3000
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
incompatible with conventional GCLs. Although higher CaCl
2
concentrations resulted in
k
c
>
k
w
for HC2 and MSB, the
increases were lower than those for the GCL bentonites. The
BPN specimens exhibited similar increases in
k
c
relative to
k
w
for 5-50 mM CaCl
2
, but exhibited
k
c
<
k
w
for 500 mM CaCl
2
.
The relationship between
k
and swell index,
SI
(ASTM D
5890), in the same solution (DIW or CaCl
2
) for the bentonites in
Fig. 3a is illustrated in Fig. 3b. The treated bentonites generally
exhibited lower
k
for a given
SI
than the GCL bentonites,
including MSB and HC2, which exhibit superior hydraulic
behavior because of the activation of osmotic swelling (Di
Emidio et al. 2011). However, the BPN exhibited both a low
k
c
and a low
SI
(8 mL/2 g) in 500 mM CaCl
2
, illustrating that the
hydraulic behavior of BPN was decoupled from swell. The BPN
exhibited higher swelling in DIW relative to the other
bentonites (see Table 1) due to the presence of the super-
swelling polymer (Scalia 2012). However, polymer swelling
does not account for the low
k
c
of BPN permeated with 500 mM
CaCl
2
, given the
SI
of only 8 mL/2 g. This atypical behavior of
BPN illustrates that
SI
is not necessarily an accurate indicator of
hydraulic conductivity for chemically modified bentonites.
10
-13
10
-12
10
-11
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
0.1
1
10
100 1000
(a)
GCL
BPN
HC2
MSB
Hydraulic Conductivity,
k
w
or
k
c
(m/s)
CaCl
2
Concentration (mM)
10
-13
10
-12
10
-11
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
0 10 20 30 40 50 60 70 80
(b)
GCL
BPN
HC2
MSB
Hydraulic Conductivity,
k
w
or
k
c
(m/s)
Swell Index,
SI
(mL/2 g)
Low
SI
and Low
k
Figure 3. Hydraulic conductivity of bentonite specimens as a function
of (a) CaCl
2
concentration in the permeant liquid, and (b) swell index in
the same solution (GCL data from Jo et al. 2001, Jo et al. 2005, Lee and
Shackelford 2005, Lee et al. 2005; MSB data from Lin and Benson
2000, Katsumi et al. 2008; HC2 data from Di Emidio et al. 2011; BPN
data from Scalia 2012).
4 CONCLUSIONS
The hydraulic conductivity to water (
k
w
) and CaCl
2
solutions
(
k
c
) of three novel (chemically modified) bentonites (BPN, HC,
MSB) were compared with those of three natural Na-bentonites
commonly used in geoenvironmental containment applications.
The overall hydraulic performance is a function of not only the
magnitude of chemical resistance (
k
c
/
k
w
) but also the baseline
value of
k
w
. In terms of SB backfills, the use of BPN or HC8
generally exhibited lower
k
w
compared to MSB or Na-
bentonites, implying that less BPN or HC8 is required to create
backfills with an acceptable
k
w
. However, the backfill
containing 5.5 % BPN exhibited the highest
k
c
/
k
w
, whereas the
backfill containing 5.6 % MSB indicated low
k
w
and a
k
c
/
k
w
that
was lower than that for the 5.5 % BPN backfill. Overall, the
product of
k
w
and
k
c
/
k
w
for the backfills containing 5.5 % BPN
and 5.6 % MSB resulted in superior hydraulic performance in
terms of the providing the lowest values of
k
c
. In terms of
GCLs, all three novel bentonites (BPN, HC, MSB) showed not
only low
k
w
but also far superior resistance to chemical attack
than the natural bentonites. Thus, the potential use of
chemically modified bentonites in applications involving SB
backfills and GCLs is promising. However, the differences in
behaviors among the novel bentonites illustrated in this paper
highlight the need for further research into the specific
mechanisms affecting the performance of such novel bentonites.
5 ACKNOWLEDGEMENTS
Financial support for portions of the work presented herein was
provided by the U.S. National Science Foundation (NSF),
Arlington, VA, under grants CMMI-0757815 and CMMI-
0758334.
The opinions expressed in this paper are solely those
of the authors and are not necessarily consistent with the
policies or opinions of the NSF. The authors also appreciate the
assistance of Mike Donovan and Jerry Darlington of CETCO,
USA. Finally, the authors are grateful to Wyo-Ben, Inc., Hojun
Corp., and American Colloid Company for providing materials.
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