Impact of Organic Coatings on Frictional Strength of...

Impact of Organic Coatings on Frictional Strength ofOrganically Modified Clay

B. Bate, A.M.ASCE1; Q. Zhao, A.M.ASCE2; and S. E. Burns, F.ASCE3

Abstract: Organic matter is frequently encountered in both naturally occurring and engineered particulate media. Charged functional groupsin the organic matter can lead to cation exchange within the clay interlayer, which results in the formation of an organic coating on the claysurfaces and alters the interfacial frictional regime in the soil mass. This study investigated the triaxial shear frictional behavior of montmoril-lonite particles coated with a controlled organic phase composed of quaternary ammonium cations. Through cation exchange, organic cationswere loadedonto the clay’s interlayer exchange sites, with control on the density of organic coverage and structure of the organic cation. Resultsdemonstrated that increasing the total organic carbon content of the clay resulted in increasing frictional resistance regardless of whether theincrease in carbon content was attributable to increased density of organic loading, increased cation size, or increased cation tail length.Concentrating organic carbon in one of the quaternary ammonium cation branch positions led to measureable gains in strength compared withdistributing the carbon over all four branch positions. Measured critical-state friction angles for the organoclays ranged between 34 and 61!,whereas all tested organoclays demonstrated peak strength coupled with contractive tendencies. The presence of the organic cations in the clayinterlayer led to alteration of the structure within the interlayer and is believed to have combinedwith forces from electrostatic bonding betweenthe organic cation head groups and the clay surface, as well as chain entanglement and dewatering, to contribute to the increased frictionalresistance of the modified organoclays. DOI: 10.1061/(ASCE)GT.1943-5606.0000980. © 2014 American Society of Civil Engineers.

Author keywords: Montmorillonite; Organobentonite; Organoclay; Quaternary ammonium cation; Triaxial tests; Shear strength; Criticalstate; Friction angle.

Introduction

Two types of organic matter are frequently encountered in geo-technical systems: natural organic matter (NOM), which has ahighly heterogeneous structure with variable properties, and engi-neered organic matter, which has a relatively simplified structurewith predictable properties. Because both the structure and densityof organic matter within a soil influence the frictional behavior, itis difficult to systematically quantify the influence of NOM on soilstrength because of the extreme heterogeneity of the complex NOMstructure; however, materials composed of simplified engineeredorganic matter (e.g., quaternary ammonium cations) can be studiedsystematically and are important in applications for waste contain-ment and remediation, such as permeable reactive barriers (Lee et al.2012).

Natural organic matter is generated through the metabolism,death, and breakdown of all types of organic life forms, yieldinglarge concentrations of dissolved or particulate materials that can

interact through nonpolar forces or are geochemically reactive withcharged inorganic geologicmaterials such as soil grains as a result ofthe presence of charged functional groups [e.g., hydroxyls (R–O–H),carboxyls (R–COOH), and phenolic compounds (C6H5OH)]. Thecontent of organic matter in soils is highly variable and can rangefrom values as low as 0.5–5% (by weight) in the surface horizon ofsoils to as high as 100% in organic soils (Sparks 2003). In contrast,engineered organoclays have an organic matter phase with a con-trolled structure and can be exchanged onto clay surfaces up to thelevel of the clay’s cation-exchange capacity (CEC). Organoclayshave gained increasing attention in geotechnical engineering forcontainment applications and are present in a variety of applicationsin geotechnical engineering, such as lubricants during pipe jacking,borehole stabilizers, and components of geosynthetic clay liners andslurry walls to increase resistance to highly concentrated chemicalsolutions. Both natural and synthesized organic coatings on claysoils are of engineering interest because they can affect soil prop-erties such as hydraulic conductivity and can provide a sorptivereservoir within a soil deposit (Burns et al. 2006; Li et al. 1996).

Whereas organic compounds are known to occur extensivelythroughout geotechnical systems, their influence on the fundamentalfrictional behavior of particulate materials is not well understood.When an organic phase coats a clay mineral such as montmoril-lonite, three fundamental frictional mechanisms are affected: (1)particle-to-particle interaction within the interlayer of the clay oc-curs through the organic phase in addition to interlayer water, (2)the alignment of the interlayer water molecules is distorted by thepresence of the organic molecules, and (3) the interparticle forces arealtered (Zhao andBurns 2012a). It is important to note that even in thepresence of inorganic interlayer cations, clay particles are not indirect contact but are interacting through the diffuse double layer(Mitchell and Soga 2005). Depending on the structure and density oforganic coatings within the clay interlayer, these mechanisms are

1Assistant Professor, Dept. of Civil, Architectural, and EnvironmentalEngineering,Missouri Univ. of Science and Technology, Rolla, MO 65409-0030 (corresponding author). E-mail: [email protected]

2Graduate Research Assistant, School of Civil and EnvironmentalEngineering, Georgia Institute of Technology, Atlanta, GA 30332-0355.E-mail: [email protected]

3Professor, School of Civil and Environmental Engineering, GeorgiaInstitute of Technology, Atlanta, GA 30332-0355. E-mail: [email protected]

Note. This manuscript was submitted on January 12, 2012; approved onJune 7, 2013; published online on June 11, 2013. Discussion period openuntil June 1, 2014; separate discussions must be submitted for individualpapers. This paper is part of the Journal of Geotechnical and Geoenvi-ronmental Engineering, Vol. 140, No. 1, January 1, 2014. ©ASCE, ISSN1090-0241/2014/1-228–236/$25.00.

