Hydraulic Conductivity of Bentonite Slurry Mixed · PDF fileHydraulic Conductivity of Bentonite Slurry Mixed Sands David Castelbaum1 and Charles D ... correlated poorly with the total

Hydraulic Conductivity of Bentonite Slurry Mixed SandsDavid Castelbaum1 and Charles D. Shackelford2

Abstract: The hydraulic conductivity �k� of specimens from columns containing initially dry sands mixed with bentonite slurries wasmeasured. The mixed specimens represented a range in void ratios �0.672�e�3.94� and bentonite contents �0.61%�BC�7.65%, by dryweight�. The measured k values, which ranged from 2.4�10−7 cm /s to 6.8�10−4 cm /s, correlated poorly with the total void ratio �e� ofthe specimens, due to the complicating effect of the bentonite in the sand-bentonite slurry mixtures. However, the measured k valuescorrelated better with the void ratio of the bentonite �eb�, which is consistent with the results of previous studies involving permeation ofcompacted bentonite and sand-bentonite specimens, even though the range in values of eb in this study �42.5�eb�127� was much higherthan that previously reported. The relatively large range in eb values for the sand-bentonite slurry mixtures was also consistent with therelatively large range in measured k values, which are about one to seven orders of magnitude higher than values of k commonly reportedfor compacted sand-bentonite mixtures, despite similar bentonite contents. In terms of bentonite content, addition of more than 3%bentonite via slurry injection and mixing with the sands was successful in reducing the k of the unmixed sands �9.4�10−3 cm /s�k�5.4�10−2 cm /s� by as much as four orders of magnitude to values less than 1.0�10−6 cm /s.

DOI: 10.1061/�ASCE�GT.1943-5606.0000169

CE Database subject headings: Bentonite; Hydraulic conductivity; Sand, soil type; Slurries; Soil mixing.

Introduction

Subsurface contamination of soil and ground water by chlorinatedsolvents �e.g., tetrachloroethene, trichloroethene, carbon tetra-chloride� resulting from inadvertent leaks, spills, and improperdisposal practices has been prevalent over the past half-century orso. Such chlorinated solvents present a unique remediation prob-lem for at least two reasons �Shackelford et al. 2005�. First, chlo-rinated solvents tend to be stable under common subsurfaceconditions. Second, although the aqueous solubility of chlorinatedsolvents is relatively low, aqueous phase concentrations of thesesolvents still can be several orders of magnitude greater than es-tablished maximum contaminant levels. Past remediation tech-nologies, such as pump-and-treat, have had limited success incost-effectively restoring aquifers to typical cleanup levels �e.g.,Shackelford and Jefferis 2000�. In this regard, the relatively re-cent development and implementation of the zero-valent iron�ZVI�-clay technology as an alternative in-situ remediation ap-proach for treatment of source zones contaminated with chlori-nated solvents has shown promise �e.g., Shackelford et al. 2005;Wadley et al. 2005�.

The ZVI-clay technology involves injecting and mixing ofboth clay and granular ZVI suspended in water-based slurry di-rectly into a source zone contaminated with chlorinated solvents.The ZVI serves as the reactive media that drives reductive

1Former Graduate Research Assistant, Dept. of Civil and Environmen-tal Engineering, Colorado State Univ. E-mail: [email protected]

2Professor, Dept. of Civil and Environmental Engineering, 1372 Cam-pus Delivery, Colorado State Univ., Fort Collins, CO 80523-1372 �corre-sponding author�. E-mail: [email protected]

Note. This manuscript was submitted on August 11, 2008; approvedon June 5, 2009; published online on June 10, 2009. Discussion periodopen until May 1, 2010; separate discussions must be submitted for indi-vidual papers. This paper is part of the Journal of Geotechnical andGeoenvironmental Engineering, Vol. 135, No. 12, December 1, 2009.
©ASCE, ISSN 1090-0241/2009/12-1941–1956/$25.00.
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dechlorination of chlorinated solvents �Gillham and O’Hannesin1994; Wadley et al. 2005�. The mixing process serves to �1� ho-mogenize soils and contaminants in source zones and �2� facilitateintroduction of a mixture of ZVI and clay. Applications of theZVI-clay technology have involved mixing via high pressure jet-ting and shallow soil mixing using large-diameter �e.g., 2.4 m or8 ft� hollow-stem augers �Shackelford et al. 2005; Wadley et al.2005�.

The clay in the ZVI-clay technology serves several useful pur-poses. First, the clay in the clay-water slurry facilitates suspensionof the granular ZVI, thereby improving the uniformity in the dis-tribution of the ZVI and reducing the mechanical energy requiredto mix the soils �Wadley 2002�. Second, the addition of the claycan enhance sorption of the contaminants, which reduces the mo-bility of the contaminants and increases the time for reaction be-tween the ZVI and contaminants. Finally, mixing of the clay intothe contaminated subsurface can decrease the hydraulic conduc-tivity of the treated zone, which can reduce contaminant mass fluxfrom the treated zone via advection �i.e., hydraulically drivencontaminant transport� by diverting upstream groundwater flowaround the treated zone and lowering the amount of contaminantmass flux directly emanating from the treated zone into the sur-rounding ground water. Initial development and field applicationsof the ZVI-clay technology used kaolin clay. However, more re-cent field applications have involved the use of sodium bentonite,primarily due to the more extensive use of sodium bentonite infield applications and the lower cost of bentonite.

Given the aforementioned considerations, the primary purposeof this study was to evaluate the possible reduction in hydraulicconductivity resulting from injecting and mixing bentonite-waterslurry into three sands via a laboratory apparatus developed tomimic field application of the ZVI-clay technology. Although ex-tensive study on various factors affecting the hydraulic conduc-tivity of sand-bentonite �S-B� mixtures has been conducted �e.g.,Gipson 1985; Garlanger et al. 1987; Chapuis et al. 1992; Haugand Wong 1992; Kenney et al. 1992; Haug and Bolt-Leppin 1994;
O’Sadnick et al. 1995; Mollins et al. 1996; Alston et al. 1997;
NVIRONMENTAL ENGINEERING © ASCE / DECEMBER 2009 / 1941

ASCE license or copyright; see http://pubs.asce.org/copyright

Gleason et al. 1997; Howell and Shackelford 1997; Stern andShackelford 1998; Abichou et al. 2002a,b; Sällfors and Öberg-Högsta 2002;. Ebina et al. 2004; Kaoser et al. 2006; Teachavora-sinskun and Visethrattana 2006�, the focus of virtually all of thesestudies has been on compacted S-B mixtures being considered foruse as liners for waste containment applications as opposed toS-B mixtures that have been mixed in-place. The primary differ-ence between these two applications of S-B mixtures is themethod of preparing test specimens for hydraulic conductivitytesting, which can result in significantly higher void ratios for themixed in-place specimens relative to those associated with com-pacted S-B mixtures �Castelbaum 2007�.

Although hydraulic conductivity testing initially was per-formed on specimens from columns mixed with bentonite slurrycontaining ZVI, generation of hydrogen gas resulting from chemi-cal reaction between the ZVI and the bentonite �e.g., Reardon1995,2005� confounded the measurement of hydraulic conductiv-ity, such that this study involved measurement of hydraulic con-ductivity on specimens from columns mixed with only bentoniteslurry �i.e., without ZVI�. In addition, no contamination was usedin the study both for safety reasons and because the lack of ZVIobviated the need to consider mixing involving contaminatedmedia. Although the lack of ZVI and contaminated media maylimit the direct application of the results to the typical scenariosinvolving implementation of the ZVI-clay technology, the resultsof the study provide insight into the expected hydraulic behaviorof S-B mixtures at relatively high void ratios, and may be usefulin other applications involving similar materials and conditions,such as disposal of dredged materials �e.g., Lacasse et al. 1977�and slurried mineral wastes �e.g., Carrier et al. 1983�.

