One-dimensional consolidation behavior ofcement-treated organic soil

Antonio Bobet, Joonho Hwang, Cliff T. Johnston, and Marika Santagata

Abstract: This paper addresses the effects of cement treatment on the one-dimensional (1D) consolidation behavior of ahighly organic soil (LOI 40%60%, where LOI is the loss on ignition), based on 1D constant rate of strain and incremen-tal loading tests. The effects of Portland cement addition are evaluated for dosages ranging from 8% to 100% by dry massof soil, corresponding to values of the cement factor of 24 and 296 kg of cement per cubic metre of untreated soil, withinthe range used in deep mixing practice. Additional parameters investigated are the impact of curing surcharge and duration.The most evident effect of the treatment is the development of a cementation-induced preconsolidation stress: the greater thecement dosage, the greater the preconsolidation stress and the greater the vertical effective stress that can be sustained atany void ratio. The results also provide a consistent picture of the effects of cement treatment on stiffness, hydraulic conduc-tivity, coefficient of consolidation, and creep. Comparison to data obtained for the untreated soil demonstrates the stablenature of the structure generated as a result of treatment. The consolidation results are complemented by pH measurements,extraction tests, elemental analyses, and Fourier transform infra-red (FTIR) spectroscopy analyses, which provide insightinto the interaction between soil organic matter and cement.

Key words: peat, organic matter, cement treatment, deep mixing, consolidation, creep.

Rsum : Cet article discute des effets du traitement au ciment dun sol hautement organique (LOI 4060 %, o LOI est la perte au feu) sur son comportement en consolidation unidimensionnelle (1D), partir dessais en chargement en 1D taux de dformation constant et incrmental. Les effets de lajout de ciment Portland sont valus pour des dosages variantde 8 100 %, ce qui correspond des valeurs de facteur de ciment de 24 et 296 kg de ciment par mtre cube de sol nontrait, ceci tant des valeurs typiquement utilises dans le mlange en profondeur. Les paramtres additionnels qui sont tu-dis sont limpact de la surcharge durant le curage et sa dure. Leffet le plus vident du traitement est le dveloppementdune contrainte de pr-consolidation induite par la cimentation : plus le dosage de ciment est leve, plus la contrainte depr-consolidation est leve, et une plus grande contrainte effective verticale peut tre soutenue pour tout indice des vides.Les rsultats fournissent aussi une image des effets du traitement au ciment sur la rigidit, la conductivit hydraulique, le co-efficient de consolidation, et le fluage. Les rsultats obtenus sont compars des donnes provenant du sol non trait, cequi dmontre la nature stable de la structure gnre suite au traitement. Les rsultats de consolidation sont complmen-ts par des mesures de pH, des essais dextraction, des analyses lmentaires et des analyses de spectroscopie infrarouge transforme de Fourier ( FTIR ). Ces analyses fournissent des informations sur linteraction entre la matire organique dusol et le ciment.

Motscls : tourbe, matire organique, traitement au ciment, mlange en profondeur, consolidation, fluage.

[Traduit par la Rdaction]

IntroductionPeats and other soils high in organic matter are prone to

large settlements, which derive both from their high compres-sibility and the significant impact of secondary consolidation.As a result, construction on these soils poses significant chal-lenges to the geotechnical engineering profession. Several op-

tions are generally available to modify and improve theground conditions: strengthening of the foundation, elimina-tion of the problem soils, treatment of the problem soils, andrelocation of the project. In many cases, the only option isstrengthening the foundation or elimination of part of theproblem soils, as the other options are impractical or too ex-pensive. A widely used approach to the improvement of theengineering properties of soft soils is the deep mixingmethod (DMM), which was developed in Sweden and Japanin the late 1960s for treatment of soft soils. It was introducedin North America in the late 1980s and has been widely usedsince. Although still limited compared to Europe, there havebeen applications of this method in North America to treat-ment of organic soils; for example, the stabilization of a 3.5to 5 m thick organic silty clay deposit for construction of arailroad embankment in a section of the HudsonBergenLight Rail Transit System, New Jersey (Esrig et al. 2003),and the stabilization of a 2.5 to 7.5 m thick organic clay layer

Received 28 May 2010. Accepted 17 February 2011. Publishedat www.nrcresearchpress.com/cgj on 12 July 2011.

A. Bobet and M. Santagata. School of Civil Engineering,Purdue University, 550 Stadium Mall Drive, West Lafayette, IN47907, USA.J. Hwang. Asian Development Bank, 6 ADB Avenue,Mandaluyong City 1550, Philippines.C.T. Johnston. Department of Agronomy, Purdue University,915 W State Street, West Lafayette, IN 47907, USA.

Corresponding author: Marika Santagata (e-mail: mks@purdue.edu).

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Can. Geotech. J. 48: 11001115 (2011) doi:10.1139/T11-020 Published by NRC Research Press

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(with organic content from 4% to over 30%) for the I-95 high-way widening in Alexandria, Va. (Lambrechts et al. 2003).The principle of deep mixing is the mixing of hardening

agents (generally lime or cement) with soil in situ in con-trolled proportions to produce columns of hardened materialthat display higher strength and stiffness and generally lowerhydraulic conductivity than the original soil. Depending onthe configuration of the deep mixing system used, the binderagents can be introduced in slurry (wet method) or dry (drymethod) form. The binding agents are mixed with soil at thedistal end or along the drill shaft by pure rotation of the mix-ing augers or by a combination of rotation and injection ofthe binder in slurry form at high pressure (Bruce and Bruce2003). The properties of deep mixed soils are controlled by avariety of factors including: (i) physical and chemical proper-ties of the soil and groundwater, (ii) type and amount ofbinder, (iii) curing period, and (iv) mixing effectiveness. Todate, significant research has focused on investigating the fac-tors influencing the improvement of the strength of inorganicclays and the geotechnical properties of these soils after treat-ment. However, despite the considerable work performed inSweden over the years (e.g., hnberg and Holm 1999;hnberg et al. 2001; Holm 2005), the recent EuroSoilStabproject, and additional recent contributions exploring the ef-fectiveness of different binders (e.g., Hayashi and Nishimoto2005; Hernandez-Martinez and Al-Tabbaa 2005, 2009) andthe durability of the treatment (e.g., Butcher 2005), there re-main many questions on the effectiveness of this method intreating organic soils. They arise from concerns about the in-terference of organics with cement hydration reactions, andon the dissolution of organic matter in high pH environ-ments. These issues are compounded by the fact that theterm organic soils is generally used to refer to a broad setof geomaterials, with often contrasting geotechnical proper-ties. Moreover, assessments of the effectiveness of the treat-ment have, to date, typically focused on the measuredincrease in compressive strength (most commonly from un-confined tests) and, to a lesser degree (e.g., Cortellazzo andCola 1999; Tremblay et al. 2001; Hebib and Farrell 2003),on the one-dimensional (1D) compression behavior. Studieson other critical properties including coefficient of consolida-tion and creep coefficient remain very limited.The objective of this paper is to fill this void by presenting

results from an extensive experimental program of constantrate of strain (CRS) and incremental loading (IL) tests con-ducted on a soil with approximately 50% organic content.The data presented also include chemical data that provideinsight into the interaction of the soils organic matter withcement. The work complements the results presented by San-tagata et al. (2008), which focused on the 1D consolidationbehavior of the same organic soil in both the undisturbed (testson specimens obtained from block samples) and reconstitutedstate. While in the field the effectiveness of the treatmentwould be measured as improvement over the undisturbedfield conditions, the reconstituted results are used in this pa-per as the reference against which the changes in mechani-cal and chemical properties of the soil treated with cementare assessed. The choice of this baseline removes the effectsof the soils initial conditions, which are necessarily site de-pendent.