228 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / JANUARY 2014

J. Geotech. Geoenviron. Eng. 2014.140:228-236.

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http://dx.doi.org/10.1061/(ASCE)GT.1943-5606.0000980mailto:[email protected]:[email protected]:[email protected]:[email protected]

shown to either decrease (Mitchell and Soga 2005; Terzaghi 1996)or increase (Cheng et al. 2007; den Haan et al. 1995) the frictionalinteractions of the particulate medium. Additionally, because theoccurrence, weight, and structure of natural organic material arehighly heterogeneous, it is difficult, if not impossible, to quantify theeffects of naturally occurring organic matter via a systematic method.Therefore, the focus of this study was on the synthetic organiccompounds (i.e., quaternary ammonium cations) that are used for soilmodification in geotechnical engineering, with implications inter-preted for NOM. The work studied the behavior of a clay mineralthat was coated with a known, controlled organic phase andquantified the frictional interactions as a function of both the densityof coating (including lateral extent and thickness) and structure oforganic cations. The cations were exchanged onto mineral surfacesunder controlled laboratory conditions, and quantification of thefrictional behavior was performed experimentally.

Background

Although organic soils are typically regarded as highly compressiblematerials with low frictional strength (Mitchell and Soga 2005;Terzaghi 1996), in many cases, high-effective-stress friction anglesof organic soils (ranging from 25 to 90!) have been reported (Chenget al. 2007; den Haan et al. 1995; Hight et al. 1992). Cheng et al.(2007) used computed X-ray tomography and electron microscopyto study the cause of the high friction angle measured for Dutchorganic soils. The high strength was attributed to dense subhorizontallaminae, which contained angular and platy particles of mediumsilt size, as well as lenslike structures outside the dense laminae.Hight et al. (1992) measured a friction angle of 34! for Bothkennarclay, which contains a relatively large organic content of 2–4%;however, in this case, the high strength was attributed to the domi-nant angular silt fraction. Peat, a general category of soil characterizedby high organic content, has also been reported to have friction anglesas high as 49–53!, which were measured for two Italian peats andattributed to fabric effects (Cola and Cortellazzo 2005). Similarly, ithas been theorized that diatoms, residual plant fabrics, andmicrofiberscontributed to the high friction angles found in organic soils (Chenget al. 2007). Additionally, it is also well documented that the un-drained strength ratios of organic clays are higher than those ofinorganic clays (Jamiolkowski et al. 1985; Leroueil 1990).

In this study, quaternary ammonium cations (QACs), which areproduced commercially for a variety of industrial applications, wereused as the organic phase in the synthesized soil. It is important to notethat QACs were chosen because of their simplicity in structure andwell-documented chemical properties, even though NOM possessesa far more complex structure than QACs. Whereas amine is an es-sential component of NOM, QACs are rarely found in NOM. Addi-tionally, properties ofNOMare rarely dominated by long alkyl chainsof surfactant such as hexadecyltrimethylammonium (HDTMA);however, because QACs are used in commercial applications (e.g.,engineered geosynthetic clay liners) and have been studied ex-tensively (e.g., surfactant arrangement, hydraulic conductivity,and sorptive behavior), and because they represent a quantifiable,repeatable organic phase, they were chosen for testing in thisstudy.

When combined with a mineral of measurable cation-exchangecapacity, QACs preferentially displace naturally occurring inorganiccations (e.g., Na1 and Ca21) and at low concentrations bond elec-trostatically to the mineral surface, producing a soil with a basemineral solid phase bonded to a specified organic phase. As a resultof its high cation-exchange capacity and high surface area, bentoniteis frequently used as the base soil for cation exchange in organoclays.

The primary clay mineral in bentonite is montmorillonite, which iscomposed of one layer of an octahedral aluminum sheet sandwichedbetween two tetrahedral silicon sheets. Because of the isomorphicsubstitution within the crystal structure, the clay layers are nega-tively charged. The clay’s basal oxygen atoms carry the excess ofnegative charge, which results in a strong electron-donating capacity(Brigatti et al. 2006). In the tetrahedral sheet, an excess negativecharge as a result of isomorphic substitution is primarily distributedover three surface oxygen atoms of one tetrahedron, whereas in theoctahedral sheet, an excessive negative charge in a single octa-hedron is distributed principally over the 10 surface oxygen atomsof the four tetrahedra (Sposito 1984). The localization of chargethat is formed in the tetrahedral sheet of montmorillonite results instronger complexes with cations and dipolar molecules than thoseformed in the octahedral sheet (Brigatti et al. 2006; Sposito 1984),even though the excess negative charge of montmorillonite islocated primarily in the octahedral sheet (Mitchell and Soga 2005;Van Olphen 1977).