Materials and Methods

Constituent Materials

The constituent materials for the S-B mixtures evaluated in thisstudy included two sands and a processed sodium bentonite. Thetwo sands included fine-grained Ottawa sand �Grade F-58foundry sand, U.S. Silica Company, Ottawa, Ill.� with a measuredspecific gravity of solids �ASTM D 854�, Gs, of 2.67, andmedium-to-coarse-grained quartz sand �Filpro #3 well gravel,U.S. Silica Company, Mauricetown, N.J.� with a Gs of 2.65. Amixture of the two sands in equal proportions by dry weight�Gs=2.66� also was used in the study. Based on the particle-sizedistributions of the two sands and the sand mixture shown in Fig.1, the Ottawa sand and the quartz sand were referred to as “finesand” and “coarse sand,” respectively, whereas the mixture of thetwo sands was referred to as the “sand blend.” Both fine andcoarse sands and the sand blend classified as poorly graded sands�SP� according to the Unified Soil Classification System �USCS��ASTM D 2487�.

The bentonite was a powdered, polymer modified, air-floatbentonite �Hydrogel, WYO-BEN, Inc., Grey Bull, Wyo.�. Thephysical and chemical properties and mineralogical compositionof the bentonite are summarized Table 1. The bentonite classifiedas high plasticity clay �CH� according to the USCS.

Tap water was used both as the mixing liquid and the permeantliquid in this study. The measured chemical properties of the tapwater are shown in Table 2. The measured hydraulic conductivity,k, values based on permeation with the tap water in accordance
with ASTM D 2434 for the fine sand, coarse sand, and sand blend
1942 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGIN


were 1.2�10−2 cm /s, 5.4�10−2 cm /s, and 9.4�10−3 cm /s atvoid ratios, e, of 0.627, 0.700, and 0.406, respectively.

Bentonite Slurry Preparation

Bentonite in slurry form was injected and mixed into columns ofdry sand. The bentonite slurries were prepared by mixing bento-nite and tap water in varying proportions by weight. The bento-nite content of the slurry, BCs, was defined as the mass ofbentonite divided by the total mass of the slurry �i.e., bentoniteplus water�. The amount of water used to prepare the bentoniteslurry was adjusted to account for the air-dried �hygroscopic�gravimetric water content of the bentonite, which ranged from 8.5to 12.5%. The bulk bentonite was stored in sealed 19-L �5 gal�plastic buckets provided by the supplier. The desired values ofBCs for this study were 5, 6, and 7%.

The dry mass of sand in the column was used as the basis fordetermining the minimum required amount of bentonite in theslurry to be injected. The required mass of hydrated bentoniteslurry was determined from the percentage of bentonite in theslurry and the target bentonite content of the postmixed column.For example, for 1,000 g of dry sand in the column, a targetbentonite content of 3% in the postmixed column, and a 5% ben-tonite slurry, the relevant weights are 30 g of bentonite �i.e.,1 ,000 g�0.03� in 600 g of total bentonite slurry �i.e., 30 g/0.05�.The total amount of slurry prepared in this manner was increasedby approximately 20% to account for the capacity of the injectionsystem �e.g., hosing, fittings, etc.� and the samples required fortesting of the slurry.

High-speed colloidal mixers were used to produce the bento-nite slurries. Due to the volume of bentonite slurry typically pre-pared �approximately 3–4 L�, smaller 1-L batches of a bentoniteslurry were initially mixed in a 1.5-L Cuisinart blender for 2 min�Yeo 2003�. The smaller batches were combined in a 4.5-L War-ing industrial blender and mixed for another 2 min. The viscosity,specific gravity, electrical conductivity �EC�, bentonite content,and temperature of the composite batch of bentonite slurry thenwere measured to verify and/or compare consistency in these

Coarse

Medium Fine

Sand

Fines (Silt & Clay)

0

10

20

30

40

50

60

70

80

90

100 0

10

20

30

40

50

60

70

80

90

1000.00010.0010.010.1110

Fine SandCoarse SandSand BlendBentonite

PercentPassing(bydryweight)

Particle Size (mm)

PercentR

etained(by

dryweight)

4.75mm

2.00mm

0.425mm

0.075mm

Fig. 1. Particle-size distributions for sands and bentonite used in thestudy

properties between each batch of prepared slurry used for the

EERING © ASCE / DECEMBER 2009


column mixing tests. Viscosity and specific gravity were deter-mined using a Marsh Funnel and mud balance, respectively, fol-lowing procedures in the American Petroleum InstituteRecommended Practice 13B-1 �RP 13B-1�. The EC was deter-mined using an EC probe and meter �Model #150 Aplus, OrionResearch, Inc., Beverly, Mass.�, and the bentonite content wasdetermined by drying a small specimen of the bentonite slurry inan oven at 110�5°C. The bentonite slurry then was stored in asealed 4-L plastic container and allowed to hydrate for a mini-mum period ranging from 16 to 20 h. Following hydration, thebentonite slurry was mixed again in the 4.5-L Waring industrialblender for 2 min and the previously indicated properties were

Table 1. Physical and Chemical Properties and Mineralogical Compositi

Property

Specific gravity, Gsa

Atterberg limits:

Liquid limit, LL �%�

Plastic limit, PL �%�

Plasticity index, PI �%�

Principal minerals �%�: Based o

Montmorillonite

Kaolinite

Quartz

Plagioclase feldspar

Illite/mica

Mixed-layer illite/smectite

Calcite

Chlorite

Clinoptilolite

Gypsum

Cation exchange capacity, CEC �cmolc /kg� Based on aTesting

Exchangeable metals Based on aTesting�cmolc /kg�:

Ca2+

Mg2+

Na+

K+

H+

Sum

Soluble salts �mg/kg�: Based on aTestingCa2+

Mg2+

Na+

K+

CO32−

HCO3−

SO42−

Cl−

NO3−

Soil pH Based on aTesting

Soil EC �mS/m� at 25°C Based on aTesting

Note: cmolc�centimoles of charge.aAverage of three tests with 2.73�Gs�2.77.

measured again to provide a check of the values of these proper-



ties of the bentonite slurry determined after initial preparation. Acomparison of the results of the tests performed on batches ofboth the initially prepared slurries and the hydrated slurries indi-cated little change in slurry properties �see Table 3�. The preparedbentonite slurries then were used immediately thereafter in prepa-ration of the mixed specimens �i.e., without additional storage�.

Overall, excellent control between desired and measured val-ues of BCs was achieved for all prepared batches. For example,for the desired values of BCs of 5%, 6%, and 7%, the ranges inmeasured values of BCs were 4.95%�BCs�5.04%, 5.98%�BCs�6.02%, and 6.96%�BCs�7.01%, respectively. Thisslight variability in measured BCs was not expected to have an

Bentonite

Standard Measured value

ASTM D 854 2.75

ASTM D 4318

478

28

449

diffraction analyses performed byeralogy Inc., Tulsa, OK 87

—

8

2

2

—

1

—

Trace

Trace

s performed by Soil, Water, and Plantatory at Colorado State University

73.5


19.3

10.9

38.8

0.8

�0.1

69.8

s performed by Soil, Water, and Plantatory at Colorado State University 460

240

3,964

117

125

764

7,167

1,624

350


8.7


250

on of

n X-rayMin

nalyseLabor

nalyseLabor

nalyseLabor

nalyseLabor

nalyseLabor

appreciable effect on the test results.



