Hydraulic Conductivity and Shear Strength of Dune Sand–Bentonite Mixtures M. K. Gueddouda Department of Civil Engineering, University Amar Teledji Laghouat Algeria
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Md. Lamara Department of Civil Engineering, University Amar Teledji Laghouat Algeria
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Nabil Aboubaker Department of Civil Engineering, University Aboubaker Belkaid, Tlemcen Algeria
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Said Taibi Laboratoire Ondes et Milieux Complexes, CNRS FRE 3102, Univ. Le Havre, France
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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
INTRODUCTION Rapid 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
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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 et 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 and 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 which 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.
Characterization Tests
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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
Figure 1: X-ray diffraction analysis of bentonite of Maghnia
Table 1: Chemical composition of Maghnia bentonite (comma “,” is decimal point) 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 (comma “,” is decimal point)
•
SiO2 %
SO3%
Cl %
CaCO3%
95,87
0,91
0,36
2,5
M.O % Organic Matter ------
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).
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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 P94-068). 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
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Passing proportions by weight (%)
100
80
Dune Sand Bentonite
60
40
20
0 100
10
1
0,1
0,01
1E-3
1E-4
Particle size (mm)
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 (comma “,” is decimal point) Φ ≤ 80µm % 85
LL % 140,6
PL % 47,5
PI % 93,5
Ac
SP (m2/g)
Cu
Cc
ES (%)
B
C2 < 2 µm % 60
1,86
462
---
---
---
S
---
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 (Mitchell, 1993). Atterberg limits obtained for different mixture are presented in Figure (4); mixtures with less than 10% are non plastic soils,
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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 for bentonite content varies between 10% and 15%. Liquid Limit (LL), Plastic Limit (PL) (%)
40 LL PL
30
20
10
0
0
5
10
15
20
Bentonite content B (%) 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 relationship:
G (%) =
∆H H
× 100..........................(2)
∆H = Hf - H0 H0: initial height (before swelling). Hf: final height (after swelling). 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 and 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
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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 3% B + 97 % S 5% B + 95 % S 10% B + 90 % S 12% B + 88 % S 15% B + 85 % S
10 8 6 4 2 0 0,1
1
10
100
1000
10000
Time (min) Figure 5: Swelling evolution of S/B mixtures versus time
Table 4: physical - mechanical properties of S/B mixtures (comma “,” is decimal point) %S
%B
γd
wopt (%) 9,7
LL % ---
PL % ---
PI % ---
G % ---
Pg (kPa) ---
100
0
(kN/m3) 19,4
97
3
19,1
10,5
20,7
---
---
0,85
17
95
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 evaluation saturated hydraulic conductivity k is 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 conductivity coefficient. A specimen of 50 mm in diameter
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and 20 mm height is placed in metal ring and saturated during 24 hrs. The loading pressure was selected according to a geometric progression σi+1 /σi =2. The conductivity coefficient k [cm/s] is obtained using equation (3). In the present study, the Cv coefficient is evaluated by Taylor’s approach.
k = Cv × mv × γw........................................(3) γw: unit weight of water [9.8 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 7); 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. - The effect of applied loading pressures on hydraulic conductivity is less significant, once become more than 400 kPa. Other authors found these limiting values around 100 kPa, (Wus and Kheras (1990)) and 200 Pa (Alston et al. (1997)).
hydraulic conductivity k (cm/s)
- Olson (1986) has shown that the estimated permeability values are always less than the measured one. 0,1 400 kPa
0,01 1E-3 1E-4
0% B+100% S 3% B+97% S 5% B+95% S 10% B+90% S 12% B+88% S 15% B+85% S%
1E-5 ksat =10-6 cm/s
1E-6 1E-7
ksat =10-8 cm/s
1E-8 1E-9 1E-10
0
100 200 300 400 500 600 700 800 900 1000 normal stress σ (kPa)
Figure 6: Hydraulic conductivity vs normal stress of S/B mixtures
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0,01
σ (25 kPa) σ (50 kPa) σ (100 kPa) σ (200 kPa) σ (400 kPa) σ (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. The hydraulic conductivity for pressure equal to 25 kPa decreases 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 200 kPa. For percentage of bentonite addition more than 12%, the hydraulic conductivity is less than 10-6 cm/s under a low vertical pressure (100 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 proprieties 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