The intercalation of QACs into the interlayer space ofmontmorillonite results in a QAC-clay complex with a structure thatdiffers significantly from that of pure sodium montmorillonite. Be-cause the microscale interparticle forces between clay particles arealtered in the presence of organic cations, the frictional behavior ofQAC clays also may be altered at the macroscale and variesdepending on the type of organic cation and the cation loading on theclay surface (Burns et al. 2006). For pure sodium montmorillonite,the layer charge of the 2:1 mineral is balanced by the interlayersodium cations, whichmaintain the electrical neutrality of the system,whereas clay particles impose particle-to-particle long-range re-pulsion forces to resist external load (Callaghan and Ottewill 1974).In contrast, for QAC-coated clays, the charged organic cation headgroups are strongly bonded to the clay surface (He et al. 2004; Liuet al. 2009), resulting in partial neutralization of the negativesurface charge of the clay, as indicated by measured zeta poten-tial (Bate and Burns 2010). Zeta potential, which is the electricalpotential in the diffuse double layer and is an indicator of the netsurface charge on the particles, will also influence the develop-ment of the clay fabric during slurry consolidation (Palomino andSantamarina 2005), which, in turn, will exert influence on theinterfacial friction behavior of the clay particles. In the presence ofinterlayer organic cations, the interparticle repulsion is weakened,which can result in flocculation when the amount of adsorbedQACs is less than 100% of the cation-exchange capacity of the clay(Janek and Lagaly 2003; Van Olphen 1977; Xu and Boyd 1995b).Flocculation of clay particles results in decreased pore volumes,which is verified by the collapse of basal spacing observed incalcium montmorillonite (Sheng and Boyd 1998), N2 adsorption-desorption (He et al. 2006), decreased water content of QAC clays,and increases in effective consolidation pressure, all of whichcontribute to an increase in the shear strength of the synthesizedclay (Mitchell and Soga 2005).

Interlayer inorganic cations such as sodium and calcium affectthe frictional resistance of clays through double-layer effects andclay fabric formation. In contrast, when QACs are intercalated inthe interlayer of a 2:1 clay mineral at less than 100% of the cation-exchange capacity, the QAC chains arrange in a disordered state(liquidlike) (Vaia et al. 1994). The dominant interactions betweenQAC chains are entanglement and lateral hydrophobic interactions(Xu and Boyd 1994, 1995a), which result in energy dissipation andincreased frictional resistance. It is important to note that whenorganic cations are added to low- and medium-charge montmoril-lonite, the QACs do not interact with each other via tip-tip in-teraction, which would create an interaction regime of lubrication asa result of the ordered solidlike state of the cations (He et al. 2004;

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Wang et al. 2000). In contrast, in organoclays, the cations exist ina liquidlike disordered state at the molecular scale, which increasesthe frictional energy required to compensate carbon-chain bendingand rotation; consequently, the particle-particle interactions are gov-erned by energy-dissipative mechanisms such as chain entan-glement (Chen and Israelachvili 1992; Yoshizawa et al. 1993),which becomemore pronounced as surfactant packing shifts fromlateral monolayers to lateral bilayers, pseudotrilayers, or paraffin-type packing.

In terms of frictional interactions, a regime of particle-to-particlelubrication only dominates in self-assembled monolayer packingwhen a double-chain carbon surfactant is loaded onto a particlesurface at a very high density, resulting in tight packing of thesurfactant chains and large basal spacings (up to 60 Å) (Boschkovaet al. 2002; Lagaly et al. 2006). In the case of friction measurementsmade at low pressure, the surfactant forms a fluidlike layer thatcontrols particle-to-particle interaction; that is, the tails of the sur-factant chains interact. However, changing the characteristics ordensity of loading of the surfactant and the confining pressure canactually change the particle-interaction regime from lubrication toincreasing friction. Choosing a surfactant that exhibits disorder at themolecular level can increase friction, and interactions becomegoverned by energy-dissipative mechanisms such as chain entan-glement, which become more pronounced as the density of sur-factant packing on the surface is decreased. Decreasing the densityof packing of the surfactant can also lead to changes in the frictionalregime because the surfactant monolayer no longer acts as a unifiedfluid layer but instead leads to entanglement of the surfactantmolecules (Chen and Israelachvili 1992; Yoshizawa et al. 1993).

Although many properties of organoclays have been studied,e.g., sorption capacity, swelling, and hydraulic conductivity, littledata are available for the strength of organoclays. Burns et al. (2006)carried out direct shear tests on bentonite that had been exchangedwithHDTMAand benzyltriethylammonium (BTEA) cations, wheresurface coverage of the organic cation varied from 50% cation-exchange capacity (CEC) exchanged to 100% CEC exchanged. Themeasured direct shear peak friction angle decreased as the surfacecoverage of HDTMA was increased; however, BTEA clay showedan increase in strength as the total organic carbon content was in-creased (Burns et al. 2006). It was hypothesized that the degree ofdisorder in the packing of the benzene ring, in addition to a com-ponent of chemical adhesion, was responsible for the increase inmeasured strength for the BTEA clays. In addition, literature valueshave reported a measured direct shear friction angle of 34! fortrimenthylammonium (TMA) bentonite exchanged at 85% of CEC(Soule and Burns 2001).

This study details the results of an experimental investigationdesigned to measure the shear strength of seven organically modi-fied clays. In this work, five different QAC structures were studied,with the cations chosen to reflect the influence of increasing thecation size through equal branching at all four quaternary positionsversus increasing the cation tail-length effect through increasing thecation size at only one quaternary position. These variables werechosen because previous study of zeta potential suggested thatQACs with a single long carbon chain behaved differently fromthose with four shorter carbon branches (Bate and Burns 2010). Asthe branched-chain length was increased, the interaction betweenthe 2:1 clay mineral and head nitrogen group was weakened, asopposed to the behavior observed for organoclaysmade fromcationswith carbon concentrated in a single quaternary position,whose headgroups always remain bonded strongly to the surface. In addition, theeffect of cation loading also was studied by synthesizing clays atcation-exchange percentages of 30, 60, and 100%. The experimentalresults quantify the behavior of engineered organoclays and also

were interpreted with implications for the behavior of naturallyoccurring, organic-rich soils.