Experimental methods

MaterialsSoil for this research program was sampled at the south end

of the Celery Bog park in West Lafayette, In. Two blocksamples and several disturbed soil samples were collectedin April and July 2001 from the bottom of a 4.5 m diameterpit excavated to a depth of 2 m. Seven (samples S5S11) ofthe 11 disturbed samples collected were used for this study,and are referenced in this paper based on the sample num-ber. Borehole data (Earth Exploration, Inc. 1993) indicatethat the layer of organic soil from which the samples wereobtained is 34 m thick and underlain by marl and a siltyclay layer, and that the groundwater table lies in the upperpart of the organic soil layer. The surface material at thislocation is designated as a Houghton muck (USDA 1998).The soil is a highly organic (loss on ignition (LOI) in the

40% to 60% range), sapric (2% fiber content) soil withmildly acidic (5.65.9) pH. The inorganic fraction is entirelyfiner than 0.075 mm, with over 60% in the clay fraction,which contains highly active clay minerals (smectite and ver-miculite). The organic fraction, comprised mainly of highlyprocessed humic substances, functions as a glueing agentbetween the silt-size and clay-size particles, causing the aver-age size of the soil particles to increase. In contrast withnonhumic substances, which are easily attacked by soil mi-croorganisms and exist in the soil for a relatively short periodof time (Schnitzer and Khan 1972; Sparks 2003), humic sub-stances refer to naturally occurring, biogenic, heterogeneousorganic compounds of high molecular weight (Sparks 2003).Humic substances, which represent one of the most chemi-cally reactive fractions of the soil due to their high surfacearea and surface charge, can be divided into three fractionsbased on their solubility characteristics: humin, humic acid,and fulvic acid. Humin refers to the soil organic matter frac-tion that is insoluble in alkali and remains after extraction ofthe humic and fulvic acids with dilute alkali; fulvic acid re-fers to the colored soil organic matter that is soluble in bothalkali and dilute acid; humic acid is the dark-colored organicmatter that is soluble in alkali, but is insoluble and precipi-tates in dilute acid. The fulvic fraction, in general, has lowermolecular weight, and contains higher oxygen, but lower car-bon than humic acid. The fulvic acid fraction also containssignificantly more acidic functional groups, especially carb-oxyl acid (COOH), and is more reactive than humic acid.The distribution of the three fractions of soil organic mattervaries depending on the soil type and depth in the soil pro-file. Santagata et al. (2008) report that the soil used in thisresearch is comprised of approximately 74% humin and inor-ganic components, 24% humic acid, and less than 3% fulvicacid.All index properties of the soil show significant sample to

sample variability with the specific gravity (Gs) exhibiting alinear correlation to the LOI (Gs = 0.0104LOI(%) + 2.570),with measured values of Gs ranging between 1.9 and 2.2(Santagata et al. 2008). Similarly, both the plastic limit(PL = 114%253%) and the liquid limit (LL = 228%406%)increase with increasing organic content. The latter decreasedto 24%36% of its original value as a result of oven drying.Based on the above, the soil can be classified according tothe Unified Soil Classification System (USCS; ASTM 2006)

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as an organic clay with sand (OH). Details on soil indexproperties, mineralogy, and microstructure are provided bySantagata et al. (2008).Portland cement (PC) was selected as the binder as it is the

most economic and most widely used cement in practice. Ad-ditionally, and consistent with data from the literature (e.g.,Hernandez-Martinez and Al-Tabbaa 2009), unconfined com-pression tests (Hwang et al. 2004) on the soil treated withdifferent binders, including cement and lime, showed that ce-ment provided the most effective treatment. The amounts ofPC used ranged from 8% to 20%, 50%, and 100% by drymass of soil, corresponding to values of the cement factor(mass of cement used per unit volume of untreated soil) ofapproximately 24, 59, 148, and 296 kg/m3, respectively.These values fall in the range employed in practice (FHWA2000; Bruce and Bruce 2003), particularly when treatinghighly organic soils (e.g., hnberg and Holm 1999).

Sample preparationThe sample preparation procedure consisted of four steps:

mixing, compaction, curing, and extrusion. The mixing pro-cedure was specially designed to simulate the kneading ac-tion applied to the soil during deep mixing. A measurementof the water content was conducted 24 h prior to mixing todetermine the exact mix proportions (at the same time a lossof ignition test was also conducted). In the mixing phase, soiland water were mixed at a water content of 289%, the aver-age value of the natural water content measured on the Aprilsamples. Portland cement was then added to the mixture inthe form of a slurry with a water to cement ratio of 0.5, avalue typical in practice. The components were then mixedin a 5 L Kitchen-Aid stand mixer for 2 min.After mixing, the soil was compacted into plastic cylinders

(diameter = 7.62 cm, height = 15.24 cm) using a modifiedmechanical standard Proctor hammer with a reduced diameter(2.54 cm) to generate a kneading action (Hwang et al. 2004).A thin film of concrete form oil was applied to the cylinderwall before placing the soil, to allow for easy extraction ofthe sample. The soil mixture was compacted in three layers,applying 40 blows per layer. After compaction, the cylinderwas cut in half along the plane parallel to the ends to reducefriction along the walls during the curing and extrusion proc-esses. A 48 kPa surcharge was applied to the soil by placingdead weights on a concrete cap positioned on top of the cyl-inder, to simulate the effect of an overburden stress. Duringthe curing period the soil cylinders were immersed in tapwater to ensure continued access of the soil and cement towater, as would occur in the field. To this effect a smallopening was drilled in the bottom of the cylinder, where athin Plexiglas plate with a large number of holes was placed.This was done to ensure access of water to the entire crosssection at the bottom of the specimen during curing and tofacilitate extraction of the soil from the cylinder with mini-mal disturbance to the soil. Curing lasted 14 days for the ma-jority of the tests presented in this paper, although a limitednumber of tests were conducted on specimens cured for28 days. At the end of curing, the cylinder was removedfrom the water bath and hot water was run over the sides tofacilitate extrusion of the soil from the mold. The cylinderwas then inverted on a piece of wax paper and air pressurewas applied to the opening on its bottom. The rigid plexiglas

plate distributed the air pressure uniformly to the specimen,reducing disturbance as the sample was extruded.At least two samples were prepared for each cement con-

tent. Specimens with a diameter of 2.5 in. (1 in. = 25.4 mm)were trimmed from these samples and used to perform a pairof constant rate of strain (CRS) and incremental loading (IL)consolidation tests.Modified curing procedures were used to investigate the

role played by curing surcharge and curing duration (see sec-tion titled Effects of curing duration and curing surcharge).

