1
Hydraulic Conductivity
and Shear Strength of
Dune Sand - Bentonite Mixtures
M.K. GUEDDOUDA
Department. of civil engineering, university Amar Teledji Laghouat
Algeria
gueddouda_mk@yahoo.fr
M. LAMARA
Department. of civil engineering, university Amar Teledji Laghouat
Algeria
med.lamara@mail.lagh-univ.dz
N. ABOU-BEKR
Department. of civil engineering, university Aboubakr Belkaid , Tlemcen
Algeria
aboubekrnabil@yahoo.fr
S. TAIBI
Laboratoire Ondes et Milieux Complexes, CNRS FRE 3102, Univ.
Le Havre, France
Said.Taibi@univ-lehavre-univ.fr
+
ABSTRACT
Compacted layers of sand-bentonite mixtures have been
proposed and used in a variety of geotechnical structures as
engineered barriers for the enhancement of impervious landfill
liners, cores of zoned earth dams and radioactive waste
repository systems. In the practice we will try to get an
economical mixture that satisfies the hydraulic and mechanical
requirements. The effects of the bentonite additions are reflected
in lower water permeability, and acceptable shear strength. In
order to get an adequate dune sand bentonite mixtures, an
investigation relative to the hydraulic and mechanical behaviour
is carried out in this study for different mixtures. According to
the result obtained, the adequate percentage of bentonite should
be between 12% and 15 %, which result in a hydraulic
conductivity less than 10-6 cm/s, and good shear strength.
KEYWORDS: Dune sand, Bentonite, Hydraulic conductivity,
shear strength, insulation barriers
2
INTRODUCTION
Rapids technological advances and population needs lead to the generation of increasingly
hazardous wastes. The society should face two fundamental issues, the environment protection
and the pollution risk control. One of the actual solutions, for handling these contamination
problems, is by enclosing the wastes in a specific location, using insulated barriers. The
efficiency of these insulated barriers depends largely on their hydraulic and mechanical behaviour
along with their capacities of attenuation and retention of the contaminant. Compacted sandy soils
with small additions of bentonite (bentonite- sand mixture) have been proposed and used as an
adequate material for these insulation layers. In order to be efficient, theses insulation barriers
should fulfil some requirements (Chapuis (1990); Parker and al.1993).
 The typical thickness for these layers, ranges between 15 and 30 cm;
 Permeability at saturated state varies between 10-6 and 10-8 cm/s (Chapuis (1990); Parker and
al.1993);
 Properties of exchange and adsorption should be capable to hold some preferentially
pollutants;
 Physical stability of the material in contact with water;
 A swelling potential that ensure good contact with the host rock and permit the replenishment
of existing cracks or that will develop in the future;
 The sand must also possess some characteristic such grain size distribution, in order to prevent
bentonite leaching from the skeleton and hence ensuring the hydraulic stability of the mixture.
In Algeria, the rapid development of urban areas and the growth of oil industry in the south
regions, begin to generate enormous quantities of hazardous wastes. In order to avoid
groundwater pollution and environment degradation, an insulation barrier using dune sand, which
is an available and economical local material, enhanced by small addition of bentonite is
proposed.
In searching for an adequate mixture, an investigation study is carried out on several dune sand
calcium bentonite mixtures with different percentages of bentonite additions, which varies
between 3% to 15%. The assessment of the hydraulic and mechanical properties for these
mixtures is presented along with a comparison with a similar study on Dune sand from India
enhanced with sodium bentonite (Ameta & Wayal, 2008).
MATERIALS USED FOR STUDY
Bentonite of Maghnia
The term ‘Bentonite’ is now well established, and used to describe a clay material whose major
mineralogical components are formed by the Smectite group. As a result, bentonite is a very
expansive soil. The most important bentonite mines in Algeria are situated in the western region.
The bentonite used in this study is extracted from Maghnia mine (Hammam Boughrara, 600 km
west of the capital Algiers).
Dune Sand
Dune sand is a material available in large quantities within the Algerian south, it is known as
desert sand. The dune sand used is a local material from Laghouat region.