Materials and Methods

Wyoming bentonite (Volclay CG-50, CETCO, Hoffman Estates,Illinois), composed primarily of sodium montmorillonite, was thebase clay for the study andwas used as received. The natural organiccarbon content of the material was 0% (TOC-VCPN, Shimadzu,Kyoto, Japan), and its CEC was 69:1meq=100g (Hazen Research,Golden, Colorado). X-ray diffraction (XRD) analysis demonstratedthat the predominant mineral contained in Volclay CG-50 wasmontmorillonite (76 wt %), with secondary phases including quartz(11 wt %), feldspar (10 wt %), illite (1 wt %), and other phases intrace quantities (2 wt %). Quartz and feldspar have limited in-teraction with QACs.

Five QACs were chosen for study [chemical structures shown inBate andBurns (2010)]: tetramethylammonium (TMA, denoted 4C1)chloride [ðCH3Þ4NCl], tetraethylammonium (TEA, denoted 4C2)bromide [ðCH2CH3Þ4NBr], tetrabutylammonium (TBA, denoted4C4) bromide [ððCH2Þ3ðCH3ÞÞ4NBr], decyltrimethylammonium(DTMA, denoted 1C10) bromide [ðCH3Þ3NC10H21Br], andhexadecyltrimethylammonium (HDTMA, denoted 1C16) bro-mide [ðCH3Þ3NC16H33Br]. All cation salts were obtained fromFisher Scientific and were used as received. The water used in allexperimentation was deionized (Barnstead E-pure, ThermoFisher Scientific, Waltham, Massachusetts).

The organoclays were synthesized using the method previouslydescribed in Bate and Burns (2010). In summary, the organoclayswere synthesized in the laboratory by exposing the particle surfacesof the bentonite to an aqueous solution containing theQACat 30, 60,or 100% of the CEC of the clay. The organic compound was dis-solved in 40 L of deionized water, and 2 kg of the clay was added tothe aqueous solution. The resulting suspension was mechanicallystirred for 1 h and allowed to stand for a minimum of 24 h to allowgravity separation. The supernatant was then siphoned off, andthe solids were rinsed with deionized water to remove any saltsor loosely bound cations, and the process was repeated until theconductivity of the supernatant was below 600mS=cm.Mineralogicimpurities were separated by gravity settling. The slurry was thenstored in airtight containers until tested. TMA-exchanged clayswereprepared at 30, 60, and 100% of CEC to test the effect of density ofcoverage, and TEA-, TBA-, DTMA-, and HDTMA-exchangedclays were prepared at 100% of CEC to test the effect of cationstructure and cation size.

The total organic carbon (TOC) content was measured for eachorganoclay using an organic carbon analyzer (TOC-VCPN,Shimadzu, Kyoto, Japan) and solid-sample module (SSM-5000A,Shimadzu, Kyoto, Japan). The sample was combusted at 680!C inthe presence of an oxidation catalyst, and the resulting CO2 wasmeasured using a nondispersive infrared (NDIR) gas analyzer. Aknown reference material, anhydrous dextrose powder (C%5 40%by mass; Fisher Scientific), was used as the calibration source, andcalibrations were performed daily during measurements. Themeasured TOC content in each organoclay agreed reasonably wellwith the calculated values of sorbed carbon (Table 1).

X-ray diffraction tests were performed on samples of unmodifiedbentonite and seven organoclays. The samples were prepared byoven drying, followed by grinding with a mortar and pestle untila particle size passing a #325 sieve was achieved. Approximately0.1 g of the dry powder was combined with 20 mL of deionizedwater, and the suspension was ultrasonically dispersed for 30 min.Next, the suspension was extracted with a pipette, placed on a glassplate, and covered to prevent dust from accumulating on the sample.



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Bate Bate

The sample was allowed to stand for 72 h to allow air drying at roomtemperature and humidity. X-ray diffraction patterns were generatedusingCuKa1 radiation on a PANalytical diffractometer operating at45 kV and 40 mA, filtered with a Ge single-crystal monochromator.The samples were scanned between 6 and 60! (2u) at a step size of0.02! and a 1-s count time per step.MDI Jade 8.5 software was thenused to analyze the XRD data obtained, including peak matching.

After exchanging the organic coating onto the clay surface,samples were prepared for strength testing using one-dimensionalconsolidation according to the following procedure: after cationexchange was completed, the slurry was carefully poured intoa stainless steel slurry consolidometer, taking care to avoid entrap-ment of air. The dimensions of the consolidometer were 10.2 cm(4 in.) in diameter and 45.7 cm (18 in.) in height. Initially, a seatingload of less than 3.5 kPa was applied to the slurry, and properalignment of the geotextile and filter paper was verified. Next, thecylinder was transferred to the load frame for stress-controlled one-dimensional slurry consolidation. Axial load was placed on theslurry in increments of 3.5, 7, 14, 28, 56, and 100 kPa (Geotac loadframe, Geotac, Houston, Texas) until end of primary consolidation,as judged by the Taylor square-root-of-timemethod,was achieved ateach load step. The slurry samples were monitored daily to limitsecondary compression. It is important to note that friction losseswere not measured in the slurry consolidometer and that the ultimatevertical consolidation load on the slurry was lower than the nominal100 kPa.Once the ultimate preloading stresswas achieved, the slurrywas unloaded by the same increments in the reverse order.Depending on the type of organoclay, the duration of each load stepranged from 1 day up to 3 weeks to ensure completion of primaryconsolidation. P5 filter paper (Fisher Scientific, Hampton, NewHampshire) and a nonwoven geotextile were placed on top andbottom of the samples to facilitate drainage. To maintain the ionicstrength of the pore fluid during slurry consolidation, a 0.001 MNaCl solution (ionic strength chosen to be equivalent to that of thefinal organoclay slurry pore fluid after one-dimensional consoli-dation) was used to maintain saturation. After the slurry consoli-dated, the samples were extruded from the consolidation tube, andthe specimen was divided into three parts for strength testing.Samples were trimmed with a soil lathe and wire saw into 3.6-cm-diameter (1.4-in.) and 7.6-cm-high (3-in.) samples. The specimenswere not consolidated to an equal dry density but were allowed todevelop an equilibrium fabric under the applied final consolidationload. Consequently, the final water contents of the slurry consoli-dated samples ranged between 136 and 228% (averaged value ofthree specimens; see Table 1) and were comparable for the threetrimmed specimens.