Consolidation testsAll CRS tests presented in this paper were conducted em-

ploying one of three computer-controlled CRS apparatusesmanufactured by Geotac of Houston, Tex., available in Pur-dues Bechtel Geotechnical Engineering Laboratory, and fol-lowing the same procedure. First the soil specimen wasbackpressure saturated for 3648 h. The value of the backpressure varied depending on the cement content: from300 kPa for the untreated soil to 560600 kPa for the higherdosage of cement. Following backpressure saturation, thespecimen was loaded one-dimensionally at a constant dis-placement rate corresponding to a nominal strain rate i.e.,based on the initial specimen height of 0.25%1%/h, de-pending on the test, until the desired target stress was reached(2001600 kPa). This load was maintained for 23 days untildissipation of at least 95% of the excess pore pressure gener-ated during loading. Then, the specimen was unloaded to10% of the maximum load at 10% of the displacement rateemployed during loading. The rate reduction was required tolimit the negative excess pore pressure generated during un-loading. The load was again maintained for 23 days until95% of the negative excess pore pressure dissipated. Finally,in some cases the specimen was reloaded using the same dis-placement rate employed for the consolidation phase to a fi-nal target stress. At this stage a creep test lasting severalcycles was conducted. The nonlinear solution (Wissa et al.1971) was used for reduction of the CRS data to derive verti-cal effective stress, coefficient of consolidation (Cv), and ver-tical hydraulic conductivity (kv) as a continuous function ofthe strain (or void ratio) from measurements of vertical dis-placement, axial load, excess pore pressure developed at thebase of the soil specimen, and cell (back) pressure.All incremental loading tests were performed using a cell

equipped for backpressure saturation, and under single drain-age conditions with measurement of the excess pore pressureat the base of the specimen. Measurements of the base excesspore pressure were used to determine the end of primary con-solidation, and the load increments were applied allowing littleto no secondary compression. This was done to limit the im-pact of changes in soil stiffness with time in the cement-treatedsoil (Kassim and Clarke 1999). The specimen was allowed tocreep for at least one cycle of secondary compression at themaximum effective stress (15501600 kPa) reached ineach test.Table 1 summarizes the consolidation testing program. For

each of the tests performed it provides the sample sourcenumber, LOI measured on the soil prior to mixing, cementcontent, test type, strain rate used (CRS test only), curing du-ration and surcharge, and water content (w) and void ratio (e)at the start of consolidation (i.e., end of curing). As shown in

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the table, CRS tests on the untreated soil were conducted us-ing rates in the 0.25%1%/h range, while all tests on the ce-ment-treated soil employed the same rate of 0.5%/h. Withone exception (test 10 in Table 1), the maximum pore pres-sure ratio (Duh/sv, where Duh is the excess pore pressureand sv is the vertical stress) measured in all the CRS testsperformed on the untreated soil remained below the 30%threshold fixed by ASTM D4186 (ASTM 1989) (all testswere performed prior to the 2006 revision of the standard).For all tests on the cement-treated soil the maximum porepressure ratio remained below 5%.Note that the values of the cement content provided in Table 1

deviate somewhat from the target nominal values of 8%,20%, 50%, and 100%. For simplicity the latter values (50%and 100%) are used in the remaining text.Calculation of the phase relations of the treated soil

presents some challenges, in particular due to the need to es-timate the specific gravity of the treated soil, which decreasescontinuously during hydration (e.g., Lorenzo and Bergado2004). In this work, calculation of the void ratio of thecement-treated organic soil was performed treating cementand soil as separate phases, i.e., calculating the mass and vol-ume of each of these phases separately. With this assumptionthe masses of the two solid phases were calculated from thepost-test oven-dried mass and the mix proportions used toprepare each cement-treated soil specimen (accounting forthe amount of water reacting with the cement); while themass of the water phase was derived from the difference be-

tween initial total mass and final dry mass. For calculation ofthe volume of the hydrated cement a value of 2.15 was usedfor the specific gravity (Lea 2004). For the soil, the specificgravity was derived from the linear relationship existing be-tween Gs and LOI for this particular soil (Santagata et al.2008), based on measurements of the LOI conducted priorto mixing the soil with cement. The void ratio was then cal-culated as the ratio of the volume of water (at the end of thecuring stage all specimens were found to be essentially 100%saturated) and the sum of the volumes of the two solidphases. Figure 1 presents average volume phase diagrams forpre-consolidation (end of curing) conditions for specimensprepared with 20%, 50%, and 100% cement by dry mass ofthe soil. Note from Fig. 1, and from the data in Table 1, thatthere is a trend of decreasing void ratio at the start of consol-idation (i.e., at end of curing), with increasing cement con-tent. This trend is consistent with the cement and wateradded during mixing, the water consumed by the cement hy-dration reaction, and the deformations measured under thecuring surcharge. The deformation increased with decreasingcement dosage.As shown in Table 1, each test has been assigned a se-

quential reference number. This is the same reference numberused in a previous publication (Santagata et al. 2008) focusedon the behavior of the untreated soil. Tests 15 performed onintact block samples of the natural soil are omitted fromTable 1, as they are not directly relevant to the research pre-sented in this paper.

Table 1. Summary of experimental program.

Test No.Fieldsample LOI (%) PC (%) Test type

Strain rate(%/h)

Curing duration(days)

Curing sur-charge (kPa)

w (%) endof curing

e end ofcuring

6 S5 45.3 0 CRS 0.5 14 48 179.2 4.037 S5 49.2 0 CRS 0.5 14 48 197.0 4.268 S5 43.6 0 CRS 0.5 14 48 219.5 4.269 S7 54.4 0 CRS 0.25 14 48 206.6 4.3210 S7 55.2 0 CRS 1 14 48 214.8 4.4511 S7 54.4 0 CRS 0.1 14 48 181.9 4.2012 S9 36.6 0 CRS 0.1 14 48 156.3 3.8013 S9 37.5 0 CRS 1 14 48 155.4 3.6414 S7 52.5 0 IL 14 48 194.4 4.3815 S8 46.0 0 IL 14 48 178.5 3.6716 S9 40.9 18.7 CRS 0.5 14 48 169.0 3.6517 S9 40.7 18.7 IL 14 48 166.9 3.7418 S10 41.7 8.1 CRS 0.5 14 48 174.4 3.7519 S10 41.7 8.1 IL 14 48 180.2 3.9520 S10 44.7 51.4 CRS 0.5 14 48 167.6 3.5721 S10 44.7 51.4 IL 14 48 168.0 3.5722 S10 44.8 103.4 CRS 0.5 14 48 125.2 2.5723 S10 44.8 103.4 IL 14 48 126.0 2.5824 S11 48.0 100.8 CRS 0.5 14 48 129.0 2.6925 S10 45.0 52.6 CRS 0.5 28 48 171.1 3.6526 S10 44.8 52.6 IL 28 48 168.0 3.6527 S11 42.7 100.8 CRS 0.5 28 48 130.1 2.7328 S11 42.3 100.8 IL 28 48 133.0 2.7629 S10 44.2 52.6 CRS 0.5 14 96 157.7 3.3530 S10 43.1 52.6 IL 14 96 158.6 3.4031 S11 44.7 42.1 CRS 0.5 14 192 137.1 3.0132 S11 44.6 48.4 IL 14 192 143.6 3.11

