Hydraulic Conductivity of Bentonite Slurry Mixed Sands
David Castelbaum1 and Charles D. Shackelford2
Abstract: The hydraulic conductivity 共k兲 of specimens from columns containing initially dry sands mixed with bentonite slurries was
measured. The mixed specimens represented a range in void ratios 共0.672ⱕ e ⱕ 3.94兲 and bentonite contents 共0.61% ⱕ BCⱕ 7.65%, by dry
weight兲. 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 共e兲 of
the specimens, due to the complicating effect of the bentonite in the sand-bentonite slurry mixtures. However, the measured k values
correlated better with the void ratio of the bentonite 共eb兲, which is consistent with the results of previous studies involving permeation of
compacted bentonite and sand-bentonite specimens, even though the range in values of eb in this study 共42.5ⱕ eb ⱕ 127兲 was much higher
than that previously reported. The relatively large range in eb values for the sand-bentonite slurry mixtures was also consistent with the
relatively 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 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 共9.4⫻ 10−3 cm/ s ⱕ k
ⱕ 5.4⫻ 10−2 cm/ s兲 by as much as four orders of magnitude to values less than 1.0⫻ 10−6 cm/ s.
DOI: 10.1061/共ASCE兲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 chlorinated
solvents 共e.g., tetrachloroethene, trichloroethene, carbon tetrachloride兲 resulting from inadvertent leaks, spills, and improper
disposal practices has been prevalent over the past half-century or
so. Such chlorinated solvents present a unique remediation problem for at least two reasons 共Shackelford et al. 2005兲. First, chlorinated solvents tend to be stable under common subsurface
conditions. Second, although the aqueous solubility of chlorinated
solvents is relatively low, aqueous phase concentrations of these
solvents still can be several orders of magnitude greater than established maximum contaminant levels. Past remediation technologies, such as pump-and-treat, have had limited success in
cost-effectively restoring aquifers to typical cleanup levels 共e.g.,
Shackelford and Jefferis 2000兲. In this regard, the relatively recent development and implementation of the zero-valent iron
共ZVI兲-clay technology as an alternative in-situ remediation approach for treatment of source zones contaminated with chlorinated solvents has shown promise 共e.g., Shackelford et al. 2005;
Wadley et al. 2005兲.
The ZVI-clay technology involves injecting and mixing of
both clay and granular ZVI suspended in water-based slurry directly into a source zone contaminated with chlorinated solvents.
The ZVI serves as the reactive media that drives reductive
1
Former Graduate Research Assistant, Dept. of Civil and Environmental Engineering, Colorado State Univ. E-mail: [email protected]
2
Professor, Dept. of Civil and Environmental Engineering, 1372 Campus Delivery, Colorado State Univ., Fort Collins, CO 80523-1372 共corresponding author兲. E-mail: [email protected]
Note. This manuscript was submitted on August 11, 2008; approved
on June 5, 2009; published online on June 10, 2009. Discussion period
open until May 1, 2010; separate discussions must be submitted for individual papers. This paper is part of the Journal of Geotechnical and
Geoenvironmental Engineering, Vol. 135, No. 12, December 1, 2009.
©ASCE, ISSN 1090-0241/2009/12-1941–1956/$25.00.
dechlorination of chlorinated solvents 共Gillham and O’Hannesin
1994; Wadley et al. 2005兲. The mixing process serves to 共1兲 homogenize soils and contaminants in source zones and 共2兲 facilitate
introduction of a mixture of ZVI and clay. Applications of the
ZVI-clay technology have involved mixing via high pressure jetting and shallow soil mixing using large-diameter 共e.g., 2.4 m or
8 ft兲 hollow-stem augers 共Shackelford et al. 2005; Wadley et al.
2005兲.
The clay in the ZVI-clay technology serves several useful purposes. First, the clay in the clay-water slurry facilitates suspension
of the granular ZVI, thereby improving the uniformity in the distribution of the ZVI and reducing the mechanical energy required
to mix the soils 共Wadley 2002兲. Second, the addition of the clay
can enhance sorption of the contaminants, which reduces the mobility of the contaminants and increases the time for reaction between the ZVI and contaminants. Finally, mixing of the clay into
the contaminated subsurface can decrease the hydraulic conductivity of the treated zone, which can reduce contaminant mass flux
from the treated zone via advection 共i.e., hydraulically driven
contaminant transport兲 by diverting upstream groundwater flow
around the treated zone and lowering the amount of contaminant
mass flux directly emanating from the treated zone into the surrounding ground water. Initial development and field applications
of the ZVI-clay technology used kaolin clay. However, more recent field applications have involved the use of sodium bentonite,
primarily due to the more extensive use of sodium bentonite in
field applications and the lower cost of bentonite.
Given the aforementioned considerations, the primary purpose
of this study was to evaluate the possible reduction in hydraulic
conductivity resulting from injecting and mixing bentonite-water
slurry into three sands via a laboratory apparatus developed to
mimic field application of the ZVI-clay technology. Although extensive study on various factors affecting the hydraulic conductivity of sand-bentonite 共S-B兲 mixtures has been conducted 共e.g.,
Gipson 1985; Garlanger et al. 1987; Chapuis et al. 1992; Haug
and Wong 1992; Kenney et al. 1992; Haug and Bolt-Leppin 1994;
O’Sadnick et al. 1995; Mollins et al. 1996; Alston et al. 1997;
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Materials and Methods
Constituent Materials
The constituent materials for the S-B mixtures evaluated in this
study included two sands and a processed sodium bentonite. The
two sands included fine-grained Ottawa sand 共Grade F-58
foundry sand, U.S. Silica Company, Ottawa, Ill.兲 with a measured
specific gravity of solids 共ASTM D 854兲, Gs, of 2.67, and
medium-to-coarse-grained quartz sand 共Filpro #3 well gravel,
U.S. Silica Company, Mauricetown, N.J.兲 with a Gs of 2.65. A
mixture of the two sands in equal proportions by dry weight
共Gs = 2.66兲 also was used in the study. Based on the particle-size
distributions of the two sands and the sand mixture shown in Fig.
1, the Ottawa sand and the quartz sand were referred to as “fine
sand” and “coarse sand,” respectively, whereas the mixture of the
two sands was referred to as the “sand blend.” Both fine and
coarse sands and the sand blend classified as poorly graded sands
共SP兲 according to the Unified Soil Classification System 共USCS兲
共ASTM D 2487兲.
The bentonite was a powdered, polymer modified, air-float
bentonite 共Hydrogel, WYO-BEN, Inc., Grey Bull, Wyo.兲. The
physical and chemical properties and mineralogical composition
of the bentonite are summarized Table 1. The bentonite classified
as high plasticity clay 共CH兲 according to the USCS.
Tap water was used both as the mixing liquid and the permeant
liquid in this study. The measured chemical properties of the tap
water 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
Coarse
Sand
Medium
Fines (Silt & Clay)
Fine
0
90
10
80
20
70
30
60
40
50
50
40
60
30
10
0.075 mm
20
0.425 mm
4.75 mm
70
2.00 mm
Percent Passing (by dry weight)
100
Fine Sand
Coarse Sand
Sand Blend
Bentonite
0
10
1
0.1
0.01
Particle Size (mm)
0.001
80
Percent Retained (by dry weight)
Gleason et al. 1997; Howell and Shackelford 1997; Stern and
Shackelford 1998; Abichou et al. 2002a,b; Sällfors and ÖbergHögsta 2002;. Ebina et al. 2004; Kaoser et al. 2006; Teachavorasinskun and Visethrattana 2006兲, the focus of virtually all of these
studies has been on compacted S-B mixtures being considered for
use as liners for waste containment applications as opposed to
S-B mixtures that have been mixed in-place. The primary difference between these two applications of S-B mixtures is the
method of preparing test specimens for hydraulic conductivity
testing, which can result in significantly higher void ratios for the
mixed in-place specimens relative to those associated with compacted S-B mixtures 共Castelbaum 2007兲.
