the optimum, in spiteof the similar shapes of the curves. The difference can bemainly attributed to the different textures of the soil in thoseconditions.

Modelling of wetting paths for soilscompacted at the SPO

To check the validity of the model proposed in the previ-ous sections (“Correlation between the properties of clayeysoils at standard and modified Proctor optimum and the liq-uid limit” and “Description and modelling of the wettingpaths of different soils, starting from the SPO or MPO”),predictions of the wetting paths of two soils were derivedfrom the correlations and compared to the experimental data.The soils were chosen for their very different liquid limits:the white kaolin, with a liquid limit of 61%, and the mixtureof 70% Jossigny silt and 30% Hostun sand, with a liquidlimit of 20%. The parameters derived from eqns. [2], [3],[4], [8], and [9] are indicated in Table 2, the void ratio anddegree of saturation being calculated assuming a standard valueof 26.5 kN/m3 for the specific unit weight of grains, with

[12] e =26.5

1dγ

−

The equations of the straight lines in the [log(uc), e],[log(uc), S], and [log(uc), w] planes are therefore

[13] e = e Cu

uopt ms

c

copt

+

log

[14] w w Du

u= +

opt ms

c

copt

log

[15] Swe

= 0.027

Figure 14 shows that there is a general agreement betweenthe results of these calculations and the experimental pointsfor the two materials. In spite of some divergences, the or-ders of magnitude are correct with respect to all of the pa-rameters concerned. Therefore, it can be concluded that theapproach can be useful for engineers, even if the proposedcorrelations still need be improved by the addition of newexperimental data.

Conclusions

The aim of this paper was to give a simplified overview ofthe behaviour of compacted clayey soils on drying–wettingpaths and to provide correlations to model this behaviour.The lack of available data restricted this approach mainly tothe standard and modified Proctor optimum water contentand maximum dry density, but attention was also given tomaterials compacted on both sides of the optimum watercontent under the same stress (or energy) and, to a limitedextent, to soils compacted in other conditions.

Some interesting and useful features of these paths havebeen highlighted.

(1) The reversibility of the drying–wetting paths of soilscompacted under standard or modified Proctor conditionsand possibly, also, in other conditions was noted; however,there is probably a minimum compaction stress required toobserve the reversibility of the paths.

(2) It is possible to derive the wetting curve of a soil fromcompaction tests in which the negative pore-water pressureis measured.

(3) Soils compacted at a water content higher than the op-timum follow the same wetting paths as those compacted atthe optimum.

(4) The linearity of drying and wetting paths starting fromthe optimum when plotted against the logarithm of the nega-tive pore-water pressure was noted.

(5) The importance of the liquid limit as a classification pa-rameter was confirmed. Correlations have been establishedbetween the liquid limit and the main properties of the soils atthe SPO and MPO: water content, dry density, negative pore-water pressure, and swelling and drying indices.

It was shown with two examples of soils (white kaolinand a Jossigny silt – Hostun sand mixture) compacted to theProctor optimum water content and maximum dry density,that the model based on these correlations can provide a sat-isfactory, though simplified, description of the wetting paths.However, these correlations are based on tests of Europeansoils. One should be very careful when applying them toother types of soils, such as residual soils for instance; inthat case, the liquid limit may not be the relevant parameter,and the correlations may be different (Williams et al. 1983).

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List of symbols

Cms swelling coefficient with respect to void ratioDms swelling coefficient with respect to water content

e void ratioeopt void ratio at Proctor optimum water content and maxi-

mum densityGs specific density of grainsIP plasticity indexS degree of saturation

ua pore air pressureuc negative pore-water pressure or suction (uc = ua – uw)

ucd negative pore-water pressure of desaturationucopt negative pore-water pressure at Proctor optimum water

content and maximum densityucSL shrinkage limit negative pore-water pressure

uw pore-water pressurew water contentwi initial water content of specimens

wL liquid limitwopt Proctor optimum water content

wP plastic limitγdmax specific weight at Proctor optimum water content and

maximum densityγ s specific weight of grainsγw specific weight of water

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Aspects of the behaviour of compacted clayey soils on drying and wetting paths

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