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Chemical testspH measurements, fractionation tests, elemental analyses

using a LECO analyzer, and Fourier transform infrared spec-troscopy (FTIR) analyses were conducted to complement theconsolidation experiments.pH measurements on the untreated soil were conducted us-

ing a Corning 44 pH meter, a pH glass electrode, and a calo-mel (reference) electrode in accordance with ASTM D4972(ASTM 2007). The same device was used for measuring thepH of the cement-treated soil. For these tests an amount ofsoil corresponding to a dry mass of 15 g was mixed with acement slurry (with water to cement ratio of 0.5), and waterwas added to achieve an overall water to solids ratio of 4.Measurements were conducted 1 h after mixing, as well as 1and 7 days later. Between measurements the treated soil waskept in a tightly sealed glass container to prevent any mois-ture loss.Fractionation of both the untreated and treated soil was

performed to quantify the percentage of humin, humic acid,and fulvic acid present (see section titled Materials). Forthis purpose, samples were prepared following the same pro-cedure employed for the samples used for the consolidationtests, except that no curing surcharge was applied. Instead,after compaction the samples were stored in a glass jar for7 days. At this time a sample of mass corresponding to 15 gof dry soil was fractionated employing the following proce-dure: (i) the samples were treated with a 0.1 mol/L HCl sol-ution (with solution to solids ratio of 5) to remove anycarbonates so that the organic matter could be easily sepa-rated from the mineral portion of the soil; (ii) after continu-ous shaking for 48 h the samples were centrifuged at2000 revolutions/min for 15 min and the supernatant solutiondecanted; (iii) humic and fulvic acids were extracted using ahighly basic solution (pH > 12); (iv) the humic acid was thenseparated from the fulvic acid using a solution with pH < 1;(v) the humic acids, which can be very strongly bonded tomineral matter, were purified using an HF solution to elimi-nate any additional inorganic matter. Note that this is thesame procedure used on the untreated soil (Santagata et al.2008), except that it avoids pretreatment with a 0.1M HF sol-ution, as this step, which is used on the untreated soil toeliminate silicates, would promote dissolution of some of thecementitious products.

The elemental (C, H, N) composition of each of the frac-tions was determined using a LECO CHN-2000 analyzer. Inthis technique the carbon, nitrogen, and hydrogen content aredetermined from analysis of the gases released by the com-bustion of the soil sample.The different fractions extracted from both the untreated

and cement-treated soil were further characterized employingFTIR. This technique is especially suited for the characteriza-tion of humic substances that have no regularly repeatingstructural units, and are, instead, made of a variety of func-tional groups. FTIR spectroscopy is based on the fact thatmolecules vibrate at discrete energies in the infrared (IR) re-gion of the electromagnetic spectrum. When the frequency ofthe incident IR radiation matches the frequency of the partic-ular vibrational mode (bending or stretching) of a molecule,IR absorption occurs. FTIR analyses for this research wereconducted using a Perkin-Elmer 1600 spectrophotometer.Each specimen, prepared in solid form using potassium bro-mide (KBr), was scanned 640 times from 370 to 4000 cm1(the vibration of most organic molecules occurs in the mid-IR spectral region) with an 0.5 cm1 interval and 2 cm1 res-olution.

Results and discussion

Overview of effects of cement on 1D consolidationbehaviorResults from tests on soil with 0% (reconstituted samples)

and 20% PC (59 kg/m3) are presented in Fig. 2 to illustratethe consistency in results between CRS and IL tests, and tohighlight some of the changes in 1D consolidation behaviorinduced by the addition of cement. As shown in Fig. 2a, thecompression curves for 20% PC obtained from CRS and ILconsolidation tests coincide, indicating reproducibility in thebehavior of the cement-treated soil as well as no significantstrain-rate effects (the two tests necessarily involve strainingof the soil at different rates). Similar observations apply tothe untreated soil (see also Santagata et al. 2008). Figure 2ais a plot of void ratio with effective stress, Fig. 2b includesthe change of permeability with void ratio, Fig. 2c showsdata of coefficient of consolidation with effective stress, andFig. 2d of constrained modulus with effective stress. Figure 2aindicates that the most notable effect of treatment on thecompression behavior is the increase in the preconsolidationpressure, s 0p, which, based on determinations using thestrain energy method (Becker et al. 1987), increases byabout 80% (from an average value of 50.2 kPa for the un-treated reconstituted soil to approximately 94 kPa). As a re-sult of the development of this cementation-induced s 0p, thecompression curve of the treated soil is shifted to higher ef-fective stress, with the treated soil able to carry almosttwice the vertical effective stress sustained by the untreatedsoil at the same void ratio.Beyond s 0p the slope of the compression curve for the 20%

PC-treated soil is essentially constant, with values of thecompressibility index (Cc = 1.291.33) in the range of thosemeasured on the reconstituted untreated soil. The effects oftreatment are, instead, apparent in the swelling and reloadingindices measured during unloadreload stages. In the over-consolidation ratio (OCR) 12 range both the swelling and

Fig. 1. End of curing volume phase relations for 20%, 50%, and100% cement-treated soil. Hydr., hydrated.

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the reloading indices decrease by more than 30% comparedto the values measured for the untreated soil specimens.Figure 2b shows that, at a given void ratio, the average hy-

draulic conductivity of the treated soil is approximately 4.6times greater than that of the reconstituted soil. Despite thisdifference, the slope of the linear portion of the hydraulicconductivity (k) plot (Ck = De/Dlogk) remains almost un-changed with treatment. For the soil treated with 20% PCthe average value of Ck/Cc is 0.66, in the range of the dataobtained for the untreated reconstituted soil (0.580.67).Note also the slight difference (15%20%) between thecurves of hydraulic conductivity versus void ratio obtainedfrom the IL and CRS consolidation tests. The effects of treat-ment on permeability are also highlighted in Fig. 3, whichshows the cumulative excess pore pressure generated duringloading and normalized by the measured strain rate for both0% and 20% PC. It is seen that only 20% of the excess porepressures produced in the reconstituted soil are generated inthe treated soil at the same void ratio. Given that the con-strained modulus is not affected by treatment in the normallyconsolidated region (as discussed later), the results presentedin Fig. 3 indicate an increase in the hydraulic conductivity ofthe soil with the addition of cement.Figure 2c shows the variation of the coefficient of consol-

idation with stress level for both the treated (IL and CRS

data) and untreated organic soil. Some discrepancy betweenthe IL and CRS data is observed, with the Cv values obtainedfrom the CRS test being consistently two times higher thanthe IL data. More importantly, the results show that the coef-ficient of consolidation increases with the addition of PC byapproximately one order of magnitude over the stress range

Fig. 2. Effect of 20% cement addition on (a) compression behavior, (b) hydraulic conductivity, (c) coefficient of consolidation, and (d) con-strained modulus of highly organic soil. mv, coefficient of volume compressibility.

Void ratio, e1.52.02.53.03.54.0

Exce

sspo

repr

essu

re/s

train

rate

,

u

h/

a(kP

a/%

/h)

0

50

100

150

200

0% PCNo.13(CRS)

19% PCNo.16(CRS)

.

Fig. 3. Effect of 20% cement addition on excess pore pressure gen-erated in CRS test.

Bobet et al. 1105

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investigated. This is a reflection of the increase in k with thetreatment noted above.The values of the constrained modulus of the treated soil

are plotted against vertical effective stress in Fig. 2d. In theoverconsolidated region, the treated soil exhibits generallystiffer response with higher constrained modulus, but onceconsolidation proceeds past the preconsolidation pressure,the constrained modulus of the treated soil coincides withthat of the untreated reconstituted soil.