Isotropically consolidated undrained triaxial compression testswere performed using an automated load frame (Geotac, Houston,Texas). All shear tests were strain-rate-controlled tests and were

performed in accordance with ASTM D4767-11 (ASTM 2011).Based on the time-deformation readings recorded during late-stageslurry consolidation, a strain rate of 0.5% per hour was selected fortesting. Samples were sheared at nominal (actual) effective confiningpressures of 50 (50:96 2:2), 100 (98:06 2:7), and 200 kPa(199:16 2:5 kPa) with a backpressure of 140 kPa during shear, andall samples recorded a minimum B-value of 0.95 before shearing.Corrections for cross-sectional area change, membrane stiffness, andfriction between loading rod and bushing were accounted for in thecalculation of applied stress. The corrected heights and diameters ofthe samples were used in the shearing-test calculation by assumingdeformation of a right circular cylinder. The inflow and outflow fluidsused in sample compression tests were 0.001 M NaCl solution tomaintain consistent background ionic strength between the samplesformed from clay slurry and the bulk solution during triaxial testing (i.e., there was no decrease in ionic strength after sample preparation).To prevent corrosion within the panel controls, the salt solution wasisolated from the deaired (Nold deaerator, SerialNo. 790,WalterNoldCo., Natick, Massachusetts) water by two P620000 bladder accu-mulators (TrautweinSoil TestingEquipment, Houston, Texas).Watercontents of the samples at the end of slurry consolidation and post-shear varied depending on the organic cation tested (Table 1).

Results

The results of the experimental strength tests were analyzed ac-cording to changes in four different variables: (1) increasing thecation loading through an increase in the quantity of organic carbonthat was exchanged (i.e., increasing % CEC exchanged), (2) in-creasing the length of all four quaternary ammonium branches si-multaneously (TMA→ TEA → TBA), (3) increasing the length ofonly one branch on the QAC (TMA → DTMA → HDTMA), and(4) increasing the total organic content of the organoclay samples.The stress-strain relationships were analyzed in terms of Cambridgestress variables: effective mean normal stress p9, p95 ðs19 1 2s39Þ=3,deviatoric stress q, q5 ðs19 2s39Þ, and axial strain (vertical strain) ɛ.

The coefficient of consolidation Cv for the seven organoclayswas analyzed using the Taylor square-root-of-time method withdouble drainage. To compare the different organoclay samples asquantitatively as possible, two case scenarios were examined foreach of the seven organoclay samples. First, the data were analyzedat the end of one-dimensional slurry consolidation of the samples(nominal vertical loading of 100 kPa), and second, the data wereanalyzed during the consolidation stage of the triaxial shear test at200 kPa. Data from the slurry consolidation tests performed onorganoclays exchanged at 100% CEC showed similar magnitudesof Cv, ranging from ∼0:53 1026 m2=min to ∼13 1025 m2=min.Data from the 200-kPa consolidation stage of the triaxial testresulted in similar orders of magnitude for the measured values of

Table 1. Physical and Mechanical Properties of Tested Organoclays

Organiccations

CEC exchanged(%)

Liquid limit/plasticityindex (%)

Measured/theoreticalTOC (%)

Water contenta

(%)

Water contentb (%)fcs9

(degrees)50 kPa 100 kPa 200 kPa

TMA, 4C1 30 — 1.0/1.0 164 144 121 105 34.060 — 1.5/1.9 162 141 122 101 38.5

100 266/184 2.5/3.2 184 172 152 126 41.1TEA, 4C2 100 140/74 5.4/6.1 156 136 120 92 46.7TBA, 4C4 100 118/46 8.7/11.4 136 131 107 82 48.4DTMA, 1C10 100 205/98 7.9/9.5 228 206 190 146 52.6HDTMA, 1C16 100 219/130 15.1/13.2 148 147 116 99 60.8aAfter one-dimensional slurry consolidation, before isotropic consolidation (nominal 100 kPa).bAfter isotropic consolidation.



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Cv (∼33 1027 m2=min to ∼63 1025 m2=min), which increased asthe percent CEC exchanged increased, with the long-chain organiccarbon cations (DTMA and HDTMA) resulting in a higher value ofthe coefficient of consolidation compared with branched cations(TEA and TBA). It is important to note that the Cv values obtainedfor slurry consolidation and triaxial consolidation were at differentlevels of stress and should not be compared directly. Increases in themagnitude of the coefficient of consolidation may indicate increasedstructuring within the organoclay and are also related to increasedhydraulic conductivity of the organoclay, which has been demon-strated to increase as the total organic content of the clay is increased(Lorenzetti et al. 2005).