Although hydraulic conductivity testing initially was performed on specimens from columns mixed with bentonite slurry
containing ZVI, generation of hydrogen gas resulting from chemical reaction between the ZVI and the bentonite 共e.g., Reardon
1995,2005兲 confounded the measurement of hydraulic conductivity, such that this study involved measurement of hydraulic conductivity on specimens from columns mixed with only bentonite
slurry 共i.e., without ZVI兲. In addition, no contamination was used
in the study both for safety reasons and because the lack of ZVI
obviated the need to consider mixing involving contaminated
media. Although the lack of ZVI and contaminated media may
limit the direct application of the results to the typical scenarios
involving implementation of the ZVI-clay technology, the results
of the study provide insight into the expected hydraulic behavior
of S-B mixtures at relatively high void ratios, and may be useful
in other applications involving similar materials and conditions,
such as disposal of dredged materials 共e.g., Lacasse et al. 1977兲
and slurried mineral wastes 共e.g., Carrier et al. 1983兲.
90
100
0.0001
Fig. 1. Particle-size distributions for sands and bentonite used in the
study
were 1.2⫻ 10−2 cm/ s, 5.4⫻ 10−2 cm/ s, and 9.4⫻ 10−3 cm/ s at
void ratios, e, of 0.627, 0.700, and 0.406, respectively.
Bentonite Slurry Preparation
Bentonite in slurry form was injected and mixed into columns of
dry sand. The bentonite slurries were prepared by mixing bentonite and tap water in varying proportions by weight. The bentonite content of the slurry, BCs, was defined as the mass of
bentonite divided by the total mass of the slurry 共i.e., bentonite
plus water兲. The amount of water used to prepare the bentonite
slurry was adjusted to account for the air-dried 共hygroscopic兲
gravimetric water content of the bentonite, which ranged from 8.5
to 12.5%. The bulk bentonite was stored in sealed 19-L 共5 gal兲
plastic buckets provided by the supplier. The desired values of
BCs for this study were 5, 6, and 7%.
The dry mass of sand in the column was used as the basis for
determining the minimum required amount of bentonite in the
slurry to be injected. The required mass of hydrated bentonite
slurry was determined from the percentage of bentonite in the
slurry and the target bentonite content of the postmixed column.
For example, for 1,000 g of dry sand in the column, a target
bentonite content of 3% in the postmixed column, and a 5% bentonite slurry, the relevant weights are 30 g of bentonite 共i.e.,
1 , 000 g ⫻ 0.03兲 in 600 g of total bentonite slurry 共i.e., 30 g/0.05兲.
The total amount of slurry prepared in this manner was increased
by approximately 20% to account for the capacity of the injection
system 共e.g., hosing, fittings, etc.兲 and the samples required for
testing of the slurry.
High-speed colloidal mixers were used to produce the bentonite slurries. Due to the volume of bentonite slurry typically prepared 共approximately 3–4 L兲, smaller 1-L batches of a bentonite
slurry were initially mixed in a 1.5-L Cuisinart blender for 2 min
共Yeo 2003兲. The smaller batches were combined in a 4.5-L Waring industrial blender and mixed for another 2 min. The viscosity,
specific gravity, electrical conductivity 共EC兲, bentonite content,
and temperature of the composite batch of bentonite slurry then
were measured to verify and/or compare consistency in these
properties between each batch of prepared slurry used for the
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Table 1. Physical and Chemical Properties and Mineralogical Composition of Bentonite
Property
a
Specific gravity, Gs
Atterberg limits:
Liquid limit, LL 共%兲
Plastic limit, PL 共%兲
Plasticity index, PI 共%兲
Principal minerals 共%兲:
Montmorillonite
Kaolinite
Quartz
Plagioclase feldspar
Illite/mica
Mixed-layer illite/smectite
Calcite
Chlorite
Clinoptilolite
Gypsum
Cation exchange capacity, CEC 共cmolc / kg兲
Exchangeable metals
共cmolc / kg兲:
Ca2+
Mg2+
Na+
K+
H+
Sum
Soluble salts 共mg/kg兲:
Ca2+
Mg2+
Na+
K+
CO2−
3
HCO−3
SO2−
4
Cl−
NO−3
Soil pH
Standard
Measured value
ASTM D 854
ASTM D 4318
2.75
478
28
449
Based on X-ray diffraction analyses performed by
Mineralogy Inc., Tulsa, OK
Based on analyses performed by Soil, Water, and Plant
Testing Laboratory at Colorado State University
Based on analyses performed by Soil, Water, and Plant
Testing Laboratory at Colorado State University
87
—
8
2
2
—
1
—
Trace
Trace
73.5
19.3
10.9
38.8
0.8
⬍0.1
69.8
Based on analyses performed by Soil, Water, and Plant
Testing Laboratory at Colorado State University
Based on analyses performed by Soil, Water, and Plant
Testing Laboratory at Colorado State University
Based on analyses performed by Soil, Water, and Plant
Testing Laboratory at Colorado State University
Soil EC 共mS/m兲 at 25° C
460
240
3,964
117
125
764
7,167
1,624
350
8.7
250
Note: cmolc⫽centimoles of charge.
a
Average of three tests with 2.73ⱕ Gs ⱕ 2.77.
column mixing tests. Viscosity and specific gravity were determined using a Marsh Funnel and mud balance, respectively, following procedures in the American Petroleum Institute
Recommended Practice 13B-1 共RP 13B-1兲. The EC was determined using an EC probe and meter 共Model #150 Aplus, Orion
Research, Inc., Beverly, Mass.兲, and the bentonite content was
determined by drying a small specimen of the bentonite slurry in
an oven at 110⫾ 5 ° C. The bentonite slurry then was stored in a
sealed 4-L plastic container and allowed to hydrate for a minimum period ranging from 16 to 20 h. Following hydration, the
bentonite slurry was mixed again in the 4.5-L Waring industrial
blender for 2 min and the previously indicated properties were
measured again to provide a check of the values of these proper-
ties of the bentonite slurry determined after initial preparation. A
comparison of the results of the tests performed on batches of
both the initially prepared slurries and the hydrated slurries indicated little change in slurry properties 共see Table 3兲. The prepared
bentonite slurries then were used immediately thereafter in preparation of the mixed specimens 共i.e., without additional storage兲.
Overall, excellent control between desired and measured values of BCs was achieved for all prepared batches. For example,
for the desired values of BCs of 5%, 6%, and 7%, the ranges in
measured values of BCs were 4.95% ⱕ BCs ⱕ 5.04%, 5.98%
ⱕ BCs ⱕ 6.02%, and 6.96% ⱕ BCs ⱕ 7.01%, respectively. This
slight variability in measured BCs was not expected to have an
appreciable effect on the test results.
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Table 2. Chemical Properties of Tap Watera
Property
sections were used as the cells for hydraulic conductivity testing,
as shown in Fig. 3. The inside diameter of all column segments
was 101.6 mm 共4.0 in.兲 to accommodate the mixing blades, which
were slightly smaller in diameter. The base section of the assembled column allowed the injection ports and auger blades to
pass completely through the materials immediately above the
base section 共i.e., the target mixing zone兲. The spacer sections
were used as needed so that the nuts on the top plate used to
compress the o-rings and tighten the column only needed to be
advanced approximately 25 mm. The total number of 134-mmthick sections in an assembled column was determined based on
the length of column needed to accommodate the estimated postmixed total volume of sand and bentonite slurry for a given set of
test conditions. Further details on the column mixing equipment
are provided in Castelbaum 共2007兲.