Effect of cement dosage on 1D consolidation behavior andstructure of cement-treated soilHaving highlighted the general effects that the addition of

cement has on the 1D compression behavior, this section ad-dresses the role played by the cement dosage. Specifically,Fig. 4 presents the data for CRS tests conducted with cementdosage varying between 0% (untreated reconstituted soil) andapproximately 100% (cement factor, 296 kg/m3). Similar toFig. 2, Fig. 4a plots void ratio with effective stress, Fig. 4bvoid ratio with permeability, Fig. 4c coefficient of consolida-tion with effective stress, and Fig. 4d constrained moduluswith effective stress. While, as discussed for 0% and 20%PC, the results of the CRS and IL tests are found to be con-sistent, for clarity, the data for the latter are not included inthis figure. Key results are summarized in Table 2.The compression curves of the reconstituted and PC-treated

soil are shown in Fig. 4a. The figure shows that the greaterthe cement dosage, the greater the cementation-induced pre-consolidation stress, the more extended the reloading region,and the greater the vertical effective stress that can be sus-tained at a given void ratio.The figure also highlights the trend of decreasing value of

the void ratio at the start of consolidation (i.e., at end of cur-ing; see data in Table 1) with increasing cement content. Thistrend is consistent with the cement and water added duringmixing, the water consumed by the cement hydration reac-tion, and the deformations measured under the curing sur-charge.While the preconsolidation stress increases with treatment,

the compression index in the normally consolidated regiondoes not show significant changes with cement addition. Asshown in Table 2, there is no clear trend with cement dosageand, regardless of the cement content, the values of Cc fallwithin the range observed for the untreated reconstituted soil.Figure 4b shows the increase in hydraulic conductivity of

the organic soil with treatment. Although the initial void ratiodecreases gradually with increasing cement content, the hy-draulic conductivity of the treated soil increases with cementpercentage at any given void ratio. With 8% PC the effect ofthe treatment is modest, with a 18%26% increase in k at thesame void ratio. However, a much greater increase in hy-draulic conductivity is observed with higher PC contents,and for 100% PC the hydraulic conductivity is over one orderof magnitude greater than that of the untreated soil. This ob-servation is in agreement with past findings (Broms and Bo-man 1979; Brandl 1981; Buensuceso 1990; Townsend andKlym 1996; Cortellazzo and Cola 1999; Kang 2011). The in-crease in hydraulic conductivity with treatment is caused bythe change in the fabric of the soil as a result of the cementreactions. First, the released Ca2+ and OH increase the pHand the ionic strength of the solution; the increase in the Ca2+

concentration reduces the double-layer thickness of the phyl-losilicate minerals present in the organic soil, promoting floc-culation and aggregation of particles. Second, the formation ofthe CSH and CAH gels binds soil particles together, fur-ther contributing to aggregation and flocculation (Tremblay etal. 2001; Al-Rawas 2002) of the soil particles. As a result ofthese influences a more open fabric is formed, with macro-pores, which serve as the main channels for flow, having sizethat increases with increasing cement content. These hypothe-ses are supported by previous observations of the microstruc-ture of cement-treated clay conducted by Chew et al. (2004)using scanning electron microscopy (SEM) and mercury in-trusion porosimetry. SEM observations show that the additionof cement produces a flocculated structure, and both analysesindicate an increase in the size of the openings formedamongst soil particle clusters with increasing cement content.Particle-size analyses on untreated and cement-treated soilconducted by both Rao and Rajasekaran (1996) and Chew etal. (2004) also demonstrate an increase in particle size withcement content.Note that it is not expected that the same trend of increas-

ing hydraulic conductivity with cement content would extendto the larger cement dosages used in cutoff walls. In suchcases the high cement dosages employed would ultimatelycause a decrease in kv.Values of Ck and Ck/Cc derived from the data presented in

Fig. 4 are summarized in Table 2. It is observed that for ce-ment contents up to 50% there is no clear trend between ce-ment content and either Ck or Ck/Cc. In fact, for both theseparameters the data for the cement-treated soil fall within therange observed for the untreated reconstituted soil. However,with 100% cement, the values of both Ck and Ck/Cc fallclearly below the range measured on all other specimens.The decrease in Ck/Cc is especially of interest, as it reflectsthe fact that a smaller percentage of the voids that contributeto the compressibility of the soil contribute to its hydraulicconductivity. This suggests that the fabric formed as a resultof treatment with 100% cement involves some internal notconnected pore space. Similar observations are reported byKang (2011) for a cement-treated inorganic clay.The coefficient of consolidation is plotted against vertical

effective stress in Fig. 4c. In general, at the same vertical ef-fective stress, Cv increases with treatment. Similar to pre-vious observations regarding s 0v and kv, the effects oftreatment with 8% PC are small. The increase in Cv is dueto the increase in both hydraulic conductivity and constrainedmodulus (discussed later). Similar results have been obtainedfor other soils treated with lime and cement (Broms 1999;Cortellazzo and Cola 1999; Kassim and Clarke 1999; Kang2011).The effect of cement treatment on the constrained modulus

is illustrated in Fig. 4d, which shows that while cement treat-ment leads to an increase in the constrained modulus in theoverconsolidated region, the relationship between modulusand effective stress is independent of cement content in thenormally consolidated region.The average increase in preconsolidation pressure with

treatment, shown in Fig. 4a, is summarized in Fig. 5, whichshows that the increase in s 0p is relatively modest for low val-ues of cement (50% and 88% increase in s 0p with 8% and

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20% PC, respectively), but much sharper at higher dosages,as s 0p increases by approximately 800% and 2300% with50% and 100% PC, respectively.Further comparisons between the reconstituted and treated

soils can be made within the framework proposed by Burland(1990), which relies on the normalization of the compressioncurves through the use of the void index, Iv. Iv is definedbased on e100 and e

1000, the intrinsic void ratios, i.e., the

void ratios of the soil in the reconstitutedremolded state atstresses of 100 and 1000 kPa, respectively, as follows:

1 Iv e e100

e100 e1000

With this normalization the compression curves of recon-stitutedremolded soils all fall around a unique line termedthe intrinsic compression line (ICL), and the in situ state fora variety of sedimentary natural clays in the normally con-solidated state plots in a band to the right of the ICL. Thebest-fit curve through these data is termed the sedimentationcompression line (SCL). At the same void ratio the SCL liesat vertical effective stresses approximately five times higherthan the ICL, implying that most naturally sedimented clayscan support five times higher vertical effective stress than re-constituted clays. Burland (1990) as well as others haveshown that the in situ state of some clays can fall substan-tially above the SCL.

Fig. 4. Effect of cement treatment (8%100% PC) on: (a) compression behavior, (b) hydraulic conductivity, (c) coefficient of consolidation,and (d) constrained modulus of highly organic soil.

Table 2. Summary of results on cement treated soil.

PC (%) Cc, 2 4s 0P Cr, OCR 42 Ck Ck/Cc Ca Ca/Cc0 1.548 0.207 0.266 0.055 0.95 0.09 0.60 0.04 0.123 0.0958 1.444 0.057 0.89 0.64 0.106 0.08520 1.326 0.026 0.87 0.66 0.080 0.06350 1.699 0.013 0.91 0.61 0.070 0.042100 1.382 0.005 0.73 0.54 0.029 0.021

Note: Ca, creep coefficient.

