Effect of compaction pressure on consolidation behaviour of unsaturated silty soil

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    Effect of compaction pressure on consolidationbehaviour of unsaturated silty soil

    A.R. Estabragh, A.A. Javadi, and J.C. Boot

    Abstract: The effect of compaction pressure on subsequent soil behaviour during isotropic consolidation has been in-vestigated by conducting controlled-suction triaxial tests on samples of an unsaturated compacted silty soil. A compre-hensive set of laboratory experiments was carried out in a double-walled triaxial apparatus on samples of unsaturatedsoil that were prepared using two different compaction pressures. The axis translation technique was used for creatingthe desired suctions in the samples. In the experiments, the soil samples were subjected to isotropic consolidation un-der constant suctions. The results show that different compaction pressures produce different fabrics in a soil and there-fore affect the behaviour of the soil. The results also show that the value of yield stress and the location of theloadingcollapse (LC) yield curve are functions of soil fabric. Furthermore, it is shown that the slopes of normal con-solidation lines for densely and loosely compacted samples differ in unsaturated conditions but are the same in satu-rated soils. A comparison is made between the behaviour of the dense and loose samples, and the difference in thebehaviour is explained.

    Key words: suction, unsaturated soil, compaction, consolidation, soil fabric.

    Rsum : On a tudi leffet de la pression de compactage sur le comportement subsquent du sol durant une conso-lidation isotrope au moyen dessais triaxiaux succion contrle sur des chantillons dun sol limoneux non saturcompact. On a ralis un vaste ensemble dexpriences en laboratoire dans un appareil triaxial double paroi sur deschantillons de sol non satur qui ont t prpars en utilisant deux diffrentes pressions de compactage. La techniquede translation daxe a t utilise pour crer les succions dsires dans les chantillons. Dans les expriences, leschantillons de sol ont t soumis une consolidation isotrope sous des succions constantes. Les rsultats montrent quedes pressions diffrentes de compactage produisent diffrentes fabriques dans un sol, et en consquence, influence lecomportement du sol. Les rsultats montrent galement que la valeur de la limite lastique et la localisation de lacourbe limite deffondrement (LC) sont fonction de la fabrique du sol. De plus, on montre que les pentes des lignes deconsolidation normale pour les chantillons compacts dans des tats denses ou meubles (qui seront appels ici denseset meubles respectivement) diffrent dans les conditions non satures, mais elles sont les mmes dans les sols saturs.On a fait une comparaison entre le comportement des chantillons denses et meubles, et la diffrence de comportementa t explique.

    Mots cls : succion, sol non satur, compactage, consolidation, fabrique du sol.

    [Traduit par la Rdaction] 550

    Estabragh et al.1. Introduction

    Compacted soils are commonly used in the constructionof several soil structures. The behaviour of compacted unsat-urated soil is not fully understood because of its unsaturatedstate. An understanding of the mechanical behaviour of

    these unsaturated soils is therefore vital for the effectivedesign and analysis of many foundations, slopes, embank-ments, and retaining structures. There are relatively few ex-perimental data on the mechanical behaviour of compactedunsaturated soils, and therefore more information is neededin this respect for a better understanding of compacted un-

    Can. Geotech. J. 41: 540550 (2004) doi: 10.1139/T04-007 2004 NRC Canada

    540

    Received 19 April 2002. Accepted 8 December 2003. Published on the NRC Research Press Web site at http://cgj.nrc.ca on 2 July2004.

    A.R. Estabragh1 and A.A. Javadi. Department of Engineering, School of Engineering, Computer Science and Mathematics,University of Exeter, Exeter EX4 4QF, Devon, UK.J.C. Boot. Department of Civil and Environmental Engineering, University of Bradford, Bradford, West Yorkshire, BD7 1DP, UK.

    1Corresponding author (e-mail: A.R.Estabragh@exeter.ac.uk).

  • saturated soils and their behaviour. Over the past 10 years asignificant amount of research has been aimed at under-standing the behaviour of unsaturated soils. It is generallyaccepted that the mechanical behaviour of unsaturated soilsdoes not obey the principle of effective stress (Jennings andBurland 1962; Wheeler and Karube 1995) and requires theuse of two independent stress state variables. In the three-dimensional case, four stress parameters are required to ex-plain the stress state at any point within an unsaturated soil.For axisymmetric conditions the number of stress parame-ters reduces to a total of three, which are usually chosen asmean net stress (p), deviator stress (q), and suction (s) anddefined as

    [1] p u = + 1 323

    a

    [2] q = 1 3

    [3] s = ua uw

    where 1 and 3 are the axial and radial total stresses, re-spectively; and ua and uw are the pore-air and pore-waterpressures, respectively.