Measured value
Ionic species 共mg/L兲:
13.6
Ca2+
2.1
Mg2+
3.1
Na+
0.8
K+
⬍0.01
B3+
⬍0.1
CO2−
3
−
42.1
HCO3
10.8
SO2−
4
3.2
Cl−
⬍0.1
NO−3
⬍0.1
NO−3 -N
pH
7.9
43
Hardness as CaCO3 共mg/L兲
35
Alkalinity as CaCO3 共mg/L兲
Total dissolved solids 共mg/L兲
76
EC 共mS/m兲 at 25° C
9.01
a
Based on analyses performed by Soil, Water, and Plant Testing Laboratory at Colorado State University.
Test Column Preparation
Test columns were prepared by weighing the column assembly
empty and weighing out approximately 4.5 kg of sand, which
typically resulted in an initial height of dry sand in the mixing
zone of approximately 240 mm. The sand was poured through a
funnel and allowed to fall freely into the assembled column. The
sides of the column were tapped lightly to facilitate settling of the
sand within the column. The total height of the sand within the
assembled column was measured, and the mixing zone height was
determined by subtracting 80 mm to account for the base section.
Finally, the assembled column with sand was weighed and then
secured in the column-mixing platform. The resulting distribution
of the sand within the column was assumed to be uniform.
Column Mixing Equipment
The mixing of test columns of sand with simultaneous injection
of the bentonite slurry was performed using the vertical mixing
platform that was specially manufactured for this study as shown
in Fig. 2. The mixing platform provides for three key independently controlled operations similar to field-scale soil mixing
equipment: 共1兲 vertical movement of the hollow-stem auger; 共2兲
rotation of the hollow-stem auger; and 共3兲 injection of the bentonite slurry 共i.e., injection slurry兲. The rate of vertical movement of
the auger was fixed at approximately 102 mm/min, while rates of
auger rotation 共reversible兲 and slurry injection could be varied
from 0 to 20.7 revolutions per minute 共rpm兲 and 60 to 670 mL/
min, respectively. The highest auger rotation rate 共i.e., 20.7 rpm兲,
which also is within the typical range of auger rotation rates used
for field applications, was used for all column tests performed in
this study. The slurry injection rate was varied depending on the
column height and the volume of slurry to be injected as described subsequently.
Test specimens of the bentonite slurry-sand mixtures for hydraulic conductivity measurements were prepared and mixed directly inside a specially manufactured clear acrylic column
consisting of several 134-mm-thick sections sandwiched between
a variable number of spacer sections 共20–40 mm thick兲 at the top
and a single 80-mm-thick section at the base. The 134-mm-thick
Column Mixing Procedures
Illustrations of the column mixing scenario including pictures
showing slurry injection during mixing and a mixed specimen
within a permeameter segment of the column prior to permeation
are provided in Fig. 4. The column mixing sequence consisted of
auger rotation and slurry injection during downward movement
共i.e., advancing兲 through the mixing zone, and reverse auger rotation only 共i.e., without slurry injection兲 during upward movement 共i.e., retraction兲. The movement of the auger blades
completely through the mixing zone in a single direction 共i.e.,
either downward or upward兲 was considered as a single pass.
Slurry was injected only during advancement of the auger such
that the slurry injected was immediately mixed with the soil by
the auger blades located just above the slurry injection ports. The
injection rate was adjusted so that the required volume of injected
slurry and, therefore, the target quantity of bentonite, was deliv-
Table 3. Average Values of Measured Properties of Bentonite Slurries Used in the Study
Viscosity
共s兲
Column
1
2
3
4
5
6
7
Specific gravity
of slurry
EC @25° C
共mS/m兲
Temperature,
T 共°C兲
Bentonite content of
the slurry, BCs 共%兲
Initial
Hydrated
Initial
Hydrated
Initial
Hydrated
Initial
Hydrated
Initial
Hydrated
56
57
56
55
54
127
808
50
50
51
48
50
96
314
1.030
1.030
1.030
1.030
1.030
1.035
1.040
1.030
1.030
1.030
1.030
1.030
1.035
1.040
114.6
115.9
116.9
118.2
119.3
139.2
156.0
118.7
120.1
120.8
121.6
122.4
144.1
157.6
31.3
30.9
30.5
31.2
31.0
30.6
32.0
28.1
28.2
28.5
28.4
27.7
28.6
28.6
4.99
4.98
4.96
4.97
4.99
6.01
6.97
4.97
4.99
4.98
4.96
4.99
6.01
6.98
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Fig. 2. 共Color兲 Laboratory-scale mixing equipment for injecting and mixing zero-valent iron-clay slurries for remediation of soils contaminated
with chlorinated solvents
ered to the mixing zone during the downward pass共es兲 of the
auger. Several combinations of test conditions resulted in required
injection rates that exceeded the ability to deliver the minimum
volume of slurry in a single downward pass. Therefore, two
downward passes with injection and two upward passes without
injection 共i.e., four passes兲 were performed for each test column.
Once the slurry injection rate was determined and set, the rotating auger was advanced down through the inside of the assembled column until the auger injection ports were at the same
elevation as the top of the mixing zone 共i.e., top of the sand
column兲. The injection slurry was poured into the injector reservoir, all injector system connections were secured, the injector
motor was turned on, and the injection system fittings, hose and
auger were filled with slurry. Advancement of the auger down
through the mixing zone commenced when the injection slurry
was flowing out of the injection ports. The auger was advanced
until the visible injection/mixing front 关see Fig. 4共a兲兴 passed completely through the mixing zone and approximately 20 to 40 mm
into the base section. At this point, slurry injection and auger
advancement were stopped, auger rotation was reversed, and the
auger was retracted upward through the mixing zone. The auger
was retracted until the injection ports again were at the same
elevation as the top of the mixing zone. This procedure was repeated for the second downward and upward passes. However,
the slurry injection rate for the second downward pass was reduced to account for the increased height 共i.e., expansion兲 of the
mixing zone due to the volume of slurry injected during the first
downward pass. The auger was retracted completely from the
assembled column at the end of the second upward pass through
the mixing zone.
Column Disassembly and Testing Procedures
After mixing was complete, the assembled, mixed test column
was removed from the mixing platform and the postmixed height
of the test column was determined. The top plate, spacer sections,
and any empty and/or 134-mm-thick column sections partially
filled with sand-slurry mixture were removed. The sand-slurry
mixture in partially filled sections and the base section was discarded. The 134-mm-thick sections completely filled with sandslurry mixture then were removed one section at a time by sliding
a thin metal plate between two adjacent sections. The removed
section then was fitted with filter papers, porous stones, end caps
共i.e., assembled into the rigid-wall permeameters兲 and weighed
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Fig. 3. 共Color兲 Photograph of disassembled parts and schematic cross section of rigid-wall permeameters
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Column
Hollow Stern Auger
Mixed Soil
Direction of
travel of auger
Auger Blades
Injection Port
Injection/Mixing
Front
Unmixed Soil
(a)
(b)
(c)
Fig. 4. 共Color兲 Illustrations of 共a兲 column mixing scenario; 共b兲 slurry injection and mixing blades of auger; and 共c兲 mixed specimen within
permeameter segment of column prior to permeation
关see Fig. 4共c兲兴. The assembled permeameters were connected to a
controlled source of compressed air for hydraulic conductivity
testing via panel pressure boards 共models M100000 and
M116000, Trautwein, Houston兲.
Measurement of Hydraulic Conductivity
Hydraulic conductivity 共k兲 testing of the sand-slurry mixtures was
performed with tap water as the permeant liquid using the fallingheadwater/rising tailwater procedure described in ASTM D 5084.
However, permeation was terminated when a minimum of four
consecutive values of k were within ⫾10% of the mean value and
the ratio of the volumetric outflow to inflow rates, Qout / Qin, was
1.0⫾ 0.1, which are more stringent termination criteria than indicated in ASTM D 5084 共i.e., k within ⫾25% of the mean value,
and Qout / Qin = 1.0⫾ 0.25兲.