3
Characterization Tests

Mineralogical and chemical analysis
X-ray diffraction is one of the most widely used methods for identification of clay minerals and
studying their crystal structure within the soils. Diffraction test carried out on bentonite, showed
that the predominant clay mineral is the montmorillonite group as shown by the spectre in the
Figure 1, beside it reveals also the presence of quartz, calcite and traces of kaolinite and illite.
Results of chemical analysis showed that Maghnia bentonite is composed mostly of silicate with
17 % of aluminates (Table 1). The ratio of SiO2/Al2O3 around 3,77, which is in agreement with
reported values for bentonite between 2 and 5,5. For dune sand and according to the chemical
analysis (Table 2), the major component is silicate SiO2 (95%).
M: Montmorillonite: MgO.AlO3.5SiO2.H2O
I: Illite: K(Al Fe)2.AlSi3O10(OH)2 H2O
K: Kaolinite: Al2Si2O5(OH)4
C: Calcite: Ca CO3
Q: Quartz: SiO2
I
M
I
MM
I
I
I
M
Q
M
M
Q
M
C CK K
Q
Q
Q
C
C
I
I
K K CC
C
C
M
M
I
M
M
K IK
I
Figure 1: X-ray diffraction analysis of bentonite of Maghnia
Table 1: Chemical composition of Maghnia bentonite
SiO2 %
Al2O3%
Na2O %
CaO %
K2O %
MgO %
Fe2O3 %
P.F%
65,2
17,25
3
5
1,7
3,1
2,1
2,65
Table 2: Chemical composition of the dune sand
SiO2 %
SO3%
Cl %
CaCO3%
M.O %
Organic Matter
95,87
0,91
0,36
2,5
------
4

Scanning electronic microscope (SEM)
The image obtained by SEM with 5000 times magnification (figure 2.a), has been realised on fine
powdered bentonite. It shows that Maghnia bentonite is composed by white sheet like
agglomerates enclosing between them large void space. As visualized by SEM at large scale, the
dune sand is formed by rounded shapes grains with a slightly angularities, (figure 2.b).
a. Bentonite of Maghnia
b.Dune sand
Figure 2: S.E.M of the bentonite of Maghnia and the dune sand of Laghouat

Identification Tests
Bentonite
The grain size distribution of the bentonite is shown in Figure 3. It is very fine clay; about 60%
of particles have a diameter less than 2 µm (table 3). The value of the Plasticity Index indicates
that the bentonite of Maghnia is highly plastic; this is confirmed by large specific surface (Sp) of
462 m2/g. According to the Skempton et al. classification (1953), based on the activity, the
bentonite of Maghnia presents a high percentage of calcite Montmorillonite (Ca+2). (NF P94068). In addition, for Maghnia bentonite, the corrected activity parameter (Seed and al. 1962)
determined using equation (1) exceeds 1,75 which indicates the bentonite is a very active clay.
Ac 
PI
..........................(1)
C2  n
C2: Percentage of grains less than 2 µm
n = 5 for natural soils; n =10 for reconstituted soil
oportions by weight (%)
100
80
60
Dune Sand
Bentonite
5
Figure 3: Grain size distribution of bentonite and dune sand
Dune Sand
The grain size distribution of dune sand is plotted in Figure 3. Values corresponding to uniformity
and curvature coefficients are Cu = 1,67 and Cc=1,1, respectively. Thus, the dune sand is
classified as poorly graded fine sand according to the Unified Soil Classification System (USCS).
The physical characteristics of bentonite and dune sand are summarized in table 3.
Table 3: Tests of identification of bentonite and dune sand
C2 < 2 µm
  80m
B
%
60
S
---
PL
%
47,5
PI
%
93,5
Ac
SP (m2/g)
Cu
Cc
ES (%)
%
85
LL
%
140,6
1,86
462
---
---
---
2
---
---
---
---
1,4
1,67
1,1
86,17
PROPERTIES OF DUNE SAND - BENTONITE
MIXTURES
Dune sand – bentonite mixtures used in this study are: 3% B + 97% S, 5% B + 95% S,
10% B + 90% S, 12% B + 88% S, 15% B + 85% S
(B: Bentonite, S: dune sand).