The presence of the organic cations in the clay interlayer resultedin alterations of the basal spacing of the modified clay minerals(Table 2). The unmodified clay (inorganic cations at room humidity)had a basal spacing of 15.3 Å. In comparison, the five organoclaysmade with smaller, branched cations (TMA, TEA, and TBA) hadbasal spacings that ranged between 14.4 and 15.1Å. These branched-cation clays demonstrated a monolayer of surface coverage by theintercalated organic cation,which resulted in a basal spacing similar tothat produced by the inorganic cations (Zhao and Burns 2012a). Incontrast, as the size of the cation was increased by increasingthe length of the tail in only one quaternary position (DTMA andHDTMA), the basal spacing shifted from a dual monolayer/bilayerarrangement for DTMA (14.3–17.1 Å) to a pseudotrilayers ar-rangement for HDTMA (20.0 Å) (Zhao and Burns 2012a).

All organoclays tested in consolidated undrained triaxial com-pression demonstrated peak shear strength, with positive pore-waterpressure generation, as exhibited by the stress-strain behavior ofDMTA [Fig. 1(a)]. Stress-strain behavior of other tested organo-clays was similar (data not shown). The pronounced peak behaviorwas present despite the relatively low levels of overconsolidation[overconsolidation ratio (OCR) , 2] to which the specimens weresubjected during consolidation and preparation. Although the sam-ples exhibited peak behavior, there was no recorded tendency towarddilation because no negative pore-water pressures were recorded[Fig. 1(b)]. It was also noted that the peak shear strength was reachedmore rapidly (i.e., at smaller strains) at low effective confiningpressures than at high effective confining pressures [Fig. 1(a)]. Inp9-q space, the peak and critical-state friction angles were cal-culated by regression analysis from stress-strain curves at threedifferent confining pressures [Fig. 1(c)]. The coefficient of de-termination (r2) ranged from 0.9854 to 1.0000 for peak friction

angles and from 0.9648 to 0.9995 for critical-state friction angles,indicating good correlation. Sample replicates, which were testedin duplicate for the 60- and 100HDTMA organoclays, yieldedsimilar test results.

Although peak strength is not commonly observed in recon-stituted, normally consolidated soils, it is encountered in organicsoils (Cheng et al. 2007; den Haan et al. 1995). Peak frictionalbehavior is indicative of structuring within the soil (Poulos 1988),which in this case is most likely caused by the interfacial attractive

Fig. 1. Consolidated undrained triaxial compression strength testresults for 100% DTMA: (a) stress-strain diagram; (b) pore-waterpressure diagram; (c) q-p9 diagram for 100% DTMA clay

Table 2. Basal Spacing and Electrokinetic Properties of QACs andOrganoclays

ClayBasal spacing

(Å)Head radiusb

(Å)

Fullystretched

N, last C ontail/head H,tail Hb (Å)

Zetapotentialc

(mV)

Unmodifiedbentonite

15.3 — — 238:3

30TMA 15.1 2.41–2.50 — 232:760TMA 14.4 2.41–2.50 — 233:2100TMA 14.4 2.41–2.50 — 231:7100TEA 14.5 3.55–3.58 — 221:0100TBA 15.0 5.66–5.97 — 215:2100DTMA 14:3=17:1a 2.41–2.50 12.69/15.09 213:0100HDTMA 20.0 2.41–2.50 18.95/21.37 20:2aBimodal pattern observed, secondary peak at 17.1 Å.bMaterials Studio 5.5.cZeta potential data (Bate and Burns 2010).



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Bate Bate

Bate Bate

forces between the exchanged organic cations and the clay mineral(Zhao and Burns 2012b), in addition to the small overconsolidationdeveloped during slurry and triaxial consolidation.

The normal consolidation lines (NCLs) (100- and 200-kPaincrements) and critical-state lines (CSLs) were plotted by using thep9 values at the end of isotropic consolidation, the p9 values at thecritical state, and the void ratios from phase relationships (Fig. 2).Initially, after isotropic consolidation, all organoclays were on thecontractive side of the critical-state line, which is typical of normallyconsolidated fine-grained soil behavior. In the e-logðp9Þ plot, theNCLs were roughly parallel with the CSLs, although deviation wasnoted in some instances (100TMA, 100DTMA, and 100HDTMA)as higher pressures were reached. Typically, the NCLs and CSLsmeasured for consolidated clays will be approximately parallel, incontrast to the behavior of sands, which can exhibit nonparallel linesdepending on the initial state of the sand (Jefferies and Been 2006;Rutledge 1947; Terzaghi 1996). Sadrekarimi and Olson (2011)observed that NCLs and CSLs became steeper as a result of thecrushing of sandy soils at high stress. Nonplastic and low-plasticitysilts also exhibit behavior more similar to sands, with nonparallelNCLs and CSLs (Boulanger and Idriss 2006; Romero 1995). It isbelieved that the divergence in the NCLs and CSLs observed athigh pressures for organoclays is attributable to the large defor-mation-induced structure change and aggregate size changes withinthe clay fabric at critical state. It is noted that the plasticity of thetested organoclays was very low compared with that of unmodifiedbentonite (Table 1).

In all cases, the measured critical-state friction angle was sig-nificantly influenced by the soil’s TOC content, with the mea-sured frictional resistance for all tested clays increasing as the TOCcontent increased (Fig. 3). The high friction angles measured forthe organoclays are in contrast to the low critical-state friction anglereported for unmodified sodium montmorillonite (approximately11!) (Mesri and Olson 1970). The predominant mode of failure inthe testing programwas the formation of either onemajor shear planeor multiple shear planes.