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While Burlands (1990) framework was developed for nat-ural soils, Kang and Santagata (2006) suggest that the intrin-sic values of the untreated reconstituted soil may be used tonormalize the data of cement-treated soil, and thus the frame-work can provide insight into the behavior and the degree ofstructuring of cement-treated soils. Figure 6a, which presentsthe compression curves of all cement-treated specimens inthe void index (Iv)logs 0p space, illustrate the results of sucha normalization. Also included in this figure are the data forthe reconstituted soil, which fall on the ICL, and the com-pression curves for the intact natural soil based on tests onblock samples (Santagata et al. 2008). It is seen that the nor-malized compression curves of the cement-treated soil are ap-proximately parallel to the intrinsic and sedimentationcompression lines, and that as the cement dosage increasesthe curves shift to the right, reflecting, as discussed earlier,an increased cement-induced structure. While the curves for8% and 20% cement fall between the ICL and the SCL, thecurves for 50% and 100% cement fall very close to the SCL,suggesting a degree of structuring similar to that typical ofmany natural sedimentary clays. Similar observations werereported by Kang and Santagata (2006) for a cement-treatedinorganic clay.Figure 6a also provides insight into the nature of the struc-

ture generated in the cement-treated clay. Burland (1990)shows that for natural clays whose stress state lies close orabove the SCL, the post-yield compression curve is markedlysteeper than the SCL, and that at high stresses it tends to ap-proach the ICL as a result of the progressive collapse of thesoil structure. Baudet and Stallebrass (2004) refer to the struc-ture associated with this type of behavior as a meta-stablestructure. As shown in Fig. 6a, and discussed in more de-tail by Santagata et al. (2008), this is the behavior observedwhen testing the intact block samples. In contrast, Fig. 6ashows that upon first loading, the post-yield compressioncurves of the 50% and 100% cement-treated soil clay re-main practically parallel to the SCL. This is evidence ofthe stable nature of the structure generated as a result ofcement treatment.Post-yield structure degradation can be described using the

model proposed by Liu and Carter (1999, 2000), which ex-presses the void ratio of structured soil at any given stress

level in the virgin compression region as the sum of twoterms:

2 e e De e Deis 0ps 0v

b

where e* is the void ratio of the reconstituted soil at the samevertical effective stress; De is the difference between the voidratio of the structured soil and that of the reconstituted soil ata given vertical effective stress s 0v; Dei is the value of De de-termined in correspondence to the preconsolidation stress s 0p;and b, which is termed the compression destructuring index,is a parameter that quantifies the rate of reduction in De withstress level. Data presented by Liu and Carter (2000) suggestthat for natural soft clays, b generally falls in the 0.31 range(although higher values of b are reported). Much smaller va-lues of b (

and secondary consolidation. Moreover, for a given soiltype Ca/Cc is known to vary within a fairly limited range(Mesri and Castro 1987). As shown in Table 2 and Fig. 8,Ca/Cc decreases markedly with cement addition, from val-ues at the high end of the range typical of peats and mus-kegs, to values characteristic of inorganic clays (50% PC),and finally to values generally measured on granular soils(100% PC). This variation in Ca/Cc reflects the modifica-tions in the nature of the new geomaterial formed as aresult of treatment with different percentages of cement.Given the small change in the compression index with ce-ment addition noted above, the decrease in Ca/Cc is mainlydue to the reduction of the secondary compression indexwith cement content.

Effects of curing duration and curing surchargeThe importance of the curing process on the properties of

chemically stabilized soils has been highlighted by many re-searchers. Hwang et al. (2004) observed that stiffness and un-confined compressive strength of organic soils treated withPC and lime increased with increasing curing time and cur-ing surcharge. Hebib and Farrell (2003) observed that the un-confined strength of PC-stabilized peat increased when thesoil was preloaded during curing. They also observed thatthe unconfined compressive strength was higher for pre-

loaded soils, and that it increased with increasing curingtime. hnberg et al. (2001) observed that the unconfinedcompressive strength of cement-treated peat increased whenthe surcharge was applied immediately after treatment.The beneficial effects of prolonged curing are likely to be

twofold. First, the increase in the duration of secondary com-pression will lead to an increase in the aging-induced precon-solidation stress. Second, prolonged curing will allow anincreased time for the cement hydration reactions to takeplace. In this research, the effects of curing time were eval-uated by conducting experiments on the organic soil treatedwith 50% and 100% cement cured for 14 and 28 days. Thesecement contents were selected because they yielded the mostsignificant changes in engineering properties, and because, asdiscussed in the section titled pH conditions promoting for-mation of calcium silicate gel, the pH values measured withthese cement contents indicated conditions that would pro-mote continued hardening of the calcium silicate gel. The re-sults obtained from testing these samples indicate no effect ofcuring period on the compression behavior of the 50% ce-ment soil samples. For 100% cement the effects are small,with a slight increase in the preconsolidation stress (7%)and in the coefficient of consolidation (

the continuous hardening of the CSH gel during the timeframe investigated. Overall, the small changes in behaviorobserved with increased curing time justify the selection ofthe 14 day curing period used for the majority of the tests.The effect of the curing surcharge was investigated exclu-

sively for soil treated with 50% cement using surcharges of

48, 96, and 192 kPa, which represent approximately 2.5, 5,and 10 m thick fills with a unit weight of 19 kN/m3. Figure 9presents the compression curves for six tests (two for eachof the stress levels investigated), some of which also in-volved unloadreload stages. Focusing on the first loadingstage, it is seen that while the specimens cured under ahigher surcharge have a lower initial void ratio, all the com-pression curves reach the same virgin compression line.This is consistent with previous observations by Tremblayet al. (2001) that the virgin compression line of cement-treated soil is independent of the initial curing void ratio.The average preconsolidation stress increases from 403.4 1.7 kPa (surcharge = 48 kPa) to 471.4 5.0 kPa (96 kPa),i.e., an increase of 68 kPa for a surcharge increase of48 kPa, to 672.7 24.4 kPa (192 kPa), i.e., an increase of269.3 kPa for a surcharge increase of 144 kPa. The im-provement in the preconsolidation stress is mainly due tothe decrease in the end of curing void ratio that dictates atwhich stress level upon reloading in the consolidation cellthe virgin compression line is reached. This implies thatthe yield stress of the treated soil can be improved by cur-ing the soil under a higher surcharge for the same duration.Analysis of the data for the three different curing stresses

also shows that the constrained modulus and hydraulic con-ductivity increase with increasing curing surcharge in theoverconsolidated region, while the effects of surcharge arenegligible in the normally consolidated region. As a result,the coefficient of consolidation also increases with surcharge,with the increase in the constrained modulus as the main fac-tor.

Chemistry of cement-treated soilThe pH measurements, fractionation tests, LECO elemen-

tal analyses (C, H, N), and FTIR analyses were conducted tocomplement the consolidation experiments. The overall goalof this portion of the experimental work was to gain insightinto the interaction between the organic matter and the ce-ment, and in particular to assess the stability of the organicmatter following treatment, evaluate the reactivity of the dif-ferent functional groups present, and investigate the composi-tional changes of the different fractions of the treated soil.

Fig. 7. Effect of cement treatment (8%100% PC) on creep coeffi-cient.

Fig. 8. Effect of cement treatment (8%100% PC) on Ca/Cc.

Fig. 9. Effect of surcharge stress on compression behavior of cementtreated soil.