Permeation was initiated upwards through the specimens. As
such, maintaining the hydraulic gradient, i, at or below unity 共i.e.,
i ⱕ 1兲 typically was necessary to avoid a quick condition due to
the relatively low densities 共i.e., low effective stresses兲 of the
specimens tested. After upward permeation was terminated, the
direction of permeation was reversed 共i.e., downward permeation兲
to assess the possible effects on k due to the direction of perme-
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Table 4. Summary of Column Mixing Program
Column
Type of
sand
Target bentonite
content in
mixing zone,
BCt 共%兲
Bentonite content
of slurry,
BCs 共%兲
1
2
3
4
5
6
7
Fine
Coarse
Blend
Fine
Fine
Fine
Fine
3
3
3
1
5
3
3
5
5
5
5
5
6
7
ation, and permeation was continued until the termination criteria
were achieved. The applied hydraulic gradients for downward
permeation ranged from two to five 共2 ⱕ i ⱕ 5兲. By comparison,
hydraulic gradients lower than unity 共i ⬍ 1兲 typically are encountered in field applications involving groundwater flow 共e.g., Ingebritsen et al. 2006兲.
As a result of the low magnitudes of applied hydraulic gradients, volume changes within the specimens during permeation
were expected to be negligible. However, visual observation of
the specimens contained within the clear plastic permeameters
indicated that some settling had occurred in all but two of the
specimens during permeation, presumably due to the loose nature
of the mixed specimens 关e.g., see Fig. 4共b兲兴. In these cases, the
change in the length of the bentonite slurry mixed specimen, ⌬L,
relative to the original length, Lo 共=120 mm; see Fig. 3兲, ranged
from 1.3% 共Column 4, bottom specimen兲 to as much as 16.7%
共Column 5, upper middle specimen兲. In these cases, the actual,
measured length, L 共=Lo − ⌬L兲 of the soil specimen contained
within the permeameter was used to calculate the hydraulic conductivity as well as in determining the final volume of the specimen for the purpose of determining void ratios.
Bentonite Content Determination
Following completion of permeation, the permeameters were disassembled, and the bentonite content and void ratio were determined for each specimen. The bentonite contents of the specimen
mixtures were determined using the test method described by
Abu-Hassanein et al. 共1996兲, which is based on EC. In this procedure, the S-B mixture for each section was transferred to a
925-mL, baffled, stainless steel “dispersion cup” 共ASTM D 422兲.
Then, approximately 200 mL of tap water were added to the S-B
mixture in the dispersion cup, and the resulting mixture was mechanically stirred for 2 min with the mixer. After stirring, the sand
particles settled to the bottom of the dispersion cup and the diluted bentonite slurry was slowly poured into a 1,000-mL sedimentation cylinder. This procedure was repeated three additional
times with care to ensure that any clods of the S-B mixture were
stirred thoroughly and dispersed. Tap water was added to the
bentonite slurry in the sedimentation cylinder until the 1,000-mL
calibration mark was reached. The cylinder was sealed with a
rubber stopper and the bentonite slurry was mixed by manually
rotating the cylinder vertically for 1 min. After this mixing was
completed, the EC of the bentonite slurry was measured using the
EC probe and meter.
The EC reading was converted to a bentonite concentration,
CB 共g/L兲, through the use of EC-CB calibration curves developed
using 1-L slurries with known values for CB 关see Abu-Hassanein
et al. 共1996兲兴. Sand-specific EC-CB calibration curves were developed because the type and characteristics of the native soil to
which the bentonite is added can affect the EC of the clay slurry
共Abu-Hassanein et al. 1996兲. However, the different sands used in
this study did not have a significant effect on the EC of the bentonite slurry 共Castelbaum 2007兲.
Testing Program
The entire column mixing program is summarized in Table 4. A
total of seven columns were mixed using the fine, coarse, or
blended sand mixed with slurries consisting of 5, 6, or 7% bentonite contents, BCs, to provide mixtures of sand and bentonite
slurry containing target bentonite contents, BCt, of 1, 3, or 5%.
Results
Void Ratio and Bentonite Content
Values for the void ratio 共e兲 and measured 共actual兲 bentonite contents 共BC兲 for each of the 19 column test sections resulting from
the seven mixed columns are summarized in Table 5. All values
of e shown in Table 5 were determined after permeation. The
resulting values of e ranged from 0.672 to 3.94, whereas values of
BC ranged from 0.61% to 7.65%. Higher values of e and BC
occurred near the top of Columns 1–3 and 5, whereas the distributions in both e and BC with depth for Columns 4, 6, and 7 were
relatively uniform. In the cases of Columns 1–3 and 5, higher
values of e and BC near the top were attributed to the vertical
expansion of the mixtures that occurred during mixing due to the
lack of confining stress near the top of the mixed zone 共Castelbaum 2007兲. The increase in confining stress with depth restricted
the amount of expansion with depth, typically resulting in lower
values for e and BC with depth. In the cases of Columns 4, 6, and
7, less expansion occurred during mixing of the sand columns
with the bentonite slurries, such that only two sections were recovered for hydraulic conductivity testing, versus three sections
recovered from Columns 1–3 and four sections recovered from
Column 5. As previously indicated, column sections that were
only partially filled with the mixture of sand and bentonite slurry
were not permeated.
The initial heights of the columns of dry sand prior to injection
of the bentonite slurry and mixing, Ho, and the final heights of the
bentonite slurry mixed columns, H f , as well as the percentages of
expansion, are shown in Fig. 5. The amount of expansion that
occurred during mixing was directly related to the magnitude of
the difference between the bentonite content of the slurry 共BCs兲
injected into the sand and the target bentonite content 共BCt兲 for
the mixed specimen in the column 共Table 4兲. The amount of bentonite 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 was
only 1% for Column 4, 3% for Columns 1–3, and 5% for Column
5. Thus, the amount of injected bentonite slurry and, therefore, the
extent of expansion varied in the order Column 4
⬍ Columns 1 – 3 ⬍ Column 5. Also, although the BCt was 3% for
Columns 1–3 and Columns 6 and 7, the BCs for Columns 1–3 was
5%, whereas the BCs for Columns 6 and 7 was 6% and 7%,
respectively. Thus, the amount of injected bentonite slurry and
extent of expansion for Columns 6 and 7 was less than that for
Columns 1–3.
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Table 5. Measured Test Results for Individual Column Sections
Hydraulic conductivity,
k 共cm/s兲
Column
1
2
3
4
5
6
7
Type of
sand
Specimen location
within mixed
column
Void ratio,
e
Measured bentonite
content,
BC 共% dry wt.兲
Upward
permeation,
ku
Downward
permeation,
kd
Geometric
mean,
kgm a
ku / kd
Fine
Top
1.77
2.53
1.1⫻ 10−3
4.2⫻ 10−4
6.8⫻ 10−4
2.6
1.2
−7
−7
5.9⫻ 10−7
2.0
4.3
4.2⫻ 10
Log 共ks / kgm兲
Middle
1.00
2.08
8.3⫻ 10
Bottom
0.924
1.27
7.7⫻ 10−5
6.0⫻ 10−5
6.8⫻ 10−5
1.3
2.2
−6
−7
−6
Top
3.53
5.75
2.6⫻ 10
4.3
4.6
Middle
0.847
0.98
2.2⫻ 10−5
2.8⫻ 10−5
2.5⫻ 10−5
0.79
3.3
Bottom
0.796
0.76
1.8⫻ 10−5
2.1⫻ 10−5
1.9⫻ 10−5
0.86
3.4
−6
−7
−7
Coarse
6.1⫻ 10
Top
2.77
5.10
1.1⫻ 10
3.8
4.2
Middle
0.979
2.21
6.9⫻ 10−7
1.9⫻ 10−7
3.6⫻ 10−7
3.6
4.4
Bottom
0.672
1.23
3.8⫻ 10−7
1.5⫻ 10−7
2.4⫻ 10−7
2.5
4.6
−4
−4
−4
1.3
1.6
0.71
3.0
Blend
Fine
2.9⫻ 10
1.3⫻ 10
2.6⫻ 10
5.6⫻ 10
3.0⫻ 10
Top
0.750
0.61
3.4⫻ 10
Bottom
0.732
0.75
9.2⫻ 10−6
1.3⫻ 10−5
1.1⫻ 10−5
−7
−7
−7
Top
3.94
7.65
9.9⫻ 10
2.8
4.3
Upper Middle
1.62
4.08
8.4⫻ 10−7
3.0⫻ 10−7
5.0⫻ 10−7
2.8
4.4
Lower Middle
1.26
3.02
8.7⫻ 10−7
2.7⫻ 10−7
4.8⫻ 10−7
3.2
4.4
−7
−7
−7
Fine
3.6⫻ 10
Bottom
0.850
1.83
7.8⫻ 10
3.7
4.5
Top
0.883
1.98
4.2⫻ 10−7
2.3⫻ 10−7
3.1⫻ 10−7
1.8
4.6
Bottom
0.968
2.02
7.1⫻ 10−7
1.9⫻ 10−7
3.7⫻ 10−7
3.7
4.5
−7
−7
−7
2.6
4.2
3.5⫻ 10−7
3.0
4.5
Fine
Fine
Top
0.957
2.34
1.1⫻ 10
Bottom
0.951
2.23
6.0⫻ 10−7
2.1⫻ 10
6.0⫻ 10
4.2⫻ 10
2.0⫻ 10−7
4.0⫻ 10
6.8⫻ 10
b
a
kgm = 共kukd兲0.5.