 Atterberg Limits
In soil mechanics, the fine materials are classified on the basis on Atterberg limits which can
provide information for macroscopic behaviour (Mitchel, 1993). Atterberg limits obtained for
different mixtures are presented in figure (4); mixtures with less than 10% bentonite are non
plastic soils, for percentage of bentonite additions between 10% and 12%, the soils become as
little plastic clayey soils, while with percentage of 15% the soil appears to be a plastic. Evolution
of consistence limits according to percentage of bentonite additions follows a parabolic law
(figure.4). The plastic index ranges from 5,9 to 12,5 when bentonite content varies between 10%
and 15%.
stic Limit (PL) (%)
40
LL
PL
30
6
Figure 4: Relationship between Atterberg Limits and Bentonite content.

Swelling test
Swelling tests are carried out using a classical œdometer. Dimensions of samples are 50 mm in
diameter and 20 mm in height. The test is realised according to the free swelling method
(Serratrice & Soyez, (1996)). The dune sand – bentonite mixture samples are prepared by a static
compaction (velocity of 1 mm/min) for water contents and dry densities corresponding to the
optimum Proctor conditions. The total free swelling (G %) is computed using the following
relation :
H
G (%) 
∆H = Hf - H0
H0: initial height (before swelling).
Hf: final height (after swelling).
H
 100 .......... .......... ......( 2)
Results of free swelling are grouped in table 4. Swelling evolution of S/B mixtures over time is
shown in figure 5. The free swell of the bentonite is approximately 47,5%, while for S/B mixtures
varying between 0, 85% and 8,70% for bentonite content 3 to 15%. As expected the free swell is
proportional with bentonite additions.
For swelling pressure test (Pg), we used the constant volume method (Serratrice & Soyez,
(1996)). Results of swell pressure of S/B mixtures are indicated in table 4. The swelling pressures
of S/B mixtures increases from 17 to 178 kPa for bentonite content 3 to 15%. When bentonite
content addition is more than 10%, swelling pressure is over 100kPa. Results of physical and
mechanical properties of S/B mixtures are presented in table 4.
Swelling (G %)
12
10
8
6
4
2
3% B + 97 % S
5% B + 95 % S
10% B + 90 % S
12% B + 88 % S
15% B + 85 % S
7
Figure 5: Swelling evolution of S/B mixtures versus time
Table 4: physical - mechanical properties of S/B mixtures
%B
d
wopt (%)
100
0
(kN/m3)
19,4
97
3
95
%S

9,7
LL
%
---
PL
%
---
PI
%
---
G
%
---
Pg
(kPa)
---
19,1
10,5
20,7
---
---
0,85
17
5
18,8
11,5
21
---
---
2,22
38
90
10
18,3
12,8
27
21,1
5,9
5,90
124
88
12
17,8
14,0
28
18,2
9,8
7,30
150
85
15
17
15,2
34
21,5 12,5 8,70
178
0
100
12,1
34,0
140,6
47,5 93,1 47,5
852
Hydraulic conductivity
An indirect method for evaluating saturated hydraulic conductivity k are based on results of
oedometer test (Olson and Daniel, 1981). In this method the coefficients of volume change mv
[m2/kN] and consolidation Cv [m2/s] deduced from compressibility and consolidation curves
respectively, are used to obtain the hydraulic conductivity. A specimen of 50 mm in diameter and
20 mm height is placed in metal ring and saturated during 24 hours. The loading pressure was
selected according to a geometric progression i+1 /i =2. The hydraulic conductivity k [cm/s] is
obtained using equation (3). In the present study, the Cv coefficient is evaluated by Taylor’s
approach.
k  Cv  mv  w.......... .......... .......... .......... (3)
w :unit weight of water [kN/m3]
Evolution of saturated hydraulic conductivity of S/B mixture as function of loading pressures is
shown in Figure 6. According to the results obtained we can note that:
- For all soils, the hydraulic conductivity varies inversely with the loading pressures (figure 6);
For examples, the saturated hydraulic conductivity for dune sand with 0% addition of bentonite
varies between 1,1 10-3 to 1, 9 10-5 cm/s; whereas for mixtures of 15 % addition the values range
from to 7,41 10-7 to 4, 58 10-10 cm/s when the loading pressure varies from 25 to 800 kPa..