    The fabric or structure of a soil consists of different parti-cles that join together to form the mass of the soil. The fab-ric of compacted soils is generally explained in terms of thecompaction conditions and can therefore be estimated byspecifying a few control variables, namely compaction watercontent with regard to the optimum, energy of compaction(or attained density), and compaction method. It is generallyaccepted that the role of fabric in the behaviour of soils ismore important for unsaturated soil than for saturated soil(Alonso et al. 1987). In unsaturated soils the fabric controlsthe conditions of water, especially its suction potential. Infact, mineralogical composition affects the adsorption com-ponent of matric suction, and the internal geometry controlsthe capillary component (Alonso et al. 1987). The knowl-edge of the fabric of unsaturated soils plays an importantrole as an aid to understanding its mechanical response andto facilitating qualitative predictions of different environ-mental factors such as the effect of changes in pore-waterchemistry. Gens (1995) explained some aspects of the com-paction procedure, such as the compaction water content andcompaction pressure, which have a significant influence onthe subsequent mechanical behaviour of compacted fine-grained soils. The influence of compaction procedure onsubsequent mechanical behaviour is commonly attributed tothe different forms of soil fabric that are produced when thecompaction procedure is varied (Seed and Chan 1959;Barden and Sides 1970). Although the advent of direct ob-servational methods such as the scanning electron micro-scope or porosimeters has modified significantly theaccepted ideas on compacted soil fabric, behaviour differ-ences are still generally attributed to different initial soil fab-rics set up by the compaction process. It is often implied thatthese changes in soil fabric are of such fundamental impor-tance to the subsequent soil behaviour that different compac-tion procedures effectively produce entirely different soils.This would mean that some or all of the soil constants in anelastoplastic model would take different values dependingon the compaction procedure (Sivakumar and Wheeler

    2000). Many authors have reported on the basis of micros-copy that substantial differences in the fabric of compactedfine-grained soil can result from change in compaction watercontent. Delage et al. (1996) suggested that there are twolevels of soil fabric (microfabric and macrofabric) for siltysoils when compacted dry of optimum. They indicated thatthe most important differences occur in the macrofabric.There is a question of whether the influence of compactionprocedure can be explained entirely on the basis of theinitial state of the soil or different compaction procedureseffectively lead to different soils. Alonso et al. (1992) origi-nally argued, on the basis of experimental data reported byLawton et al. (1989, 1991), that the influence of compactionprocedure could be explained entirely in terms of the effecton initial state. Subsequent work suggested, however, thatthe value of specific volume () achieved during compactionalso affects soil model parameters such as slope of the nor-mal consolidation line ((s)) and elastic swelling index (k)(Alonso et al. 1995). Gens (1995) indicated that any effectsof the compaction procedure on the mechanical behaviourwhich cannot be explained by variation in the initial soilstate are presumably attributable to difference in soil fabric.

    1.1. ConsolidationThe application of load to an unsaturated soil sample will

    result in the generation of excess pore-air and pore-waterpressures. The excess pore pressures will dissipate with timeand will eventually return to their original values beforeloading. The dissipation process of pore pressure is calledconsolidation and results in a volume decrease or settlement(Fredlund and Rahardjo 1993). In saturated soils, the instan-taneously applied total stress is first supported by the porewater and the soil skeleton is progressively loaded duringpore-pressure dissipation.

    Barden and Sides (1970) developed a modified Rowe cellto conduct controlled-suction one-dimensional consolidationtests on unsaturated compacted soils. The suction was con-trolled or measured using the axis translation technique.Subsequently, this type of apparatus was successfully usedby many other researchers such as Fredlund and Morgen-stern (1977) and Escario and Juca (1989).