The effect of density of cation coverage was tested using TMAclays synthesized at organic cation exchanges of 30, 60, and 100%of CEC [Fig. 4(a)]. The critical-state friction angle increased from34.0 to 38.5 to 41.1! for 30, 60, and 100% CEC exchanged TMAorganoclays, with increasing organic loading resulting in a steadilyincreased friction angle. Similarly, the effect of the size of the or-ganic cation was tested using clays that were synthesized by in-creasing the chain length on all four branches of the quaternaryammonium position, with one, two, and four carbon chains in eachof the four branches (TMA4C1, TEA4C2, andTBA4C4) [Fig. 4(b)].The critical-state friction angle increased from 41.1 to 46.7 and48.4! for 100%CEC exchanged TMA, TEA, and TBA organoclays,respectively, with most of that increase occurring in the transitionfrom TMA to TEA, followed by a small increase in the transitionfrom TEA to TBA. The effect of increasing the length of the carbontail was tested using organic cationswith 1, 10, and 16 carbons in oneof the tail positions (TMA 4C1, DTMA 1C10, and HDTMA 1C16)[Fig. 4(c)]. Gains in the critical-state friction angle were observedas the carbon tail length was increased. Critical-state friction angleincreased from 41.1 to 52.6 and 60.8! for 100% CEC exchangedTMA, DTMA, and HDTMA organoclays, and the increase wasapproximately linear with the length of carbon tail elongated. In allcases, increasing the TOC content on the clay surface resulted in anincrease in the measured frictional resistance of the clay; however,the effectwasmost pronouncedwhen the length of the cation tail wasincreased (i.e., TMA, DTMA, and HDTMA).

Discussion

When compared with unmodified bentonite, the organoclays testedin this study exhibited a marked increase in frictional resistance thatcan be attributed to multiple mechanisms within the structure of the

Fig. 2. Normal consolidation lines (NCLs) and critical-state lines(CSLs) for tested organoclays: (a) increasing branch size (e.g., TMA,TEA, and TBA); (b) increasing single-tail length (e.g., TMA, DTMA,and HDTMA)

Fig. 3. Critical-state friction angle for tested organoclays as a functionof measured total organic carbon



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modified clays. Most distinctly, increasing the TOC content of theclay soils resulted in an increased critical-state friction angle (Fig. 3).However, it is important to note that the amount of water retained byeach organoclay after slurry consolidation varied significantly forthe organoclays tested (e.g., TMA versus TEA/TBA or TMA/DTMAversus HDTMA). Analysis of the trends of measured mass of or-ganic QACs sorbed to the clay surface relative to the mass of water

retained (mQAC=mwater) demonstrated that both the level of con-solidation stress and the total organic content affected the final watercontent of the soil (Fig. 5), as well as the relative positions of theNCL lines of the organoclays (Fig. 2). At low levels of total organiccontent (TMA organoclays with TOC , 4%), the ratio of mass oforganic content to mass of water demonstrated slight dependence onthe level of consolidation stress [Fig. 5(a)]; however, at total organiccontents greater than 6% (e.g., TEA, TBA, DTMA, and HDTMAorganoclays), mQAC=mwater increased as the confining stress wasincreased from50 to 100 to 200 kPa, indicating that significantly lesswaterwas retained at high stress and high organic content.Moleculardynamics (MD) simulations have shown that when organoclayswere synthesized with TMA cations, the TMA cation form discretepillars that prop open the interlayer but do not saturate the claysurfacewith organic cations, leaving clay surface-area sites availablefor water sorption (Zhao and Burns 2012a). Similar patterns wereobserved in MD simulation for DTMA organoclays synthesiz-ed with low-charge montmorillonite, which formed a transitionalmonolayer/bilayer organic interlayer structure, resulting in partialcoverage of the clay surface with somewater sorption sites available(Zhao andBurns 2012a). This structure is also supported by theXRDdata measured in this study. In contrast, theMD simulations showedthat HDTMA cations formed a pseudotrilayer structure that resultedin essentially complete coverage of the clay surface area, leavinglittle area available for water sorption (Zhao and Burns 2012a).Whereas TEA and TBA cations were not studied in the MD sim-ulations, it is believed that the TEA and TBA cations, which have

Fig. 4. Deviatoric stress as a function of effective mean normal stress:(a) as a function of increasing surface coverage of TMA; (b) as a func-tion of increasing cation branch size for TMA, TEA, and TBA; (c) as afunction of increasing cation tail length for TMA, DTMA, and HDTMA

Fig. 5. Critical-state friction angle as a function of mQAC=mwater ratioand cation structure



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relatively large molecular size compared with TMA, also blockwater access to surface sorption sites and therefore behave similarlyto HDTMA, even though the measured basal spacing for these claysdid not indicate bilayer/pseudotrilayer arrangements. It follows thatthe organoclays formed from cations that exhibited only partialsurface coverage by the organic cations (e.g., TMA and DTMA)were able to sorb larger quantities of interlayer water, resulting inlower strength than the organoclays that were formed with cationsthat blocked water access to surface sorption sites (e.g., DTMA,HDTMA, TEA, and TBA) [Fig. 5(b)]. It is worth noting that withthe exception of DTMA bentonite, the NCL curves with differentorganic cations followed similar trends: as retained water increased,void ratio increased (e.g., TMA, TEA, and TBA and TMA andHDTMA) (Fig. 2). In the case of DTMA bentonite, the void ratiowas almost the same as that of TMA, although its retained waterwas much lower than that of TMA bentonite. It is believed that thefabric of DTMA bentonite contributed to the higher void ratio.