Column Mixing Equipment

The mixing of test columns of sand with simultaneous injectionof the bentonite slurry was performed using the vertical mixingplatform that was specially manufactured for this study as shownin Fig. 2. The mixing platform provides for three key indepen-dently controlled operations similar to field-scale soil mixingequipment: �1� vertical movement of the hollow-stem auger; �2�rotation of the hollow-stem auger; and �3� injection of the bento-nite slurry �i.e., injection slurry�. The rate of vertical movement ofthe auger was fixed at approximately 102 mm/min, while rates ofauger rotation �reversible� and slurry injection could be variedfrom 0 to 20.7 revolutions per minute �rpm� and 60 to 670 mL/min, respectively. The highest auger rotation rate �i.e., 20.7 rpm�,which also is within the typical range of auger rotation rates usedfor field applications, was used for all column tests performed inthis study. The slurry injection rate was varied depending on thecolumn height and the volume of slurry to be injected as de-scribed subsequently.

Test specimens of the bentonite slurry-sand mixtures for hy-draulic conductivity measurements were prepared and mixed di-rectly inside a specially manufactured clear acrylic columnconsisting of several 134-mm-thick sections sandwiched betweena variable number of spacer sections �20–40 mm thick� at the topand a single 80-mm-thick section at the base. The 134-mm-thick

Table 2. Chemical Properties of Tap Watera

Property Measured value

Ionic species �mg/L�:

Ca2+ 13.6

Mg2+ 2.1

Na+ 3.1

K+ 0.8

B3+ �0.01

CO32− �0.1

HCO3− 42.1

SO42− 10.8

Cl− 3.2

NO3− �0.1

NO3−-N �0.1

pH 7.9

Hardness as CaCO3 �mg/L� 43

Alkalinity as CaCO3 �mg/L� 35

Total dissolved solids �mg/L� 76

EC �mS/m� at 25°C 9.01aBased on analyses performed by Soil, Water, and Plant Testing Labora-tory at Colorado State University.

Table 3. Average Values of Measured Properties of Bentonite Slurries U

Column

Viscosity�s�

Specific gravityof slurry

Initial Hydrated Initial Hydrated Ini

1 56 50 1.030 1.030 11

2 57 50 1.030 1.030 11

3 56 51 1.030 1.030 11

4 55 48 1.030 1.030 11

5 54 50 1.030 1.030 11

6 127 96 1.035 1.035 13

7 808 314 1.040 1.040 15



sections were used as the cells for hydraulic conductivity testing,as shown in Fig. 3. The inside diameter of all column segmentswas 101.6 mm �4.0 in.� to accommodate the mixing blades, whichwere slightly smaller in diameter. The base section of the as-sembled column allowed the injection ports and auger blades topass completely through the materials immediately above thebase section �i.e., the target mixing zone�. The spacer sectionswere used as needed so that the nuts on the top plate used tocompress the o-rings and tighten the column only needed to beadvanced approximately 25 mm. The total number of 134-mm-thick sections in an assembled column was determined based onthe length of column needed to accommodate the estimated post-mixed total volume of sand and bentonite slurry for a given set oftest conditions. Further details on the column mixing equipmentare provided in Castelbaum �2007�.

Test Column Preparation

Test columns were prepared by weighing the column assemblyempty and weighing out approximately 4.5 kg of sand, whichtypically resulted in an initial height of dry sand in the mixingzone of approximately 240 mm. The sand was poured through afunnel and allowed to fall freely into the assembled column. Thesides of the column were tapped lightly to facilitate settling of thesand within the column. The total height of the sand within theassembled column was measured, and the mixing zone height wasdetermined by subtracting 80 mm to account for the base section.Finally, the assembled column with sand was weighed and thensecured in the column-mixing platform. The resulting distributionof the sand within the column was assumed to be uniform.

Column Mixing Procedures

Illustrations of the column mixing scenario including picturesshowing slurry injection during mixing and a mixed specimenwithin a permeameter segment of the column prior to permeationare provided in Fig. 4. The column mixing sequence consisted ofauger rotation and slurry injection during downward movement�i.e., advancing� through the mixing zone, and reverse auger ro-tation only �i.e., without slurry injection� during upward move-ment �i.e., retraction�. The movement of the auger bladescompletely through the mixing zone in a single direction �i.e.,either downward or upward� was considered as a single pass.Slurry was injected only during advancement of the auger suchthat the slurry injected was immediately mixed with the soil bythe auger blades located just above the slurry injection ports. Theinjection rate was adjusted so that the required volume of injectedslurry and, therefore, the target quantity of bentonite, was deliv-

the Study

@25°CS/m�

Temperature,T �°C�

Bentonite content ofthe slurry, BCs �%�

Hydrated Initial Hydrated Initial Hydrated

118.7 31.3 28.1 4.99 4.97

120.1 30.9 28.2 4.98 4.99

120.8 30.5 28.5 4.96 4.98

121.6 31.2 28.4 4.97 4.96

122.4 31.0 27.7 4.99 4.99

144.1 30.6 28.6 6.01 6.01

157.6 32.0 28.6 6.97 6.98

sed in

EC�m

tial

4.6

5.9

6.9

8.2

9.3

9.2

6.0



ered to the mixing zone during the downward pass�es� of theauger. Several combinations of test conditions resulted in requiredinjection rates that exceeded the ability to deliver the minimumvolume of slurry in a single downward pass. Therefore, twodownward passes with injection and two upward passes withoutinjection �i.e., four passes� were performed for each test column.

Once the slurry injection rate was determined and set, the ro-tating auger was advanced down through the inside of the as-sembled column until the auger injection ports were at the sameelevation as the top of the mixing zone �i.e., top of the sandcolumn�. The injection slurry was poured into the injector reser-voir, all injector system connections were secured, the injectormotor was turned on, and the injection system fittings, hose andauger were filled with slurry. Advancement of the auger downthrough the mixing zone commenced when the injection slurrywas flowing out of the injection ports. The auger was advanceduntil the visible injection/mixing front �see Fig. 4�a�� passed com-pletely through the mixing zone and approximately 20 to 40 mminto the base section. At this point, slurry injection and augeradvancement were stopped, auger rotation was reversed, and theauger was retracted upward through the mixing zone. The auger

Fig. 2. �Color� Laboratory-scale mixing equipment for injecting andwith chlorinated solvents

was retracted until the injection ports again were at the same



elevation as the top of the mixing zone. This procedure was re-peated for the second downward and upward passes. However,the slurry injection rate for the second downward pass was re-duced to account for the increased height �i.e., expansion� of themixing zone due to the volume of slurry injected during the firstdownward pass. The auger was retracted completely from theassembled column at the end of the second upward pass throughthe mixing zone.

Column Disassembly and Testing Procedures

After mixing was complete, the assembled, mixed test columnwas removed from the mixing platform and the postmixed heightof the test column was determined. The top plate, spacer sections,and any empty and/or 134-mm-thick column sections partiallyfilled with sand-slurry mixture were removed. The sand-slurrymixture in partially filled sections and the base section was dis-carded. The 134-mm-thick sections completely filled with sand-slurry mixture then were removed one section at a time by slidinga thin metal plate between two adjacent sections. The removedsection then was fitted with filter papers, porous stones, end caps

g zero-valent iron-clay slurries for remediation of soils contaminated
mixin
�i.e., assembled into the rigid-wall permeameters� and weighed



Fig. 3. �Color� Photograph of disassembled parts and schematic cross section of rigid-wall permeameters

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�see Fig. 4�c��. The assembled permeameters were connected to acontrolled source of compressed air for hydraulic conductivitytesting via panel pressure boards �models M100000 andM116000, Trautwein, Houston�.