    In oedometer testing, the soil sample is laterally confinedand therefore any movement of the sample takes place verti-cally, making it easier to measure volume change of an un-saturated sample than in a triaxial apparatus. However, theconsolidation characteristics of an unsaturated soil are bestobtained from triaxial tests, which can give the initial porepressures and the volume change under undrained conditions(Smith and Smith 1998). Sivakumar (1993), Zakaria (1994),and Cui and Delage (1996) conducted isotropic consolida-tion tests on unsaturated soil samples in a triaxial cell. Inconsolidation tests, each soil sample was compressed iso-tropically to a virgin state by increasing the mean net stress(p) while holding the suction (s) constant. The mean netstress (p) is usually increased by increasing the cell pressure(3) while the suction is held constant by keeping the air andwater pressures constant. Barden et al. (1969) examined theeffect of the size of the stress increment ratio when adoptingthe method of step loading. They concluded that a highstress increment ratio always caused greater compressionthan when the same total increment of load was applied in a

    2004 NRC Canada

    Estabragh et al. 541

  • larger number of smaller increments. When an increment ofexternal load is applied to a sample, excess pore-air andpore-water pressures will be generated within the sample.Any excess pore-air pressure that is generated will be dissi-pated very quickly due to the relatively high value of airpermeability (ka), and therefore the mean net stress (p) risesalmost immediately to its final value throughout the entiresample. The excess pore-water pressure, however, will takeconsiderable time to dissipate to the water back pressurevalue because of the relatively low value of water perme-ability (kw). This means that the suction increases in thesample gradually over the period of consolidation. Cui andDelage showed that a standard oedometer step-loading pro-cedure is not suitable for unsaturated soils and should not beused for investigating compressibility properties under con-trolled suction. They concluded from the test results that forunsaturated soils this procedure overestimates the coefficientof compressibility ((s)) and underestimates the value ofyield point and is invalid.

    Sivakumar (1993) explained this behaviour of unsaturatedsoils during consolidation with the help of the yield locusproposed by Alonso et al. (1990). He investigated the move-ment of the yield curves for the top and bottom of a triaxialsample during the application of a step increment in cellpressure and during the subsequent consolidation period. Atthe bottom of the sample, the pore-water pressure dissipatesquickly to the water back pressure and the soil reaches thefinal equilibrium on the related yield curve. At the top of thesample, the excess pore-water pressure occurs and, as it dis-sipates slowly during consolidation, the state of the soil atthe top of the sample will be in the elastic region and awayfrom the virgin state. The amount of excess pore-water pres-sure generated at the top of the sample could be minimizedby applying the external load slowly to allow the excesspore-water pressure at the top of the sample to dissipate dur-ing loading. This could be done by ramping the cell pressureat a rate such that the excess pore-water pressure at the topof the sample is kept within acceptable limits. An additionaladvantage of using ramped consolidation is that it gives acontinuous plot of specific volume () versus mean net stress(p) at a given suction. This plot can be used to identify thepreconsolidation pressure (pc ) and the slope of the subse-quent normal compression and unloading lines.

    In this paper, the effects of compaction pressure on theconsolidation behaviour of unsaturated soils are studiedthrough a comprehensive set of experiments carried out onsamples of a compacted silty soil. The samples were pre-pared by static compaction with two different compactionpressures. In what follows, the experimental procedure andresults are presented and discussed. A comparison is madebetween the behaviour of dense and loose samples, and thedifference in the behaviour is explained.

    2. Experimental study

    2.1. Soil propertiesThe soil used in the testing program was a silty soil with

    low plasticity. The soil comprised 5% sand, 90% silt, and5% clay and had a liquid limit of 29% and plasticity indexof 19%. The optimum water content in the standard compac-tion test was 14.5% and the maximum dry density was

    1.74 Mg/m3. According to the Unified Soil ClassificationSystem (USCS), the soil can be classified as silt with lowplasticity (ML).

    2.2. Sample preparationThe test program included a number of consolidation tests

    on samples of dense and loose soil. All of the samples (bothdense and loose) were prepared by static compaction at thesame water content but with two different compaction pres-sures. The samples were 38 mm in diameter and 76 mmhigh and were prepared at a water content of 10% (4.5% lessthan the optimum water content as determined by the stan-dard compaction test). To adequately ensure uniform and re-peatable density, preliminary static compaction tests werecarried out by preparing samples in one, three, six, and ninelayers in a compression frame. Compaction was carried outat a fixed displacement rate of 1.5 mm/min to maximum ver-tical total stresses of 1600 and 400 kPa for dense and loosesamples, respectively. For each method of static compactionand sample preparation, the dry density was...

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