8
hydraulic conductivity k (cm/s)
- The effect of applied loading pressures on hydraulic conductivity is less significant, once
become more than 400 kPa. Other researchers found these limiting values around 100 kPa, (Wus
and Kheras (1990)) and 200 Pa (Alston et al. (1997)).
- Olson (1986) has shown that the estimated permeability values are always less than the
measured one.
0,1
0% B+100% S
3% B+97% S
5% B+95% S
10% B+90% S
12% B+88% S
15% B+85% S%
400 kPa
0,01
1E-3
1E-4
1E-5
-6
ksat =10 cm/s
1E-6
1E-7
-8
ksat =10 cm/s
1E-8
1E-9
1E-10
0
100 200 300 400 500 600 700 800 900 1000
normal stress  (kPa)
Figure 6: Hydraulic conductivity of s/b mixtures vs normal stress
0,01
 (25 kPa)
 (50 kPa)
 (100 kPa)
 (200 kPa)
 (400 kPa)
 (800 kPa)
hydraulic conductivity k(cm/s)
1E-3
1E-4
1E-5
1E-6
1E-7
1E-8
1E-9
1E-10
0
3
6
9
12
15
bentonite content B(%)
Figure 7: Hydraulic conductivity vs Bentonite content of S/B mixtures
Hydraulic conductivity of the dune sand bentonite mixtures decreases with increasing bentonite
content (figure 7). The hydraulic conductivity for pressure equal to 25 kPa decreases
9
approximately three orders of magnitude when 12% bentonite content or more is used. For high
bentonite content 12%, the saturated hydraulic conductivity is less than 10-6 cm/s.
The target values relative to saturated hydraulic conductivity for containment liners, which should
be between 10-6 and 10-8 cm/s, can be achieved for percentages of bentonite content greater than
10%, with an applied normal pressure over 100 kPa. For percentage of bentonite addition more
than 12%, the hydraulic conductivity is less than 10-6 cm/s under a low vertical pressure (25 kPa).

Comparative study
A similar study on dune sand bentonite mixtures from India is carried out by Ameta & Wayal
(2008), using different percentages of bentonite additions ranging from 2% to 10 %. The index
properties of bentonite suggest that it is a sodium bentonite with a plastic index around 203 %.
Surprisingly the dune sand from India has the same grain size distribution as the dune sand from
Laghouat region (south of Algeria) as shown in figure 8.
The results of hydraulic conductivity obtained for different mixtures by the two studies are
presented in figure 9. From these results, we can see that the fundamental condition of
permeability, relative to isolation barrier (10-6 cm/s< k <10-8 cm/s), is met for mixtures with
percentage of bentonite additions between 6 % and 8 %, whereas in our study these percentage
are between 10 % and 12 %. These discrepancies are attributed to the nature of bentonite used, a
calcium bentonite in the present study but a sodium bentonite for the study conducted by, Ameta
& Wayal (2008).
We can conclude that due to its important swelling behavior, sodium bentonite additions require
less quantities of soil to impervious isolation barriers compared to calcium bentonite.
Passing proportions by weight (%)
100
Dune Sand
Dune Sand
80
Algeria
Inde
60
40
20
0
100
10
1
0,1
0,01
1E-3
1E-4
Particle size (mm)
Figure 8: Grain size distribution of dune sand from (Algeria and India)
hydraulic conductivity, k (cm/s)
10
0,01
12% B + 88 % S (Algeria)
10% B + 90 % S (Algeria)
6% B + 94 % S (Inde)
8% B + 92 % S (Inde)
1E-3
1E-4
 = 400kPa
1E-5
-6
k=10 cm/s
1E-6
1E-7
-8
k=10 cm/s
1E-8
1E-9
1E-10
0
200
400
600
800
1000
normal stress  (kPa)
Figure 9: Hydraulic conductivity vs Bentonite content of S/B mixtures from (-Algeria and India-)

Direct shear strength of sand - Bentonite mixtures
As stated above another important property, for these insulation barriers, is their mechanical
behaviour. In this section the shear strength for different mixtures of dune sand – bentonite is
investigated. The unconsolidated undrained (U.U) direct shear test is used. To ensure
homogenization, soil is mixed with water and warped in plastic bags during 24 hours
(recommended by Gleason and al. (1997) and Daniel (1994)). For all types of mixtures,
specimens are prepared using the static compaction technique at optimum proctor conditions with
sample dimensions are (6 6) cm2 and thickness of 2 cm.