Measurement of Hydraulic Conductivity

Hydraulic conductivity �k� testing of the sand-slurry mixtures wasperformed with tap water as the permeant liquid using the falling-headwater/rising tailwater procedure described in ASTM D 5084.However, permeation was terminated when a minimum of four

Hollo

Au

In

Inje

U

Dire

ctio

nof

trav

elof

auge

r

(b)

Fig. 4. �Color� Illustrations of �a� column mixing scenario; �b� slupermeameter segment of column prior to permeation

consecutive values of k were within �10% of the mean value and



the ratio of the volumetric outflow to inflow rates, Qout /Qin, was1.0�0.1, which are more stringent termination criteria than indi-cated in ASTM D 5084 �i.e., k within �25% of the mean value,and Qout /Qin=1.0�0.25�.

Permeation was initiated upwards through the specimens. Assuch, maintaining the hydraulic gradient, i, at or below unity �i.e.,i�1� typically was necessary to avoid a quick condition due tothe relatively low densities �i.e., low effective stresses� of thespecimens tested. After upward permeation was terminated, thedirection of permeation was reversed �i.e., downward permeation�

mn

rn Auger

Soil

lades

Port

Mixingnt

d Soil

(a)

(c)

ection and mixing blades of auger; and �c� mixed specimen within

Colu

w Ste

Mixed

ger B

jection

ction/Fro

nmixe

rry inj

to assess the possible effects on k due to the direction of perme-



ation, and permeation was continued until the termination criteriawere achieved. The applied hydraulic gradients for downwardpermeation ranged from two to five �2� i�5�. By comparison,hydraulic gradients lower than unity �i�1� typically are encoun-tered in field applications involving groundwater flow �e.g., Inge-britsen et al. 2006�.

As a result of the low magnitudes of applied hydraulic gradi-ents, volume changes within the specimens during permeationwere expected to be negligible. However, visual observation ofthe specimens contained within the clear plastic permeametersindicated that some settling had occurred in all but two of thespecimens during permeation, presumably due to the loose natureof the mixed specimens �e.g., see Fig. 4�b��. In these cases, thechange in the length of the bentonite slurry mixed specimen, �L,relative to the original length, Lo �=120 mm; see Fig. 3�, rangedfrom 1.3% �Column 4, bottom specimen� to as much as 16.7%�Column 5, upper middle specimen�. In these cases, the actual,measured length, L �=Lo−�L� of the soil specimen containedwithin the permeameter was used to calculate the hydraulic con-ductivity as well as in determining the final volume of the speci-men for the purpose of determining void ratios.

Bentonite Content Determination

Following completion of permeation, the permeameters were dis-assembled, and the bentonite content and void ratio were deter-mined for each specimen. The bentonite contents of the specimenmixtures were determined using the test method described byAbu-Hassanein et al. �1996�, which is based on EC. In this pro-cedure, the S-B mixture for each section was transferred to a925-mL, baffled, stainless steel “dispersion cup” �ASTM D 422�.Then, approximately 200 mL of tap water were added to the S-Bmixture in the dispersion cup, and the resulting mixture was me-chanically stirred for 2 min with the mixer. After stirring, the sandparticles settled to the bottom of the dispersion cup and the di-luted bentonite slurry was slowly poured into a 1,000-mL sedi-mentation cylinder. This procedure was repeated three additionaltimes with care to ensure that any clods of the S-B mixture werestirred thoroughly and dispersed. Tap water was added to thebentonite slurry in the sedimentation cylinder until the 1,000-mLcalibration mark was reached. The cylinder was sealed with arubber stopper and the bentonite slurry was mixed by manuallyrotating the cylinder vertically for 1 min. After this mixing wascompleted, the EC of the bentonite slurry was measured using theEC probe and meter.

The EC reading was converted to a bentonite concentration,CB �g/L�, through the use of EC-CB calibration curves developed

Table 4. Summary of Column Mixing Program

ColumnType of

sand

Target bentonitecontent in

mixing zone,BCt �%�

Bentonite contentof slurry,BCs �%�

1 Fine 3 5

2 Coarse 3 5

3 Blend 3 5

4 Fine 1 5

5 Fine 5 5

6 Fine 3 6

7 Fine 3 7

using 1-L slurries with known values for CB �see Abu-Hassanein



et al. �1996��. Sand-specific EC-CB calibration curves were devel-oped because the type and characteristics of the native soil towhich the bentonite is added can affect the EC of the clay slurry�Abu-Hassanein et al. 1996�. However, the different sands used inthis study did not have a significant effect on the EC of the ben-tonite slurry �Castelbaum 2007�.

Testing Program

The entire column mixing program is summarized in Table 4. Atotal of seven columns were mixed using the fine, coarse, orblended sand mixed with slurries consisting of 5, 6, or 7% ben-tonite contents, BCs, to provide mixtures of sand and bentoniteslurry containing target bentonite contents, BCt, of 1, 3, or 5%.

Results

Void Ratio and Bentonite Content

Values for the void ratio �e� and measured �actual� bentonite con-tents �BC� for each of the 19 column test sections resulting fromthe seven mixed columns are summarized in Table 5. All valuesof e shown in Table 5 were determined after permeation. Theresulting values of e ranged from 0.672 to 3.94, whereas values ofBC ranged from 0.61% to 7.65%. Higher values of e and BCoccurred near the top of Columns 1–3 and 5, whereas the distri-butions in both e and BC with depth for Columns 4, 6, and 7 wererelatively uniform. In the cases of Columns 1–3 and 5, highervalues of e and BC near the top were attributed to the verticalexpansion of the mixtures that occurred during mixing due to thelack of confining stress near the top of the mixed zone �Castel-baum 2007�. The increase in confining stress with depth restrictedthe amount of expansion with depth, typically resulting in lowervalues for e and BC with depth. In the cases of Columns 4, 6, and7, less expansion occurred during mixing of the sand columnswith the bentonite slurries, such that only two sections were re-covered for hydraulic conductivity testing, versus three sectionsrecovered from Columns 1–3 and four sections recovered fromColumn 5. As previously indicated, column sections that wereonly partially filled with the mixture of sand and bentonite slurrywere not permeated.

The initial heights of the columns of dry sand prior to injectionof the bentonite slurry and mixing, Ho, and the final heights of thebentonite slurry mixed columns, Hf, as well as the percentages ofexpansion, are shown in Fig. 5. The amount of expansion thatoccurred during mixing was directly related to the magnitude ofthe difference between the bentonite content of the slurry �BCs�injected into the sand and the target bentonite content �BCt� forthe mixed specimen in the column �Table 4�. The amount of ben-tonite slurry that had to be injected to achieve BCt decreased and,therefore, the amount of corresponding expansion also decreased,as the difference between BCs and BCt increased. For example,although Columns 1–5 all involved the use of BCs of 5%, BCt wasonly 1% for Column 4, 3% for Columns 1–3, and 5% for Column5. Thus, the amount of injected bentonite slurry and, therefore, theextent of expansion varied in the order Column 4�Columns 1–3�Column 5. Also, although the BCt was 3% forColumns 1–3 and Columns 6 and 7, the BCs for Columns 1–3 was5%, whereas the BCs for Columns 6 and 7 was 6% and 7%,respectively. Thus, the amount of injected bentonite slurry andextent of expansion for Columns 6 and 7 was less than that for
Columns 1–3.


cm /

Hydraulic Conductivity

The results of hydraulic conductivity �k� testing of 19 test sec-tions recovered from the seven mixed columns also are summa-rized in Table 5. The results include the measured k values basedon upward permeation, ku, the measured k values based on down-ward permeation, kd, and the geometric means, kgm, of ku and kd.Several observations are apparent from the k results shown inTable 5.