b
ks = k of sand 共fine sand: ks = 1.2⫻ 10−2 cm/ s; coarse sand: ks = 5.4⫻ 10−2 cm/ s; sand blend: ks = 9.5⫻ 10−3 cm/ s兲.
Hydraulic Conductivity
The results of hydraulic conductivity 共k兲 testing of 19 test sections recovered from the seven mixed columns also are summarized in Table 5. The results include the measured k values based
on upward permeation, ku, the measured k values based on downward permeation, kd, and the geometric means, kgm, of ku and kd.
Several observations are apparent from the k results shown in
Table 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 than
values of kd for 16 of the 19 tests 共i.e., 84.2%兲. The tendency for
ku to be greater than kd may be due, in part, to the different stress
conditions imposed during upward permeation relative to downward permeation. For example, during downward permeation, the
induced seepage forces work in the same direction as gravity such
that the overall effective stresses in the specimen are expected to
be higher, resulting in lower void ratio and a lower k. In contrast,
during upward permeation, the induced seepage forces oppose
gravity, such that the overall effective stresses in the specimen are
expected to be lower, resulting in a higher void ratio and higher k.
However, given the relatively low hydraulic gradients imposed
during upward relative to downward permeation, this effect of
seepage forces was likely greater during downward permeation.
Nonetheless, the overall effect of direction of permeation was less
than one-half of an order of magnitude.
Second, all the measured values of k 共ku , kd , kgm兲 for all specimens of the S-B slurry mixtures were lower than the measured k
of the premixed sand 共i.e., ks兲, as expected. For example, the
previously reported values of ks ranged from 1.2 to 4.6 orders of
magnitude higher than the value of kgm 共Table 5兲. These lower
measured values of k for all specimens of the sand-bentonite
slurry mixtures relative to the sands alone are consistent with the
addition of high swelling bentonite to the initially dry sands,
which were free of fines.
Third, the values of kgm for the specimens representing the top
sections of the mixed columns are the highest values measured in
five of the seven columns 共Columns 1, 3–5, 7兲, which is consistent with the highest void ratios in these columns also occurring
for the top section. However, for Column 2, the value of kgm for
the top section was the lowest value measured, even though the
highest void ratio in Column 2 occurred for the top section. This
difference highlights the fact that the measured k values for the
specimens are a function not only of the void ratio but also of the
bentonite content within each section 共i.e., 5.75% for the top section versus 0.98% and 0.76% for the middle and bottom sections,
respectively兲.
Finally, the values of kgm for the S-B slurry mixtures range
from 2.4⫻ 10−7 cm/ s to 6.8⫻ 10−4 cm/ s. These kgm values are
about one to seven orders of magnitude higher than values of k
commonly reported for compacted S-B mixtures, which typically
fall within the range 10−10 cm/ s ⱕ k ⱕ 10−7 cm/ s 关e.g., see Kenney et al. 共1992兲, Stern and Shackelford 1998兴, despite similar
bentonite contents. The difference in results can be attributed primarily to the higher void ratios, in general, and the significantly
higher bentonite void ratios, in particular, of the S-B slurry mixtures evaluated in this study relative to compacted S-B mixtures,
as will be elucidated in the following discussion.
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60
Initial height,H
Final height, H
50
5
(a)
o
f
e = 0.125 + 0.429BC (r2 = 0.906)
40
Total Void Ratio, e
Column Height (cm)
4
30
20
3
2
10
1
0
1
2
3
4
5
Column Number
6
0
2
100
f
o
(b)
1.75
75
1.5
50
25
1.25
o
Relative Vertical Expansion, ∆H/H (%)
Expansion Ratio, ER (= H /H )
7
1
1
2
3
4
5
Column Number
6
0
7
Fig. 5. 共a兲 Initial and final mixed specimen heights; 共b兲 expansion
ratio and relative vertical expansion for seven columns of sand mixed
with bentonite slurry
gm
10-2
10-4
S-B Mixtures w/Fine Sand
Sand Blend
S-B Mixtures w/Coarse Sand
10-3
?
10-5
S-B Mixtures w/Sand Blend
10-4
10-6
10-5
10-7
Trend curve
10-6
10-8
gm
(m/s)
Geometric Mean Hydraulic Conductivity, k
10-3
Coarse Sand
Fine Sand
Geometric Mean Hydraulic Conductivity, k
(cm/s)
10-1
10-7
10-9
0
1
2
3
4
5
Total Void Ratio, e
Fig. 6. Geometric mean of measured hydraulic conductivity values
versus total void ratio for sand-bentonite 共S-B兲 slurry mixtures
0
2
4
6
8
10
Bentonite Content, BC (%)
Fig. 7. Bentonite content versus total void ratio for sand-bentonite
slurry mixtures
Discussion
Effect of Bentonite Void Ratio
Traditionally, the hydraulic conductivity, k, of soil has been correlated to the total void ratio, e, of the soil, with an increase in e
correlating with a semilog linear increase in k 共Lambe and Whitman 1969兲. As a result, the geometric means of the k values
measured in this study, or kgm, were plotted as a function of e in
Fig. 6.
As shown in Fig. 6, the general trend for the measured kgm
versus e of the specimens tested in this study deviates from the
traditional trend in e versus k. For example, atypically low values
of k 共i.e., k ⬇ 10−6 cm/ s兲 correlate with specimens with relatively
high values of void ratio 共i.e., e ⬎ 1.5兲, whereas atypically high
values of k 共i.e., 10−5 ⬍ k ⬍ 10−4 cm/ s兲 correlate with specimens
with relatively low values of e. These deviations are due to the
effect of the bentonite content, BC, of the specimens, where
specimens with relatively high BC 共i.e., BC⬎ 4.0% correspond to
high values of e but low measured k, and specimens with relatively low BC 共i.e., BC⬍ 1.5%兲 correspond to low values of e but
high measured k 共see Fig. 7兲.
Kenney et al. 共1992兲 present a simple model of an ideal, twoconstituent, homogeneous mixture of sand and saturated bentonite
to estimate the minimum k of compacted S-B mixtures. This ideal
mixture assumes that all water is associated with the bentonite
and that the sand particles are impervious inclusions within a
matrix of saturated bentonite and do not affect the fabric of the
bentonite. The hydraulic conductivity of such an ideal mixture
would be controlled by the hydraulic conductivity of the bentonite, which is controlled by the fabric and the void ratio of the
bentonite, eb. The fundamental definition for eb is the volume of
voids attributed to the bentonite, Vvb, divided by the volume of
solid bentonite in the mixture, Vsb, or eb = Vvb / Vsb. The eb of an
ideal saturated S-B mixture also was expressed in Kenney et al.