Two cases are considered:
- Saturated S/B mixtures after compaction at optimum proctor conditions, inundated during 24 hrs
under piston weight then sheared.
- Unsaturated S/B mixtures immediately sheared at initial water content correspond to optimum
Proctor (constant water content test).
The normal stresses used in the two cases are: 1 =100 kPa, 2 =200 kPa, 3 =300 kPa. The speed
of shearing is around 0,5 mm/min. Values of the friction angle and the cohesion are obtained
graphically according to the coulomb law:
  c  tg.......... .........( 4)
 : Shear strength (kPa)
 : Normal stress (kPa)
11
 : Friction angle (degrees)
C: Cohesion (kPa)
The results of shear strength parameters of S/B mixtures are reported in Table 5. For both
conditions saturated and unsaturated, the shear strength of sand bentonite mixtures decreases with
bentonite additions figure 10, this loss of strength is more appreciably for percentage less than 10
% while it become smaller for bentonite addition more than 10 %. The friction angle is reduced
progressively; while the cohesion increases with bentonite additions
0 % B+100 % S
3 % B+97 % S
5 % B+95 % S
10 % B+90 % S
12 % B+88 % S
15 % B+85 % S
250
200
0 % B+100 % S
3 % B+97 % S
5 % B+95 % S
10 % B+90 % S
12 % B+88 % S
15 % B+85 % S
300
shear strenght  (kPa)
shear strenght  (kPa)
300
150
100
250
200
150
100
50
50
0
0
0
100
200
0
300
100
200
300
normal stress  (kPa)
normal stress  (kPa)
a) Saturated S/B mixtures
b) unsaturated S/B mixtures
Figure 10: Mohr-coulomb failure envelopes obtained from direct shear tests
of S/B mixtures
Table 5: Mechanical parameters of dune sand -bentonite mixtures in the two
states (saturated and unsaturated)
0%B
3%B
5%B
10%B
12%B
15%B
d (kN/m3
19,4
19,1
18,8
18,3
17,8
17,0
wopt (%)
9,7
10,5
11,5
12,8
14,0
15,2
saturated mixture
C (kPa)
13
18
22
32
41
57
 (degree)
43
39
35
31
28
25
Unsaturated mixture
C (kPa)
13
21
31
48
60
71
 (degree)
43
36
31
24
18
15
According to the results (Table 5), the dune sand presents a low cohesion (13 kPa) and high
friction angle (43 °). The friction angle of S/B mixtures decreases from 43° to 25° (saturated
12
state) and from 43 to 15 ° (unsaturated state) for bentonite content 0 to 15%. The cohesion
increases from 13 to 57 kPa (saturated state) and from 13 to 71 kPa (unsaturated state).
The friction angle is inversely proportional to additional bentonite. The addition of bentonite
leads to increasing the cohesion, because of the high content of fine particles (less than 2 m).
RESULTS AND DISCUSSIONS
Figure 11 shows the correspondence between free swell and saturated hydraulic conductivity
under a low normal stress (25 kPa). The addition of small percentage of bentonite (3%) doesn't
affect the two parameters k and G (%).
The relationship between free swell and bentonite additions (3 to 15 %) seems to be linear. The
swell of mixtures with 12 and 15% bentonite content is 7,3 and 8,7%, respectively. For bentonite
content more than 10%, the hydraulic conductivity decreases. Bentonite adhered on sand particle
surfaces fill up the voids which results in a narrow water flow path and hence a decrease in
hydraulic conductivity. For percentages of bentonite more than 12%, the hydraulic conductivity is
less than 10-6 cm/s.