First, the values of the ratio of ku relative to kd, or ku /kd,ranged from 0.71 to 4.3, although values of ku were greater thanvalues of kd for 16 of the 19 tests �i.e., 84.2%�. The tendency forku to be greater than kd may be due, in part, to the different stressconditions imposed during upward permeation relative to down-ward permeation. For example, during downward permeation, theinduced seepage forces work in the same direction as gravity suchthat the overall effective stresses in the specimen are expected tobe higher, resulting in lower void ratio and a lower k. In contrast,during upward permeation, the induced seepage forces opposegravity, such that the overall effective stresses in the specimen areexpected to be lower, resulting in a higher void ratio and higher k.However, given the relatively low hydraulic gradients imposedduring upward relative to downward permeation, this effect ofseepage forces was likely greater during downward permeation.Nonetheless, the overall effect of direction of permeation was lessthan one-half of an order of magnitude.

Second, all the measured values of k �ku ,kd ,kgm� for all speci-mens of the S-B slurry mixtures were lower than the measured kof the premixed sand �i.e., ks�, as expected. For example, the

Table 5. Measured Test Results for Individual Column Sections

ColumnType of

sand

Specimen locationwithin mixed

columnVoid ratio,

e

Measured bencontent

BC �% dry

1 Fine Top 1.77 2.53

Middle 1.00 2.08

Bottom 0.924 1.27

2 Coarse Top 3.53 5.75

Middle 0.847 0.98

Bottom 0.796 0.76

3 Blend Top 2.77 5.10

Middle 0.979 2.21

Bottom 0.672 1.23

4 Fine Top 0.750 0.61

Bottom 0.732 0.75

5 Fine Top 3.94 7.65

Upper Middle 1.62 4.08

Lower Middle 1.26 3.02

Bottom 0.850 1.83

6 Fine Top 0.883 1.98

Bottom 0.968 2.02

7 Fine Top 0.957 2.34

Bottom 0.951 2.23akgm= �kukd�0.5.bks=k of sand �fine sand: ks=1.2�10−2 cm /s; coarse sand: ks=5.4�10−

previously reported values of ks ranged from 1.2 to 4.6 orders of



magnitude higher than the value of kgm �Table 5�. These lowermeasured values of k for all specimens of the sand-bentoniteslurry mixtures relative to the sands alone are consistent with theaddition of high swelling bentonite to the initially dry sands,which were free of fines.

Third, the values of kgm for the specimens representing the topsections of the mixed columns are the highest values measured infive of the seven columns �Columns 1, 3–5, 7�, which is consis-tent with the highest void ratios in these columns also occurringfor the top section. However, for Column 2, the value of kgm forthe top section was the lowest value measured, even though thehighest void ratio in Column 2 occurred for the top section. Thisdifference highlights the fact that the measured k values for thespecimens are a function not only of the void ratio but also of thebentonite content within each section �i.e., 5.75% for the top sec-tion versus 0.98% and 0.76% for the middle and bottom sections,respectively�.

Finally, the values of kgm for the S-B slurry mixtures rangefrom 2.4�10−7 cm /s to 6.8�10−4 cm /s. These kgm values areabout one to seven orders of magnitude higher than values of kcommonly reported for compacted S-B mixtures, which typicallyfall within the range 10−10 cm /s�k�10−7 cm /s �e.g., see Ken-ney et al. �1992�, Stern and Shackelford 1998�, despite similarbentonite contents. The difference in results can be attributed pri-marily to the higher void ratios, in general, and the significantlyhigher bentonite void ratios, in particular, of the S-B slurry mix-tures evaluated in this study relative to compacted S-B mixtures,

Hydraulic conductivity,k �cm/s�

Upwardpermeation,

ku

Downwardpermeation,

kd

Geometricmean,kgm

a ku /kd Log �ks /kgm� b

1.1�10−3 4.2�10−4 6.8�10−4 2.6 1.2

8.3�10−7 4.2�10−7 5.9�10−7 2.0 4.3

7.7�10−5 6.0�10−5 6.8�10−5 1.3 2.2

2.6�10−6 6.1�10−7 1.3�10−6 4.3 4.6

2.2�10−5 2.8�10−5 2.5�10−5 0.79 3.3

1.8�10−5 2.1�10−5 1.9�10−5 0.86 3.4

1.1�10−6 2.9�10−7 5.6�10−7 3.8 4.2

6.9�10−7 1.9�10−7 3.6�10−7 3.6 4.4

3.8�10−7 1.5�10−7 2.4�10−7 2.5 4.6

3.4�10−4 2.6�10−4 3.0�10−4 1.3 1.6

9.2�10−6 1.3�10−5 1.1�10−5 0.71 3.0

9.9�10−7 3.6�10−7 6.0�10−7 2.8 4.3

8.4�10−7 3.0�10−7 5.0�10−7 2.8 4.4

8.7�10−7 2.7�10−7 4.8�10−7 3.2 4.4

7.8�10−7 2.1�10−7 4.0�10−7 3.7 4.5

4.2�10−7 2.3�10−7 3.1�10−7 1.8 4.6

7.1�10−7 1.9�10−7 3.7�10−7 3.7 4.5

1.1�10−7 4.2�10−7 6.8�10−7 2.6 4.2

6.0�10−7 2.0�10−7 3.5�10−7 3.0 4.5

s; sand blend: ks=9.5�10−3 cm /s�.

tonite,wt.�

2

as will be elucidated in the following discussion.





Discussion

Effect of Bentonite Void Ratio

Traditionally, the hydraulic conductivity, k, of soil has been cor-related to the total void ratio, e, of the soil, with an increase in ecorrelating with a semilog linear increase in k �Lambe and Whit-man 1969�. As a result, the geometric means of the k valuesmeasured in this study, or kgm, were plotted as a function of e inFig. 6.

As shown in Fig. 6, the general trend for the measured kgm

versus e of the specimens tested in this study deviates from thetraditional trend in e versus k. For example, atypically low valuesof k �i.e., k�10−6 cm /s� correlate with specimens with relativelyhigh values of void ratio �i.e., e�1.5�, whereas atypically highvalues of k �i.e., 10−5�k�10−4 cm /s� correlate with specimenswith relatively low values of e. These deviations are due to theeffect of the bentonite content, BC, of the specimens, wherespecimens with relatively high BC �i.e., BC�4.0% correspond tohigh values of e but low measured k, and specimens with rela-tively low BC �i.e., BC�1.5%� correspond to low values of e buthigh measured k �see Fig. 7�.