共1992兲 as follows:
冋冉 冊
eb = Gsb
1+
册
1 ␳w
1
−
−1
r ␳dm rGss
共1兲
where Gsb = the specific gravity of solids for the bentonite; Gss
= the specific gravity of solids for the sand; r = the ratio of the dry
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200
Eq. 1 w/constant ρdm
Void Ratio for Bentonite, e
b
Column 1
Column 2
150
Column 3
Column 4
Column 5
1.45 - 1.54
Column 6
100
ρdm of Specimen
1.4
0.97
Column 7
0.59
0.71
1.6
50
ρdm =
0.54
0.5
1.18
1.02
1.34 - 1.44
1.75
1.5 1.25
1.0
0
0
2
4
6
8
10
Bentonite Content, BC (%)
Fig. 8. Bentonite content versus void ratio of bentonite for sandbentonite slurry mixtures tested in this study superimposed on curves
for constant mixture dry density, ␳dm 共g / cm3兲, based on Eq. 共1兲
masses of bentonite and sand 共i.e., BC/100兲; ␳w = the density of
water; and ␳dm = the dry density of the mixture.
In contrast to an ideal S-B mixture, Kenney et al. 共1992兲 noted
that nonideal S-B mixtures may contain insufficient and/or inadequately distributed bentonite resulting in the voids between sand
particles being partially filled with free water, forming holes or
defects. Kenney et al. 共1992兲 indicated that these water-filled
holes, which can form channels if interconnected, increase the k
of compacted S-B mixtures, and reported some values of k for the
S-B mixtures that were closer to that of the sand relative to that of
the bentonite. The results presented in Kenney et al. 共1992兲 show
that the higher values of k for S-B mixtures, as well as for specimens of bentonite only, correspond with higher values of eb.
Therefore, Kenney et al. 共1992兲 concluded that both an adequate
amount of bentonite and a sufficiently uniform distribution of the
bentonite within the mixture are required for low-permeability
sand-bentonite mixtures.
Kenney et al. 共1992兲 also indicated that, for the ideal sandbentonite mixture, the free-swell void ratio of the bentonite, eb,f-s,
represents an upper limit on the value of eb. Values of eb greater
than the eb,f-s indicate that conditions for ideal mixtures do not
exist due to water-filled voids within the mixture. The eb,f-s typically is determined from test procedures similar to those used to
determine the swell index 共i.e., ASTM D 5890兲, which typically
ranges from 25 mL/2 g to 35 mL/2 g for sodium bentonites 共Lee
and Shackelford 2005a,b; Katsumi et al. 2007; Meer and Benson
2007兲. Based on the specific gravity of solids for the bentonite
共Gsb兲 used in this study of 2.75 共see Table 1兲, this range of free
swell index values corresponds to values of eb,f-s ranging from 33
to 47.
As a result of the approach described by Kenney et al. 共1992兲,
the eb for each specimen tested in this study was calculated based
on the assumption that the volume of the voids for the mixed
specimen, Vv, equals the volume of the voids of the bentonite,
Vvb, or
Vv = Vt − Vss − Vsb = Vvb
共2兲
where Vt = the total volume of the mixed specimen; Vss = the volume of solids for the sand in the mixed specimen; and Vsb = the
volume of solids for the bentonite in the mixed specimen. The 19
values of eb calculated on the basis of Eq. 共2兲 ranged from 42.5 to
127, and were identical to those calculated on the basis of Eq. 共1兲
共Castelbaum 2007兲. For example, as shown in Fig. 8, the measured values of eb based on Eq. 共2兲 plotted as a function of bentonite content, BC, compare favorably with those based on Eq. 共1兲
for different values of ␳dm. Thus, the simple expression for eb
given by Eq. 共1兲 for compacted S-B mixtures was equally valid
for the bentonite slurry mixed sand specimens evaluated in this
study.
Only 6 of the 19 calculated values for eb for the S-B mixtures
in this study fell within the range of possible eb values based on
free swell 共i.e., 33ⱕ eb ⱕ 47兲, suggesting that the majority of the
specimens were nonideal mixtures. Based on Kenney et al.
共1992兲, such nonideal mixtures represent situations where some
of the void spaces between sand particles are not filled with bentonite, but rather are filled only with water. However, unlike the
compacted S-B specimens evaluated by Kenney et al. 共1992兲,
where water was added to the sand and bentonite mixtures during
compaction such that the bentonite likely was not fully hydrated,
the bentonite in this study added in the form of a slurry was fully
hydrated during mixing. In the latter case, the existence of isolated pores containing only water without bentonite is unlikely.
Rather, the more likely scenario is that eb ⬎ eb,f-s represents voids
between individual sand particles that actually are filled with bentonite in suspension.
For example, values of eb based on the mass of bentonite
suspended in water, eb,susp, can be calculated as a function of the
bentonite content of the suspension, BCsusp, and the specific gravity of the bentonite, as follows:
eb,susp =
共1 − BCsusp兲
Gsb
BCsusp
共3兲
where BCsusp is defined in terms of mass of bentonite per total
mass of the suspension 共i.e., water plus bentonite兲. Values of
eb,susp calculated in accordance with Eq. 共3兲 are based on the
assumption that the bentonite particles are fully suspended in
water, such that there is no settling of bentonite. Although the
definition of BCsusp given by Eq. 共3兲 is the same as that previously
defined for the bentonite content of the slurry, BCs, different designations are used here to distinguish the bentonite suspended in
the voids of the postmixed and permeated S-B specimens 共BCsusp兲
from that suspended in the prepared bentonite slurries used in the
mixing process 共BCs兲.
Based on the assumption that BCsusp is the same as BCs for the
bentonite used in this study 共i.e., Gsb = 2.75兲, the limiting values of
eb,susp according to Eq. 共3兲 are 52.3, 43.1, and 36.5 for values of
BCs 共=BCsusp兲 of 0.05 共5%兲, 0.06 共6%兲, and 0.07 共7%兲, respectively. Thus, in the case where bentonite can exist in suspension
within 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 based
on BCs 共=BCsusp兲. This upper limit on eb of 52.3 encompasses an
additional 3 calculated eb values for the bentonite slurry mixed
sand specimens evaluated in this study, such that 9 of the 19
calculated eb values were less than 52.3. Nonetheless, 10 of the
calculated eb values still fell within the range 52.3⬍ eb ⱕ 127.
Thus, the question remaining to be answered is how are such high
eb values possible?
One possible explanation, i.e., aside from experimental error,
is that the injected bentonite slurry was diluted after mixing and
permeation such that BCsusp ⬍ BCs. For example, dilution of a 5%
bentonite slurry to a 2.1% bentonite suspension within the mixed
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Fig. 9. Comparison of void ratio of bentonite versus measured hydraulic conductivity for sand-bentonite 共S-B兲 slurry mixtures from this study
versus that from Kenney et al. 共1992兲 for compacted bentonite and compacted S-B mixtures. 共Note: values for k from this study represent
geometric means of measured values, kgm; eb,f-s = free-swell void ratio.兲
specimen would result in an eb,susp of 128 based on Eq. 共3兲. Indeed, the calculated values of BCsusp based on the measured
masses of bentonite and volumes of water for all 10 permeated
specimens where 52.3⬍ eb ⱕ 127 ranged from 2.1 to 4.3%, indicating that some dilution of the bentonite slurry had occurred
during permeation. Such dilution conceivably could occur by piping of the bentonite from the specimen during permeation and/or
by the addition of permeation water to initially unsaturated specimens. However, the relatively low hydraulic gradients employed
during permeation 共i ⱕ 5兲 suggest that piping was not likely, and
no evidence of piping was observed during permeation 共e.g.,
cloudiness in the outflow兲. Although the actual degrees of saturation of the postmixed specimens prior to permeation were unknown, the results of separate mixing tests reported by
Castelbaum 共2007兲 based on the same equipment, materials, and
methods as used in this study indicate that the postmixed degrees
of saturation for 7 of the 10 column specimens with 52.3⬍ eb
ⱕ 127 ranged from 50 to 98%. Thus, the calculated values of eb
ranging from 52.3 to 127 likely can be attributed, in part, to
bentonite existing within a diluted suspension between individual
sand particles within the mixed and permeated specimens.