The variation of hydraulic conductivity and friction angle of S/B mixtures are represented
graphically in the figure 12. For low bentonite content (<5%), the mechanical behaviour is
controlled by pulverulent soil (dune sand). If additional bentonite is more than (10%), particles of
sand would not be anymore in contact and the mechanical behaviour would be controlled then by
the bentonite and its low friction angle. In addition, too much security on bentonite content added
to improve the hydraulic performance (<10-6 cm/s) can create a problem of mechanical stability
of liners. The friction angle and the hydraulic conductivity are proportional with the additional
bentonite. Hydraulic conductivity of the sand bentonite mixtures is related to the swell of the
mixtures. As swell increases, the hydraulic conductivity decreases. Addition bentonite content
more than 10% results in lower shear strength with angle of friction less than 25°.
10
k (saturated)
G%
1E-3
8
1E-4
1E-5
6
hydraulic Conductivity
1E-6
4
1E-7
1E-8
free swell
2
1E-9
1E-10
0
0
2
4
6
8
10
bentonite content B%
12
14
16
swell G(%)
hydraulic conductivity k(cm/s)
0,01
13
Figure 11: Hydraulic conductivity and free swell of S/B mixtures
1E-10
° (unsaturated case)
° (saturated case)
40
1E-9
k (cm/s)
1E-8
friction angle
35
1E-7
30
1E-6
25
1E-5
20
1E-4
15
hydraulic conductivity k(cm/s)
friction angle  (degré)
45
1E-3
hydraulic conductivity
10
0,01
0
2
4
6
8
10
12
14
16
bentonite content (B %)
Figure 12: hydraulic conductivity and friction angle of S/B mixtures
CONCLUSIONS
Several conclusions can be drawn:
- The common requirement for compacted soil liners that its hydraulic conductivity should be
less than (10-6 -10-8) cm/s. This condition is met for compacted dune sand with 12% bentonite
addition at optimum Proctor state;
- The relationship between the hydraulic conductivity and the swell of S/B mixture is well
illustrated. These two properties are inversely proportional;
- The addition of Bentonite leads to the reduction of friction angle and hence the shear strength
of S/B mixtures for both saturated and unsaturated states;
- The sodium bentonite additions require less quantities of soil to impervious isolation barriers
compared to calcium bentonite;
Finally, we can advance that using dune sand, which is a local largely available material in the
south of Algeria; with small quantities of bentonite additions can provide an economical isolation
barrier for waste disposals management.
14
List of symbol
A
C
Cc
Cu
Cv
C2
ES
G
k
LL
M.O
P.F
PI
PL
Sp
Pg
U.U
wopt
d
w
mv



activity
cohesion (kPa)
curvature coefficient
uniformity coefficient
coefficient consolidation (m2/s)
percentage of grains less than 2 µm
sand equivalent (%)
free swell (%)
saturated hydraulic conductivity (cm/s)
liquidity limit
organic Matter
fire loss
plasticity index
plasticity limit
specific surface (m2/g)
swelling pressure (kPa)
unconsolidated undrained
Optimum Proctor water content (%)
Optimum Proctor dry density (kN/m3)
unit weight of water (kN/m3)
coefficient of compressibility (m2/kN)
shear strength (kPa)
normal stress (kPa)
Friction angle (degrees)
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1.
Alston, C., Daniel, D. E. et Devroy,
D. J (1997), “Design and construction of
sand-Bentonite liner for effluent treatment lagoon, Marathon, Ontario “, Canadian
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2.
Ameta N.K and Abhay Shivaji Wayal (2008) « Effect of bentonite on permeability of dune
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3.
Chapuis R.P. (1990). “Sand – Bentonite liners: predicting permeability from laboratory
tests”. Canadian Geotechnical Journal, 27, pp.47-57.
4.
Gleason M.H., Daniel.D.E, and Eykholt, G.R (1997). “Calcium and Sodium Bentonite for
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Engineering, ASCE, 123(5): 438-445.
5.
Mitchell.J.K. (1993). “Fundamentals of soil behaviour”, 2ed, John Wiley & Sons, New York..
6.
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7.
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8.
Parker R.J., Bateman S., and Williams D. (1993) “Design and management of landfills”
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9.
Pascal Thériault, (2000). “Etude de l'influence des métaux lourds sur la conductivité
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