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pH conditions promoting formation of calcium silicate gelThe increase in the preconsolidation pressure and the shift

of the compression curves to the higher vertical effectivestress described are indications of the development of a stiffersoil structure with the treatment. The structure is developedmainly as a result of the formation of the cement hydrationproducts and their reactions with the soil particles. All thesereactions are very sensitive to the chemical environment, es-pecially the pH of the soilcement system. As soon as ce-ment gains access to water, formation of what are sometimesreferred to in the soil cement literature (e.g., Bergado et al.1996) as primary cementitious products starts. These in-clude calcium silicate hydrate (CSH), calcium aluminatehydrate (CAH), and calcium hydroxide (Ca(OH)2). Cal-cium silicate gel (CSH), which is formed from reaction ofthe cements tricalcium silicate (C3S) and dicalcium silicate(C2S) with water, is the major source of the strength of thesoilcement mixture. Its formation requires a pH greater than12.6. Following these reactions, Ca2+ and OH ions are re-leased into the pore water. The released OH ions increasethe pH of the pore water. Under high pH conditions, silica(SiO2) and alumina (Al2O3) from the clay minerals are dis-solved and combine with the released Ca2+ and OH ions toform secondary cementitious products (CSH and CAH).The amount of silica and alumina dissolved, and hence theamount of secondary cementitious products formed as a re-sult of the pozzolanic reaction, are markedly dependent onthe pH of the soilcement system.Figure 10 shows the pH values measured with different

percentages of cement 1 h and 1 day after mixing. The un-treated soil (0% PC) was slightly acidic with a pH value of5.98. Following treatment with 8% PC, the pH increased rap-idly to 11.13 1 h after treatment. No further change in pHwas observed with additional time. With 20% PC the pH in-creased to 11.98 in 1 h, and continued to increase to 12.20after 1 day, with no subsequent increase. With 50% PC, thepH increased to 12.20 in 1 h, and to 12.61 after 1 day. With100% PC, the pH increased to 12.32 in 1 h and to 12.77 after1 day. In each case, no further pH increase with time was ob-served.These measurements are in good agreement with the pre-

consolidation stress data (Fig. 5), and the sharp increase ins 0p with 50% and 100% PC can be related to the increase inpH to values above 12.6, which allows formation and contin-ued hardening of the primary cementitious products. The pHvalues measured also suggest that for the soilcement mix-tures examined in this research, the contribution to thestrength from the secondary cementitious products wasnegligible, as for values of pH < 14, no major dissolution ofsilica and alumina occurs (S. Diamond, personal communica-tion, 2010).

Interaction of organic matter and cementAs discussed in the section titled Materials, the organic

portion of the soil used in this experimental program is com-prised of humic substances that can be classified according totheir solubility as humin, humic acid, and fulvic acid. Table 3summarizes the results of the fractionation tests, includingthe data obtained for the untreated soil. As mentioned earlier,two independent measurements were conducted for each ce-ment content on two separate samples. The data presented in

Table 3 represent the average values from these two measure-ments. The results show that the amount of humic and fulvicacid fractions extracted decrease substantially with increasingcement content. For example, less than 3% of the humic acidof the untreated soil could be extracted from the soil treatedwith 20% cement. With 50% cement this percentage de-creases to less than 0.1%, and with 100% cement the amountof humic acid extracted was too small to be measured accu-rately. The fulvic acid extracted also decreased with increas-ing cement, but to a lesser degree: with 20% PC, only 10% ofthe fulvic acid was extracted, and this amount decreased to3% and 0.5% with 50% and 100% cement, respectively.Given that the addition of cement is associated with an in-

crease in the pH (see Fig. 10), and given that under highlyalkaline conditions, dissolution of humic and fulvic acids isknown to occur (see section titled Materials), the questionmay arise as to the effect of pH on the stability of the organicmatter. Loss on ignition tests conducted at the end of theconsolidation tests and elemental analyses of the three frac-tions extracted from the cement-treated samples demonstratethat no dissolution of the organic matter occurred during thecuring process, and that organic matter was not lost duringextraction and fractionation. Thus, the significant reductionof the extracted humic and fulvic acids following treatmentwith cement indicates that the organic matter was bound orat least encapsulated by the cement hydration products andwas present in the mineral fraction of the soil.As done for the soil alone, following fractionation, the hu-

mic substances were further characterized employing elemen-tal analysis and FTIR. Table 4 summarizes the masses ofcarbon, nitrogen, and hydrogen present in each of the threefractions (for cement contents greater than 20% no analysiscould be conducted on the humic and fulvic acids due to theinsufficient amounts extracted). All data pertain to sampleswith the same amount of dry soil (15 g). The data presentedin Table 4 illustrate that the C, N, and H extracted as humicacids from the 20% cement sample decreased significantly

Cement content (%)0 20 40 60 80 100 120

pH

4

6

8

10

12

1412.6

7 days after mixing

1 day after mixing

Fig. 10. Effect of cement addition on soil pH.

Table 3. Results of fractionation tests on cement treated soil.

Cementcontent (%)

Nonextracted fraction(%)

Humicacid (%)

Fulvicacid (%)

0 73.86 23.71 2.4320 99.18 0.59 0.2350 99.93 0.01 0.06100 99.99 0.00 0.01

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compared to the natural soil. In contrast, the carbon, nitrogen,and hydrogen contents of the nonextracted fraction increasedwith cement content. Again this is evidence that a significantportion of the organic matter present in the natural soil isboundencapsulated by the cement hydration products.Additional perspectives are obtained by examining the

composition of the fulvic and humic acids extracted from thenatural soil and the sample with 20% cement, as done inTable 5, in which the carbon, nitrogen, and hydrogen con-tents are expressed as a percentage of the mass of humic orfulvic acid extracted. These data indicate that the chemicalcomposition of the humic and fulvic acids extracted from thesoil treated with 20% PC changed compared to that of theacids extracted from the natural soil. Specifically, with addi-tion of cement the organic carbon percentage decreased,while the hydrogen and nitrogen percentage increased. Thissuggests that different functional groups have different reac-tivity with the cement.Further insight about the reactivity with cement of func-

tional groups of the organic matter is provided by the FTIRspectroscopy analyses, which were performed on each frac-tion of both the natural and the cement-treated soil, exceptfor the fulvic and humic acids extracted from the 100% PCtreated soil due to the small amounts available. The objec-tives of the FTIR analysis were to investigate the composi-tional changes of the different fractions of the treated soiland evaluate the reactivity of the different functional groupspresent. Two analyses were performed on each fraction forconsistency. These data are reported in Figs. 11a11c.Although the frequencies corresponding to the various vibra-tion modes (symmetric or asymmetric stretching and bendingvibration modes) of organic functional groups have beenstudied extensively, the similarity of the chemical composi-tion of organic functional groups makes it particularly diffi-cult to accurately quantify the sources responsible forchanges in the FTIR spectra; a qualitative analysis howeverprovides sufficient insight into the chemical reactions be-tween the soil and the cement.Figure 11a shows the FTIR spectra of the nonextracted

fraction of the treated soil (mineral fraction). The most sig-nificant change in the spectrum with cement treatment is thegradual increase in the intensity of the broad band in the30003600 cm1 region. This region represents the symmet-ric and asymmetric stretching of OH. According to Yousufet al. (1995), the increase in intensity of this band indicatesthe formation of Ca(OH)2 and of the CSH gel. The sharpincrease in intensity of the bands in the 1415 and 872 cm1regions is due to carbonation, i.e., the formation of calcium

carbonate (CaCO3) from the reaction of CO2 with theCa(OH)2 formed by cement hydration. The greater the ce-ment percentage, the greater the amount of Ca(OH)2 avail-able; hence, the greater the amount of CaCO3 formed bycarbonation, as reflected in the FTIR peaks. However, car-bonation is an inevitable reaction following exposure to air,and therefore has no specific significance for this study. Theband in the 15761772 cm1 region increases graduallywith respect to the baseline in the 20002500 cm1 region.In the mineral fraction this is the only band that representsthe organic functional groups, which include C=O stretchingof COOH (1720 cm1); C=O stretching of amide groups(16301660 cm1); aromatic C=C stretching and (or) asymmet-ric COO stretching (1610 cm1); as well as COO symmet-ric stretching, NH deformation, and +C=N stretching(15901517 cm1). All the organic functional groups listedhere are composed mainly of C, H, and N. The increase inthe C, H, and N contents in the nonextracted PC-treatedsoil can be interpreted as the retention in the mineral frac-tion of these functional groups by cementation.Figure 11b presents the FTIR spectra of the humic acid