Although the specimens for this study were prepared by mixing dry sand with bentonite slurries as opposed to compacting
wetted mixtures of bentonite and sand, the potential use of eb to
estimate the k of the bentonite slurry mixed sand specimens in
this study also was evaluated. The results of the evaluation are
shown in Fig. 9, where measured k is plotted versus eb for both
the compacted bentonite and compacted S-B mixtures from Kenney et al. 共1992兲 as well as for the bentonite slurry mixed sand
specimens 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 in
Group I are characterized by values of eb 共42.5ⱕ eb ⱕ 66.7兲 and k
共2.4⫻ 10−7 cm/ s ⱕ k ⱕ 1.3⫻ 10−6 cm/ s兲 that are lower than the
values of eb 共73.7ⱕ eb ⱕ 127兲 and k 共1.1⫻ 10−5 cm/ s ⱕ k ⱕ 6.8
⫻ 10−4 cm/ s兲 for the data in Group II. Thus, the calculated eb
values for the data in Group I range from slightly lower 共0.90⫻兲
to somewhat higher 共1.4⫻兲 than the upper limit of eb that can be
expected for ideal mixtures 共i.e., eb,f-s = 47兲, whereas the calculated values of eb in Group II range from 1.6 to as much as 2.7
times the upper limit in eb values expected for ideal mixtures.
However, the distinction in the two groups of data from this
study shown in Fig. 9 is more apparent in terms of k. For example, the lowest k value in Group II of 6.8⫻ 10−4 cm/ s is 523
times, or 2.72 orders of magnitude, higher than the highest k
value of 1.3⫻ 10−6 in Group I. This significant contrast in the
range of k values for the two groups of data suggests significantly
different behavior in terms of the role of the bentonite in controlling the k of the bentonite slurry mixed sand specimens. For the
1952 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / DECEMBER 2009
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10
-3
(a)
S-B Mixtures w/Fine Sand
S-B Mixtures w/Coarse Sand
S-B Mixtures w/Sand Blend
m
calc
10-7
calc
10-10
-7
-6
10
10
-5
10
10
-4
10
-3
10
calc
Calculated Hydraulic Conductivity, k
10-10
-1
10-9
10-8
10-7
10-6
10-5
-2
calc
m
10-3
10-3
10-5
calc
10-6
10-5
10-7
m
calc
k = 0.1k
m
-5
10
-6
10
-7
10
-8
10
10
-9
Trend curve
10
-6
10
-7
1
2
3
4
5
6
7
8
9
Fig. 11. Geometric mean of measured hydraulic conductivity values
versus bentonite content for sand-bentonite 共S-B兲 slurry mixtures
from this study
(m/s)
10-4
k = 10k
10
Bentonite Content, BC (%)
10-4
calc
-6
-5
0
-8
10
10-7
10-9
m
10
Measured Hydraulic Conductivity, k (m/s)
k =k
10
10
(b)
?
10-3
-4
S-B Mixtures w/Fine Sand
S-B Mixtures w/Coarse Sand
S-B Mixtures w/Sand Blend
?
(cm/s)
10-4
S-B Mixtures w/Fine Sand
S-B Mixtures w/Coarse Sand
S-B Mixtures w/Sand Blend
10
-1
10
10-2
-3
(m/s)
10-9
10
gm
10-7
(m/s)
10-8
m
10-6
-4
bentonite slurry mixtures evaluated in this study 关Fig. 10共a兲兴 as
well as the regression equation based on all the data shown in Fig.
9 关Fig. 10共b兲兴.
Effect of Bentonite Content
The kgm values for the bentonite slurry mixed sand specimens are
plotted versus BC in Fig. 11. The resulting trend in the data
shown in Fig. 11 is similar to the trend in k versus e shown in Fig.
6, due to the linear e-BC relationship 共see Fig. 7兲. That is, low
10-8
10-10
10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1
Calculated Hydraulic Conductivity, k (cm/s)
calc
10
-1
-3
10
-4
10
-5
10
-6
10
-7
10
-8
10
-9
-2
Sand Blend
10
-3
10
-4
e
b
Expected k
> 60
High
< 60
Low
T
T M
10
-5
10
-6
10
-7
T M M
B
B B
TM B
T MB
T MB
TB
T M MB
TB
(m/s)
Geometric Mean Hydraulic Conductivity, k
gm
(cm/s)
Fine Sand
10
gm
mixtures corresponding to the data in Group I, the bentonite is
loosely distributed within the voids between sand particles, but
apparently is sufficiently intact or viscous to offer some resistance
to flow. In contrast, the bentonite for the mixtures corresponding
to the data in Group II apparently is so loosely suspended that the
bentonite offers very little resistance to flow, such that the k of the
mixture approaches that of the clean sand used in the mixture.
The overall trend in k versus eb for the S-B slurry mixtures
shown in Fig. 9 is similar to that for the compacted bentonite and
compacted S-B mixtures from Kenney et al. 共1992兲, although
there is significantly more scatter in the data and the corresponding coefficients of determination, r2, are much lower. Nonetheless, the results shown in Fig. 9 suggest that eb can be used to
provide an order-of-magnitude approximation of k for the S-B
slurry mixtures tested in this study, particularly for those specimens corresponding to eb ⱕ 66.7. For example, as shown in Fig.
10, the measured k values, with one exception, are within
approximately⫾ one order of magnitude of the values of k calculated using the regression equation based on only the sand-
10
Coarse Sand
Geometric Mean Hydraulic Conductivity, k
Fig. 10. Measured hydraulic conductivity values versus calculated
hydraulic conductivity based on regressions shown in Fig. 9 for sandbentonite 共S-B兲 slurry mixtures: 共a兲 regression based on S-B slurry
mixtures from this study; 共b兲 regression based on all data combined
Variable (Column No.)
s
10-5
m
10
BC = 7 % (7)
k = 10k
10-6
Sand Blend
s
k = 0.1k
-3
Fine Sand
BC = 6 % (6)
10-4
10-5
calc
10
Coarse Sand
t
m
-2
Zone 3
BC = 5 % (5)
k =k
10
Zone 2
t
?
10-3
10-4
10
-1
BC = 1 % (4)
10-2
Calculated Hydraulic Conductivity, k
m
10
10
10-8
10-8
Measured Hydraulic Conductivity, k (cm/s)
Zone 1
-3
Sand Blend (3)
10
(m/s)
Coarse Sand (2)
10
calc
-4
Reference (1)
10
-5
(cm/s)
10
-6
gm
10
10
10-1
-7
Geometric Mean Hydraulic Conductivity, k
m
-8
Measured Hydraulic Conductivity, k
Measured Hydraulic Conductivity, k (cm/s)
-9
Geometric Mean Hydraulic Conductivity, k
Calculated Hydraulic Conductivity, k
-10
TB
Specimen Location within Mixed Column
Fig. 12. Geometric mean of measured hydraulic conductivity values
as a function of relative vertical location of specimens within postmixed columns and void ratio of bentonite 共eb兲 共T = top; M
= middle; and B = bottom兲
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values of k 共i.e., k ⱕ 10−6 cm/ s兲 were measured over a wide range
in BC 共i.e., 1.0% ⬍ BC⬍ 8.0%兲 relative to a more narrow range in
BC 共i.e., 0.5% ⬍ BC⬍ 2.0%兲 within which k was higher 共i.e., k
⬎ 10−6 cm/ s兲. Thus, k = f共BC兲 also does not account for the effect
of e on the behavior of the S-B slurry mixtures, in a similar
manner that k = f共e兲 does not account for the effect of BC as
shown in Fig. 6. However, since BC is a more practical design
parameter than eb for soil-mixing applications, extension of Fig.