Kenney et al. �1992� present a simple model of an ideal, two-constituent, homogeneous mixture of sand and saturated bentoniteto estimate the minimum k of compacted S-B mixtures. This idealmixture assumes that all water is associated with the bentoniteand that the sand particles are impervious inclusions within amatrix of saturated bentonite and do not affect the fabric of thebentonite. The hydraulic conductivity of such an ideal mixturewould be controlled by the hydraulic conductivity of the bento-nite, which is controlled by the fabric and the void ratio of thebentonite, eb. The fundamental definition for eb is the volume ofvoids attributed to the bentonite, Vvb, divided by the volume ofsolid bentonite in the mixture, Vsb, or eb=Vvb /Vsb. The eb of anideal saturated S-B mixture also was expressed in Kenney et al.�1992� as follows:

eb = Gsb��1 +1

r� �w

�dm−

1

rGss� − 1 �1�

where Gsb=the specific gravity of solids for the bentonite; Gss

0

1

2

3

4

5

0 2 4 6 8

TotalVoidRatio,e

Bentonite Content, BC (%)

10

e = 0.125 + 0.429BC (r2 = 0.906)

Fig. 7. Bentonite content versus total void ratio for sand-bentoniteslurry mixtures

0

10

20

30

40

50

60(a)Initial height,H

o

Final height,Hf

ColumnHeight(cm)

Column Number1 2 3 4 5 6 7

1

1.25

1.5

1.75

2

0

25

50

75

100(b)

Exp

ansi

onR

atio

,E

R(=

Hf/H

o)

Column Number1 2 3 4 5 6 7

Relative

VerticalE

xpansion,∆H

/Ho

(%)

Fig. 5. �a� Initial and final mixed specimen heights; �b� expansionratio and relative vertical expansion for seven columns of sand mixedwith bentonite slurry

10-7

10-6

10-5

10-4

10-3

10-2

10-1

0 1 2 3 4 5

S-B Mixtures w/Fine SandS-B Mixtures w/Coarse Sand

S-B Mixtures w/Sand Blend

10-9

10-8

10-7

10-6

10-5

10-4

10-3

GeometricMeanHydraulicConductivity,k gm(cm/s)

Total Void Ratio, e

Trend curve

Sand Blend

Fine SandCoarse Sand

?

Geom

etricMean

Hydraulic

Conductivity,

kgm(m/s)

Fig. 6. Geometric mean of measured hydraulic conductivity valuesversus total void ratio for sand-bentonite �S-B� slurry mixtures
=the specific gravity of solids for the sand; r=the ratio of the dry


masses of bentonite and sand �i.e., BC/100�; �w=the density ofwater; and �dm=the dry density of the mixture.

In contrast to an ideal S-B mixture, Kenney et al. �1992� notedthat nonideal S-B mixtures may contain insufficient and/or inad-equately distributed bentonite resulting in the voids between sandparticles being partially filled with free water, forming holes ordefects. Kenney et al. �1992� indicated that these water-filledholes, which can form channels if interconnected, increase the kof compacted S-B mixtures, and reported some values of k for theS-B mixtures that were closer to that of the sand relative to that ofthe bentonite. The results presented in Kenney et al. �1992� showthat the higher values of k for S-B mixtures, as well as for speci-mens of bentonite only, correspond with higher values of eb.Therefore, Kenney et al. �1992� concluded that both an adequateamount of bentonite and a sufficiently uniform distribution of thebentonite within the mixture are required for low-permeabilitysand-bentonite mixtures.

Kenney et al. �1992� also indicated that, for the ideal sand-bentonite mixture, the free-swell void ratio of the bentonite, eb,f-s,represents an upper limit on the value of eb. Values of eb greaterthan the eb,f-s indicate that conditions for ideal mixtures do notexist due to water-filled voids within the mixture. The eb,f-s typi-cally is determined from test procedures similar to those used todetermine the swell index �i.e., ASTM D 5890�, which typicallyranges from 25 mL/2 g to 35 mL/2 g for sodium bentonites �Leeand Shackelford 2005a,b; Katsumi et al. 2007; Meer and Benson2007�. Based on the specific gravity of solids for the bentonite�Gsb� used in this study of 2.75 �see Table 1�, this range of freeswell index values corresponds to values of eb,f-s ranging from 33to 47.

As a result of the approach described by Kenney et al. �1992�,the eb for each specimen tested in this study was calculated basedon the assumption that the volume of the voids for the mixedspecimen, Vv, equals the volume of the voids of the bentonite,Vvb, or

Vv = Vt − Vss − Vsb = Vvb �2�

where Vt=the total volume of the mixed specimen; Vss=the vol-

0

50

100

150

200

0 2 4 6 8 10

Column 1Column 2Column 3Column 4Column 5Column 6Column 7

VoidRatioforBentonite,e

b


1.40.97 0.59

1.45 - 1.54

1.34 - 1.44

1.6 0.710.54

1.02

1.18

ρdm=

0.5

1.01.251.51.75

ρdmof Specimen

Eq. 1 w/constant ρdm

Fig. 8. Bentonite content versus void ratio of bentonite for sand-bentonite slurry mixtures tested in this study superimposed on curvesfor constant mixture dry density, �dm �g /cm3�, based on Eq. �1�

ume of solids for the sand in the mixed specimen; and Vsb=the



volume of solids for the bentonite in the mixed specimen. The 19values of eb calculated on the basis of Eq. �2� ranged from 42.5 to127, and were identical to those calculated on the basis of Eq. �1��Castelbaum 2007�. For example, as shown in Fig. 8, the mea-sured values of eb based on Eq. �2� plotted as a function of ben-tonite content, BC, compare favorably with those based on Eq. �1�for different values of �dm. Thus, the simple expression for eb

given by Eq. �1� for compacted S-B mixtures was equally validfor the bentonite slurry mixed sand specimens evaluated in thisstudy.

Only 6 of the 19 calculated values for eb for the S-B mixturesin this study fell within the range of possible eb values based onfree swell �i.e., 33�eb�47�, suggesting that the majority of thespecimens were nonideal mixtures. Based on Kenney et al.�1992�, such nonideal mixtures represent situations where someof the void spaces between sand particles are not filled with ben-tonite, but rather are filled only with water. However, unlike thecompacted S-B specimens evaluated by Kenney et al. �1992�,where water was added to the sand and bentonite mixtures duringcompaction such that the bentonite likely was not fully hydrated,the bentonite in this study added in the form of a slurry was fullyhydrated during mixing. In the latter case, the existence of iso-lated pores containing only water without bentonite is unlikely.Rather, the more likely scenario is that eb�eb,f-s represents voidsbetween individual sand particles that actually are filled with ben-tonite in suspension.

For example, values of eb based on the mass of bentonitesuspended in water, eb,susp, can be calculated as a function of thebentonite content of the suspension, BCsusp, and the specific grav-ity of the bentonite, as follows:

eb,susp =�1 − BCsusp�

BCsuspGsb �3�

where BCsusp is defined in terms of mass of bentonite per totalmass of the suspension �i.e., water plus bentonite�. Values ofeb,susp calculated in accordance with Eq. �3� are based on theassumption that the bentonite particles are fully suspended inwater, such that there is no settling of bentonite. Although thedefinition of BCsusp given by Eq. �3� is the same as that previouslydefined for the bentonite content of the slurry, BCs, different des-ignations are used here to distinguish the bentonite suspended inthe voids of the postmixed and permeated S-B specimens �BCsusp�from that suspended in the prepared bentonite slurries used in themixing process �BCs�.

Based on the assumption that BCsusp is the same as BCs for thebentonite used in this study �i.e., Gsb=2.75�, the limiting values ofeb,susp according to Eq. �3� are 52.3, 43.1, and 36.5 for values ofBCs �=BCsusp� of 0.05 �5%�, 0.06 �6%�, and 0.07 �7%�, respec-tively. Thus, in the case where bentonite can exist in suspensionwithin the voids between sand particles, the upper limit on eb

increases from the upper limit of 47 based on eb,f-s to 52.3 basedon BCs �=BCsusp�. This upper limit on eb of 52.3 encompasses anadditional 3 calculated eb values for the bentonite slurry mixedsand specimens evaluated in this study, such that 9 of the 19calculated eb values were less than 52.3. Nonetheless, 10 of thecalculated eb values still fell within the range 52.3�eb�127.Thus, the question remaining to be answered is how are such higheb values possible?