fraction extracted from the untreated and treated soil. The rel-atively high intensity band in the 33003400 cm1 region ofthe untreated soil indicates the presence of phenolic OH (OHstretch of phenolic OH). The increase in the intensity of thebroad band in the 33003600 cm1 region may indicate thatpart of the Ca(OH)2 and the CSH gel were dissolved andextracted under the highly acidic environment that was cre-ated during fractionation to precipitate humic acid (pH = 1).The intensity of the 12341262 cm1 region, which is wherethe COH stretching band of phenolic OH occurs, did notchange with treatment. This is an indication that phenolicOH does not react with cement, which is in agreement withfindings from other researchers (Pollard et al. 1991). Thesmall peak in the 1718 cm1 region (corresponding to theC=O stretching band of COOH) present in the untreatedsoil disappeared in the humic acid fraction of the treatedsoil. At the same time the intensities of the bands at1400 cm1 and in the 15461600 cm1 region (which reflectthe symmetric and antisymmetric stretching of COO) in-creased. These results do not necessarily indicate the reactionof the carboxyl group (COOH) with cement, but rather thechange of the carboxyl group to carboxylate group (COO)under the highly acidic conditions used for precipitation ofthe humic acid during fractionation.Figure 11c presents the FTIR spectra of the fulvic acid. The

strong band around 3390 cm1 and the small band in the 12451251 cm1 region in the untreated soil indicate the presence of

Table 4. Mass of C, N, and H present in fractions extracted from untreated and cement treated soil.

C (g) H (g) N (g)

Cementcontent (%)

Nonextractedfraction

Humicacid

Fulvicacid

Nonextractedfraction

Humicacid

Fulvicacid

Nonextractedfraction

Humicacid

Fulvicacid

0 1.601 1.826 0.173 0.354 0.172 0.011 0.114 0.132 0.00520 2.981 0.048 0.014 0.482 0.006 0.002 0.196 0.006 0.00220 3.106 0.049 0.017 0.500 0.006 0.003 0.196 0.006 0.00250 3.126 0.540 0.180 50 3.259 0.562 0.202 100 3.210 0.630 0.150 100 3.540 0.660 0.210

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phenolic OH. Similarly, the small band at 2936 cm1 corre-sponds to the CH stretching of the phenolic CH2. The factthat these regions did not change with treatment indicatesthe low reactivity of phenolic OH and CH2 with cement.The sharp peak at 1400 cm1 in the untreated soil decreased

gradually with treatment, while the intensities in the 1560and 1630 cm1 region increased. These changes are the op-posite of the results observed in the humic acid fraction,which indicate the change of the carboxylate group(COO) to the carboxyl group (COOH) under the highlybasic solution created during fractionation.

Summary and conclusionsThe 1D consolidation behavior of a highly organic soil

treated with Portland cement was investigated by performingconstant rate of strain (CRS) and incremental loading (IL)consolidation tests on specimens obtained from soilcementsamples cured under a surcharge of 48 kPa for 14 days. Thecement dosage varied from 8% to 100% by dry mass of thesoil, corresponding to values of the cement factor in the 24296 kg/m3 range. The most evident effect of treatment withcement is the increase in the soils preconsolidation stress.The increase is fairly modest for low dosages of cement, butas large as eightfold and 25-fold for cement dosages of 50%(148 kg/m3) and 100% (296 kg/m3), respectively. As a resultof the development of this cementation-induced preconsolida-tion stress, the compression curves shift to higher effectivestress levels: the higher the cement content the greater the ef-fective stresses that the soil can sustain at the same void ra-tio. Additionally, the compressibility of the cement-treatedsoil measured in the overconsoliated region also shows a sig-nificant decrease with increasing cement. These two effectsare an indication of the effectiveness of the treatment in re-ducing settlements.In contrast to the above, the compression index (Cc) in the

normally consolidated region does not show any significantchange with cement addition. However, the structure devel-oped as a result of the addition of cement appears stable,and does not show significant degradation as a result of 1Dloading beyond the preconsolidation stress.The hydraulic conductivity and coefficient of consolidation

of the treated soil increase with the addition of cement. Theincrease in hydraulic conductivity is caused by the change inthe soil fabric, in particular the increase in size of the macro-pores serving as the main channels for flow, which arisesfrom chemical reactions with the cement. Associated withthe increase in hydraulic conductivity is the increase in coeffi-cient of consolidation: compared with the untreated reconsti-tuted soil, Cv increases by 1.4 times with 8% PC (24 kg/m3),8 times with 20% (59 kg/m3) PC, and about 38 times with50% (148 kg/m3) and 100% (296 kg/m3) PC.The effects of treatment on the creep behavior of the or-

ganic soil were investigated by performing long-term sus-tained loading tests at the maximum effective stress reachedin the consolidation tests (1600 kPa). The creep coefficient,

Table 5. Percentage of C, N, and H present in humic and fulvic acid fractions extracted from 0% and20% PC soil.

Humic acid Fulvic acid

Cement content (%) C (%) H (%) N (%) C (%) H (%) N (%)0 49.9 4.7 3.6 41.2 2.5 1.120 45.4 5.6 5.2 34.8 5.6 4.720 45.4 5.5 5.2 39.9 6.3 5.5

Fig. 11. Effect of cement addition on FTIR spectra of (a) mineralfraction, (b) humic acid, and (c) fulvic acid.

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Ca, measured at this stress level is shown to decrease withcement content, by as much as a factor of 4 with 100% PC(296 kg/m3). As a result Ca/Cc also decreases. With 50%(148 kg/m3) PC, this ratio goes from the value of 0.095measured on the untreated reconstituted soil to a value of0.04 typical of inorganic clays. With 100% PC (296 kg/m3),Ca/Cc further decreases to 0.024, falling in the range of gran-ular soils (0.02 0.01).Fractionation tests and chemical and FTIR analyses pro-

vide insight into the interaction between soil organic matterand cement. The fractionation tests show that with increasingcement dosage, decreasing amounts of humic acid and fulvicacid can be extracted from the soilcement mixture. This in-dicates that a significant portion of the organic matter isboundencapsulated in the cement matrix. Moreover, elemen-tal analyses show a change in the composition of the humicacids and fulvic acids extracted from the cement-treated soil.This suggests that not all functional groups have the same re-activity with the cement. This hypothesis is supported by theFTIR analyses, which show, for example, that phenolicgroups have limited reactivity with the cement.While the research has focused on a particular soil, the ex-

perimental approach and the fundamental conclusions on theconsolidation behavior of cement-treated organic soil and onthe interaction between organic matter and cement can be ex-trapolated to other organic soils of a similar nature.

AcknowledgmentsThis work was supported by the Joint Transportation Research

Program administered by the Indiana Department of Transporta-tion and Purdue University under project no. C-36-50U, grantSPR-2460. The contents of this paper reflect the views of the au-thors, who are responsible for the facts and the accuracy of thedata presented herein. The contents do not necessarily reflect theofficial views or policies of the Federal Highway Administrationand the Indiana Department of Transportation, nor do the con-tents constitute a standard, specification or regulation. The writerswish to acknowledge the help of Dr. G.S. Premachandra of Pur-due Universitys Agronomy Department for assistance with thefractionation and FTIR spectroscopy.

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