11 to practical use can be accomplished by defining three zones of
BC values as indicated in Fig. 11. These three zones correspond to
BC⬍ 1% 共Zone 1兲, 1% ⱕ BCⱕ 3% 共Zone 2兲, and BC⬎ 3% 共Zone
3兲.
Within Zone 1, the amount of bentonite in the mixture is insufficient and not uniformly distributed, resulting in the likelihood
of relatively high values of k 共i.e., k ⬎ 10−6 cm/ s兲. Within Zone 3,
the amount of bentonite in the mixture is both sufficient and uniformly distributed, resulting in the likelihood of relatively low
values of k 共i.e., k ⬍ 10−6 cm/ s兲. Zone 2 represents a transition
zone in which the amount of bentonite in the mixture can be
sufficient but not uniformly distributed, or insufficient and uniformly distributed, resulting in the likelihood of k ranging over
several orders of magnitude 共i.e., 10−6 cm/ s ⬍ k ⬍ 10−3 cm/ s兲.
The variability of k that occurs from Zones 1–3 is due, in part, to
additional mixing conditions related to the bentonite content of
the slurry 共BCs兲 and the effects of the slurry injection rate and
confining stress.
The variation in k of each specimen due to the relative vertical
location within the mixed test column is shown in Fig. 12. Generally, particle displacement was greatest near the top of the
mixed test columns where the effective stress was low 共Castelbaum 2007兲. As shown in Fig. 12, all specimens with eb ⬍ 60 共i.e.,
12 of 19 specimens兲 had relatively low values of k 共i.e., k
⬍ 10−6 cm/ s兲. However, four of these specimens were from the
top-most location of the mixed test columns where particle displacement was greatest and a high value of k would be expected.
Therefore, the effects of additional mixing conditions for these
specimens cumulatively resulted in an eb ⬍ 60 and k
⬍ 10−6 cm/ s.
The column with the sand blend 共i.e., Column 3, Table 5兲 had
the highest premixed dry density 共Castelbaum 2007兲, which resulted in a greater increase in effective stress with depth relative
to the columns with the fine and coarse sands 共i.e., Columns 1 and
2, respectively兲. The column with a target bentonite content, BCt,
of 5% 共i.e., Column 5兲 resulted in the greatest volume of required
slurry and highest slurry injection rate relative to the columns
with BCt values of 1% and 3% 共i.e., Columns 4 and 1, respectively兲. The columns where higher slurry bentonite contents, BCs,
were used 共i.e., 6% and 7% for Columns 6 and 7, respectively兲
resulted in decreases in the volume of required slurry and the
slurry injection rate relative to Column 1 involving a BCs of 5%s”.
Thus, these additional mixing conditions for Columns 3, 6, and 7
cumulatively contributed to decreased particle displacement predominantly near the top of the mixed column relative to Column
1 共fine sand and BCs of 5%兲 and Column 2 共sand blend and BCs
of 5%兲, whereas these additional mixing conditions for Column 5
cumulatively contributed to increased particle displacement, predominantly near the top of the mixed column, relative to Column
1 共fine sand and BCt of 3%兲 and Column 8 共fine sand and BCt of
1%兲.
However, the zone within which each specimen is located in
Fig. 12 is based on the measured BC, which must also be considered when evaluating eb and k. For example, the values of BC for
the top specimens of Columns 6 and 7 were less than 3% 共i.e.,
1.98% and 2.34%, respectively兲 and, therefore, the k values for
these specimens were within Zone 2 共i.e., the transition zone兲
shown on Fig. 12. The values of BC for the top specimens for
Columns 3 and 5 were greater than 3% 共i.e., 5.10% and 7.65%,
respectively兲 and, therefore, the k values for these specimens
were within Zone 3 shown in Fig. 12.
Limitations of Study
As previously indicated, the results of this study may be limited
with respect to field application of the ZVI-clay technology in
several ways. First, the mixing performed in this study was confined 共i.e., one-dimensional兲, whereas that in the field is unconfined 共i.e., three-dimensional兲. The greater degree of confinement
in the laboratory likely results in greater vertical expansion upon
mixing than what occurs in the field, although distributions in soil
properties with depth after mixing should be relatively consistent.
Second, the sands used in this study were initially in a dry
condition and without contamination, whereas the soil to be
mixed in the field would be contaminated and may be below the
groundwater table. Interactions between the contaminants and the
bentonite after mixing could result in higher k values than reported herein 关e.g., see Shackelford 共1994兲兴, and the existence of
high degrees of liquid saturation could impact the resulting magnitudes of e and eb after mixing.
Third, the host sands used in this study to represent the subgrade soils were limited to clean sands 共i.e., no fines兲, whereas the
types of contaminated soils to be encountered in the field are
virtually unlimited. In particular, the types and amounts of fines in
the sand are known to significantly affect the k of S-B mixtures,
with decreasing k values and/or decreasing bentonite contents
typically required to achieve a suitably low k value with increasing amount of high plasticity fines in the sand 共e.g., Alston et al.
1997兲.
Finally, the potential effect of scale always is present when
attempting to upscale laboratory results based on relatively small
specimens prepared under highly controlled conditions to larger
scale field scenarios typically subject to somewhat less controlled
conditions 共e.g., Daniel 1984兲. Thus, prudence dictates that an
appropriate measure of consideration be given when extrapolating
the results of this study to the field.
Conclusions
Values of the hydraulic conductivity, k, of specimens from columns containing initially dry sands mixed with bentonite slurries
were measured. The mixed specimens represented a range in void
ratios 共0.672ⱕ e ⱕ 3.94兲 and bentonite contents 共0.61% ⱕ BC
ⱕ 7.65%, by dry weight兲. The measured k values were found to
correlate poorly with the total void ratio 共e兲 of the specimens due
to the complicating effect of the bentonite in the S-B slurry mixtures. However, the measured k values correlated better with the
void ratio of the bentonite 共eb兲, which is consistent with the results of previous studies involving permeation of compacted bentonite and compacted S-B specimens, even though the range in
values for eb for the specimens of the S-B slurry mixtures tested
in this study 共42.5ⱕ eb ⱕ 127兲 was much higher than that previously reported for compacted bentonite or compacted S-B mixtures. The relatively high range in eb values for the specimens of
bentonite slurry mixed sand also was consistent with the relatively large range in measured k values, which are about one to
seven orders of magnitude higher than values of k commonly
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reported for compacted sand-bentonite mixtures, despite similar
bentonite contents.
In terms of BC, the measured k values for the S-B slurry mixtures correlated with three zones of BC. Zone 1 corresponded to
BC⬍ 1% and represented insufficient and nonuniform distribution
of bentonite within the voids of the S-B slurry mixtures and the
likelihood of values of k greater than 10−6 cm/ s. Zone 3 corresponded to BC⬎ 3% and represented sufficient and uniform distribution of bentonite within the voids of the S-B slurry mixtures
and the likelihood of values of k less than 10−6 cm/ s. Zone 2
represented a transitional zone corresponding to 1% ⱕ BCⱕ 3%,
wherein k ranged between 10−6 cm/ s to 10−3 cm/ s. Overall, addition of more than 3% bentonite via slurry injection and mixing
with the sands was successful in reducing the k of the unmixed
sands by as much as four orders of magnitude, from 9.4⫻ 10−3
cm/ s ⱕ k ⱕ 5.4⫻ 10−2 to k ⬍ 1.0⫻ 10−6 cm/ s.
Acknowledgments
This study was part of an ongoing effort toward development of
the ZVI-clay technology for remediation of source zones contaminated with chlorinated solvents. The financial support provided by DuPont for this development is appreciated. The support
and assistance of Dr. Tom Sale of Colorado State University also
is appreciated.
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