Another mechanism that will affect the measured strength of theorganoclays is the arrangement of cations within the interlayer.When compared with clay behavior with sodium as the interlayercation, MD simulation has shown that the presence of the organiccations in the interlayer space significantly altered the structure ofthe clay-particle interlayer, resulting in a hydrophobic region withincreased basal spacing for the long-chain cations (e.g., DTMA andHDTMA) (Zhao and Burns 2012a). This is also substantiated by theincreased basal spacingmeasured usingXRD in this study (Table 2).Additionally, unlike inorganic cations, the organic cations have anuncharged hydrophobic portion of the molecule in the C–H tails, inaddition to the region of positive charge that is centered on theammonium head group. The concentration of nonpolar carbon andhydrogen groups in the cation tails resulted in a hydrophobic drivingforce that led to preferential attraction of the organic cation to theparticle surface as a result of hydrophobic expulsion of the cationfrom water. When combined with the electrostatic attraction be-tween the positively charged head group and the negatively chargedclay mineral surface, the driving forces for sorption of the organiccations to the clay particle were strong. The impact of the adsorptionof QACs onto the clay particles was reflected in previously reportedvalues of zeta potential, which yielded a less negative zeta potentialas the organic content was increased (Table 2), indicating that moreof the positive charge was bound close to the particle surface, withinthe shear plane of the particle (Bate and Burns 2010). As zetapotential approached neutral charge, interparticle repulsion de-creased, leading to increased frictional interaction, even in instancesof higher basal spacing (e.g., DTMA and HDTMA).

Additional mechanisms that can affect the measured frictionalstrength of the organoclays include the fact that past investigationshave shown that exchanging organic cations onto the clay surfacecan result in increased chain entanglement, especially as the lengthof the carbon chains is increased (Xu and Boyd 1994, 1995a), whichwould act to increase frictional resistance in a particulate material.Additionally, increased friction can result from lateral interactionsbetween the hydrophobic cation tails, which become significant asthe length of the carbon chain increases (C-chain length approxi-mately 12 or longer) (Xu and Boyd 1994, 1995b; Zhang andSomasundaran 2006; Zhang et al. 1993). As the organic loading wasincreased or as the cation size was increased, the cations arranged inmonolayers (e.g., TMA, TEA, and TBA), transitional monolayers/bilayers (e.g., DTMA), or pseudotrilayers (e.g., HDTMA)within theinterlayer, as illustrated by themeasured basal spacings and noted byother researchers (Heinz et al. 2007; Liu et al. 2009; Zhang andSomasundaran 2006). In the case of monolayer coverage, the or-ganic cations in the interlayer were partially bound to both clayparticles (Fig. 6) (Zhao and Burns 2012a), which would contribute

to increased frictional resistance between the particles. However,as organic coverage was increased, the cation arrangement shiftedto bilayer/pseudotrilayer coverage (Fig. 6), which resulted in thepositively charged head groups associating preferentially with onlyone of the particle surfaces, and it is believed that this shifted thefrictional mechanism to include chain entanglement.

This study identified three primary variables that affected themeasured frictional strength of organoclays: TOC content, watercontent, and structure of the organic cation. As the TOC content ofthe organoclay was increased, the ratio of mass of sorbed carbon tosorbed water increased (mQAC=mwater), which resulted in increasedfrictional resistance of the organoclay [Fig. 5(b)]. It was also shownthat the structure of the organic phase affected the measured fric-tional resistance, with organoclays synthesized with carbon con-centrated in one long chain (e.g., DTMA and HDTMA) showingincreased frictional resistance when compared with organoclayssynthesized with carbon equally distributed between the fourbranched carbon groups (e.g., TEA and TBA). It is believed that theincreased frictional resistance observed in the long-chain carbonclays is attributable to entanglement of the cation chains and lateralinteractions between the hydrophobic chains (Fig. 6).

Conclusions

This study investigated the stress-strain behavior of organoclaysusing consolidated undrained triaxial tests. The study demonstratedthat in all cases, the critical-state friction angle of organic-coatedclays was higher than that of unmodified montmorillonite and thatthe critical-state friction angle increased as the total organic loadingand cation size increased,withmeasuredfcs9 ranging between 34 and60.8!. Increases in the measured frictional resistance were attributedto an increased ratio of mass of organic cation to water as the organicloading was increased and to increased chain entanglement, lateralhydrophobic interactions, and decreased distance between the cationhead groups and the charged clay surfaces that become more pro-nounced as the length of a single-cation tail is increased by addingmore carbon (Liu et al. 2009; Xu andBoyd 1994, 1995a). Interactionof these multiple mechanisms is complex, with monolayer bondingof clay platelets dominating for small cations and at low TOCcontents and tail interactions dominating at large TOC contents;however, at all levels of loading, the alteration of water and cationpacking in the interlayer combined to increase the measured fric-tional resistance of the particulate material (Liu et al. 2009; ShengandBoyd 1998).Whereas the tested organic phases did not representa one-to-one correspondence with the organic matter phases thatoccur naturally in soils, the observed trends of interlayer waterdisruption, chain entanglement, and enhanced bonding also wouldbe present in the natural organic material found in soils. Con-sequently, it is believed that part of the frictional resistancemeasuredin soils with high organic content can be attributed to the presence

Fig. 6. Monolayer and pseudotrilayer structures of organoclays withdifferent exchanged cations: (a) monolayers (100TMA); (b) pseudo-trilayers (100HDTMA)



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and physicochemical interaction of the organic phases on the particlesurfaces.

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