One possible explanation, i.e., aside from experimental error,is that the injected bentonite slurry was diluted after mixing andpermeation such that BCsusp�BCs. For example, dilution of a 5%
bentonite slurry to a 2.1% bentonite suspension within the mixed


specimen would result in an eb,susp of 128 based on Eq. �3�. In-deed, the calculated values of BCsusp based on the measuredmasses of bentonite and volumes of water for all 10 permeatedspecimens where 52.3�eb�127 ranged from 2.1 to 4.3%, indi-cating that some dilution of the bentonite slurry had occurredduring permeation. Such dilution conceivably could occur by pip-ing of the bentonite from the specimen during permeation and/orby the addition of permeation water to initially unsaturated speci-mens. However, the relatively low hydraulic gradients employedduring permeation �i�5� suggest that piping was not likely, andno evidence of piping was observed during permeation �e.g.,cloudiness in the outflow�. Although the actual degrees of satura-tion of the postmixed specimens prior to permeation were un-known, the results of separate mixing tests reported byCastelbaum �2007� based on the same equipment, materials, andmethods as used in this study indicate that the postmixed degreesof saturation for 7 of the 10 column specimens with 52.3�eb

�127 ranged from 50 to 98%. Thus, the calculated values of eb

ranging from 52.3 to 127 likely can be attributed, in part, tobentonite existing within a diluted suspension between individualsand particles within the mixed and permeated specimens.

Although the specimens for this study were prepared by mix-ing dry sand with bentonite slurries as opposed to compactingwetted mixtures of bentonite and sand, the potential use of eb to

Fig. 9. Comparison of void ratio of bentonite versus measured hydrversus that from Kenney et al. �1992� for compacted bentonite andgeometric means of measured values, kgm; eb,f-s=free-swell void rati

estimate the k of the bentonite slurry mixed sand specimens in



this study also was evaluated. The results of the evaluation areshown in Fig. 9, where measured k is plotted versus eb for boththe compacted bentonite and compacted S-B mixtures from Ken-ney et al. �1992� as well as for the bentonite slurry mixed sandspecimens in this study. The estimated range in possible eb,f-s

values also is shown in Fig. 9 for comparison.As indicated in Fig. 9, the resulting values of eb and k from

this study appear to form two separate groups of data. The data inGroup I are characterized by values of eb �42.5�eb�66.7� and k�2.4�10−7 cm /s�k�1.3�10−6 cm /s� that are lower than thevalues of eb �73.7�eb�127� and k �1.1�10−5 cm /s�k�6.8�10−4 cm /s� for the data in Group II. Thus, the calculated eb

values for the data in Group I range from slightly lower �0.90��to somewhat higher �1.4�� than the upper limit of eb that can beexpected for ideal mixtures �i.e., eb,f-s=47�, whereas the calcu-lated values of eb in Group II range from 1.6 to as much as 2.7times the upper limit in eb values expected for ideal mixtures.

However, the distinction in the two groups of data from thisstudy shown in Fig. 9 is more apparent in terms of k. For ex-ample, the lowest k value in Group II of 6.8�10−4 cm /s is 523times, or 2.72 orders of magnitude, higher than the highest kvalue of 1.3�10−6 in Group I. This significant contrast in therange of k values for the two groups of data suggests significantlydifferent behavior in terms of the role of the bentonite in control-

onductivity for sand-bentonite �S-B� slurry mixtures from this studyacted S-B mixtures. �Note: values for k from this study represent

aulic ccomp

o.�

ling the k of the bentonite slurry mixed sand specimens. For the



mixtures corresponding to the data in Group I, the bentonite isloosely distributed within the voids between sand particles, butapparently is sufficiently intact or viscous to offer some resistanceto flow. In contrast, the bentonite for the mixtures correspondingto the data in Group II apparently is so loosely suspended that thebentonite offers very little resistance to flow, such that the k of themixture approaches that of the clean sand used in the mixture.

The overall trend in k versus eb for the S-B slurry mixturesshown in Fig. 9 is similar to that for the compacted bentonite andcompacted S-B mixtures from Kenney et al. �1992�, althoughthere is significantly more scatter in the data and the correspond-ing coefficients of determination, r2, are much lower. Nonethe-less, the results shown in Fig. 9 suggest that eb can be used toprovide an order-of-magnitude approximation of k for the S-Bslurry mixtures tested in this study, particularly for those speci-mens corresponding to eb�66.7. For example, as shown in Fig.10, the measured k values, with one exception, are withinapproximately�one order of magnitude of the values of k calcu-

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1

(a)S-B Mixtures w/Fine SandS-B Mixtures w/Coarse SandS-B Mixtures w/Sand Blend

10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

MeasuredHydraulicConductivity,k m(cm/s)

Calculated Hydraulic Conductivity, kcalc(cm/s)

?

Calculated Hydraulic Conductivity, kcalc(m/s)

Measured

Hydraulic

Conductivity,

km(m/s )

km= 0.1k

calckm= 10k

calc

km= k

calc

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1

(b)S-B Mixtures w/Fine SandS-B Mixtures w/Coarse SandS-B Mixtures w/Sand Blend

10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

MeasuredHydraulicConductivity,k m(cm/s)

Calculated Hydraulic Conductivity, kcalc(cm/s)

?

Calculated Hydraulic Conductivity, kcalc(m/s)

Measured

Hydraulic

Conductivity,

km(m/s)

km= 0.1k

calckm= 10k

calc

km= k

calc

Fig. 10. Measured hydraulic conductivity values versus calculatedhydraulic conductivity based on regressions shown in Fig. 9 for sand-bentonite �S-B� slurry mixtures: �a� regression based on S-B slurrymixtures from this study; �b� regression based on all data combined

lated using the regression equation based on only the sand-



bentonite slurry mixtures evaluated in this study �Fig. 10�a�� aswell as the regression equation based on all the data shown in Fig.9 �Fig. 10�b��.

Effect of Bentonite Content

The kgm values for the bentonite slurry mixed sand specimens areplotted versus BC in Fig. 11. The resulting trend in the datashown in Fig. 11 is similar to the trend in k versus e shown in Fig.6, due to the linear e-BC relationship �see Fig. 7�. That is, low

10-7

10-6

10-5

10-4

10-3

10-2

10-1

0 1 2 3 4 5 6 7 8 9 10

S-B Mixtures w/Fine SandS-B Mixtures w/Coarse SandS-B Mixtures w/Sand Blend

10-9

10-8

10-7

10-6

10-5

10-4

10-3

GeometricMeanHydraulicConductivity,k

gm(cm/s)


?

Trend curve

Coarse Sand

Sand Blend

Fine Sand

Zone 1 Zone 2 Zone 3

Geom

etricMean

Hydraulic

Conductivity,k

gm(m/s)

Fig. 11. Geometric mean of measured hydraulic conductivity valuesversus bentonite content for sand-bentonite �S-B� slurry mixturesfrom this study

10-7

10-6

10-5

10-4

10-3

10-2

10-1

10-9

10-8

10-7

10-6

10-5

10-4

10-3

GeometricMeanHydraulicConductivity,k

gm(cm/s)

Specimen Location within Mixed Column

Reference(1)

CoarseSand(2)

SandBlend(3)

BCt=1%(4)

BCt=5%(5)

BCs=6%(6)

BCs=7%(7)

Variable (Column No.)

MT B MT B MT B BT MT MB BT BT

T

M

M

B

T

M

B

T

B

Fine Sand

Sand Blend

Coarse Sand

> 60< 60

eb Expected k

HighLow

Geom

etricMean

Hydraulic

Conductivity,k

gm(m/s)

Fig. 12. Geometric mean of measured hydraulic conductivity valuesas a function of relative vertical location of specimens within post-mixed columns and void ratio of bentonite �eb� �T=top; M=middle; and B=bottom�



values of k �i.e., k�10−6 cm /s� were measured over a wide rangein BC �i.e., 1.0%�BC�8.0%� relative to a more narrow range inBC �i.e., 0.5%�BC�2.0%� within which k was higher �i.e., k�10−6 cm /s�. Thus, k= f�BC� also does not account for the effectof e on the behavior of the S-B slurry mixtures, in a similarmanner that k= f�e� does not account for the effect of BC asshown in Fig. 6. However, since BC is a more practical designparameter than eb for soil-mixing applications, extension of Fig.11 to practical use can be accomplished by defining three zones ofBC values as indicated in Fig. 11. These three zones correspond toBC�1% �Zone 1�, 1%�BC�3% �Zone 2�, and BC�3% �Zone3�.

Within Zone 1, the amount of bentonite in the mixture is in-sufficient and not uniformly distributed, resulting in the likelihoodof relatively high values of k �i.e., k�10−6 cm /s�. Within Zone 3,the amount of bentonite in the mixture is both sufficient and uni-formly distributed, resulting in the likelihood of relatively lowvalues of k �i.e., k�10−6 cm /s�. Zone 2 represents a transitionzone in which the amount of bentonite in the mixture can besufficient but not uniformly distributed, or insufficient and uni-formly distributed, resulting in the likelihood of k ranging overseveral orders of magnitude �i.e., 10−6 cm /s�k�10−3 cm /s�.The variability of k that occurs from Zones 1–3 is due, in part, toadditional mixing conditions related to the bentonite content ofthe slurry �BCs� and the effects of the slurry injection rate andconfining stress.

The variation in k of each specimen due to the relative verticallocation within the mixed test column is shown in Fig. 12. Gen-erally, particle displacement was greatest near the top of themixed test columns where the effective stress was low �Castel-baum 2007�. As shown in Fig. 12, all specimens with eb�60 �i.e.,12 of 19 specimens� had relatively low values of k �i.e., k�10−6 cm /s�. However, four of these specimens were from thetop-most location of the mixed test columns where particle dis-placement was greatest and a high value of k would be expected.Therefore, the effects of additional mixing conditions for thesespecimens cumulatively resulted in an eb�60 and k�10−6 cm /s.

The column with the sand blend �i.e., Column 3, Table 5� hadthe highest premixed dry density �Castelbaum 2007�, which re-sulted in a greater increase in effective stress with depth relativeto the columns with the fine and coarse sands �i.e., Columns 1 and2, respectively�. The column with a target bentonite content, BCt,of 5% �i.e., Column 5� resulted in the greatest volume of requiredslurry and highest slurry injection rate relative to the columnswith BCt values of 1% and 3% �i.e., Columns 4 and 1, respec-tively�. The columns where higher slurry bentonite contents, BCs,were used �i.e., 6% and 7% for Columns 6 and 7, respectively�resulted in decreases in the volume of required slurry and theslurry injection rate relative to Column 1 involving a BCs of 5%s”.Thus, these additional mixing conditions for Columns 3, 6, and 7cumulatively contributed to decreased particle displacement pre-dominantly near the top of the mixed column relative to Column1 �fine sand and BCs of 5%� and Column 2 �sand blend and BCs

of 5%�, whereas these additional mixing conditions for Column 5cumulatively contributed to increased particle displacement, pre-dominantly near the top of the mixed column, relative to Column1 �fine sand and BCt of 3%� and Column 8 �fine sand and BCt of1%�.

However, the zone within which each specimen is located inFig. 12 is based on the measured BC, which must also be consid-ered when evaluating eb and k. For example, the values of BC for
the top specimens of Columns 6 and 7 were less than 3% �i.e.,


1.98% and 2.34%, respectively� and, therefore, the k values forthese specimens were within Zone 2 �i.e., the transition zone�shown on Fig. 12. The values of BC for the top specimens forColumns 3 and 5 were greater than 3% �i.e., 5.10% and 7.65%,respectively� and, therefore, the k values for these specimenswere within Zone 3 shown in Fig. 12.

Limitations of Study

As previously indicated, the results of this study may be limitedwith respect to field application of the ZVI-clay technology inseveral ways. First, the mixing performed in this study was con-fined �i.e., one-dimensional�, whereas that in the field is uncon-fined �i.e., three-dimensional�. The greater degree of confinementin the laboratory likely results in greater vertical expansion uponmixing than what occurs in the field, although distributions in soilproperties with depth after mixing should be relatively consistent.

Second, the sands used in this study were initially in a drycondition and without contamination, whereas the soil to bemixed in the field would be contaminated and may be below thegroundwater table. Interactions between the contaminants and thebentonite after mixing could result in higher k values than re-ported herein �e.g., see Shackelford �1994��, and the existence ofhigh degrees of liquid saturation could impact the resulting mag-nitudes of e and eb after mixing.

Third, the host sands used in this study to represent the sub-grade soils were limited to clean sands �i.e., no fines�, whereas thetypes of contaminated soils to be encountered in the field arevirtually unlimited. In particular, the types and amounts of fines inthe sand are known to significantly affect the k of S-B mixtures,with decreasing k values and/or decreasing bentonite contentstypically required to achieve a suitably low k value with increas-ing amount of high plasticity fines in the sand �e.g., Alston et al.1997�.

Finally, the potential effect of scale always is present whenattempting to upscale laboratory results based on relatively smallspecimens prepared under highly controlled conditions to largerscale field scenarios typically subject to somewhat less controlledconditions �e.g., Daniel 1984�. Thus, prudence dictates that anappropriate measure of consideration be given when extrapolatingthe results of this study to the field.

Conclusions

Values of the hydraulic conductivity, k, of specimens from col-umns containing initially dry sands mixed with bentonite slurrieswere measured. The mixed specimens represented a range in voidratios �0.672�e�3.94� and bentonite contents �0.61%�BC�7.65%, by dry weight�. The measured k values were found tocorrelate poorly with the total void ratio �e� of the specimens dueto the complicating effect of the bentonite in the S-B slurry mix-tures. However, the measured k values correlated better with thevoid ratio of the bentonite �eb�, which is consistent with the re-sults of previous studies involving permeation of compacted ben-tonite and compacted S-B specimens, even though the range invalues for eb for the specimens of the S-B slurry mixtures testedin this study �42.5�eb�127� was much higher than that previ-ously reported for compacted bentonite or compacted S-B mix-tures. The relatively high range in eb values for the specimens ofbentonite slurry mixed sand also was consistent with the rela-tively large range in measured k values, which are about one to
seven orders of magnitude higher than values of k commonly


reported for compacted sand-bentonite mixtures, despite similarbentonite contents.

In terms of BC, the measured k values for the S-B slurry mix-tures correlated with three zones of BC. Zone 1 corresponded toBC�1% and represented insufficient and nonuniform distributionof bentonite within the voids of the S-B slurry mixtures and thelikelihood of values of k greater than 10−6 cm /s. Zone 3 corre-sponded to BC�3% and represented sufficient and uniform dis-tribution of bentonite within the voids of the S-B slurry mixturesand the likelihood of values of k less than 10−6 cm /s. Zone 2represented a transitional zone corresponding to 1%�BC�3%,wherein k ranged between 10−6 cm /s to 10−3 cm /s. Overall, ad-dition of more than 3% bentonite via slurry injection and mixingwith the sands was successful in reducing the k of the unmixedsands by as much as four orders of magnitude, from 9.4�10−3

cm /s�k�5.4�10−2 to k�1.0�10−6 cm /s.

Acknowledgments

This study was part of an ongoing effort toward development ofthe ZVI-clay technology for remediation of source zones con-taminated with chlorinated solvents. The financial support pro-vided by DuPont for this development is appreciated. The supportand assistance of Dr. Tom Sale of Colorado State University alsois appreciated.

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