Hydraulic Conductivity and Shear Strength of Dune Sand–Bentonite Mixtures

M. K. Gueddouda Department of Civil Engineering, University Amar Teledji Laghouat Algeria [email protected] [email protected]

Md. Lamara Department of Civil Engineering, University Amar Teledji Laghouat Algeria [email protected]

Nabil Aboubaker Department of Civil Engineering, University Aboubaker Belkaid, Tlemcen Algeria [email protected]

Said Taibi Laboratoire Ondes et Milieux Complexes, CNRS FRE 3102, Univ. Le Havre, France [email protected] [email protected]

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 Vol. 13, Bund. H 2

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 Vol. 13, Bund. H 3

• 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% M.O % Organic Matter 95,87 0,91 0,36 2,5 ------

• 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). Vol. 13, Bund. H 4

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. PI Ac = ...... (1) C 2 − n

C2: Percentage of grains less than 2 µm; n = 5 for natural soils; n =10 for reconstituted soil

Vol. 13, Bund. H 5

100

60 Dune Sand Bentonite

Passing proportions weight (%) by

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) 2 C2 < 2 µm Φ ≤ 80µm LL PL PI Ac SP (m /g) Cu Cc ES (%) % % % % % B 60 85 140,6 47,5 93,5 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, Vol. 13, Bund. H 6

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%. 40 LL PL 30

Liquid Limit (LL), Plastic Limit (PL) (%) (PL) Limit Plastic (LL), Limit Liquid 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:

∆H G(%) = × 100...... (2) H

∆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 Vol. 13, Bund. H 7

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.

3% B + 97 % S 10 5% B + 95 % S 10% B + 90 % S 8 12% B + 88 % S 15% B + 85 % S

%) (G Swelling 6

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 (%) LL PL PI G Pg (kN/m3) % % % % (kPa) 100 0 19,4 9,7 ------

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 2 2 [m /kN] and consolidation cv [m /s] deduced from compressibility and consolidation curves respectively, are used to obtain the conductivity coefficient. A specimen of 50 mm in diameter Vol. 13, Bund. H 8

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)

3 γw: unit weight of water [9.8 kN/m ]

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)).

- Olson (1986) has shown that the estimated permeability values are always less than the measured one.

0,1 400 kPa 0% B+100% S 0,01 3% B+97% S 5% B+95% S 1E-3 10% B+90% S 12% B+88% S 1E-4 15% B+85% S% 1E-5 -6

k =10 cm/s 1E-6 sat

hydraulic conductivity (cm/s) k 1E-7 k =10-8 cm/s 1E-8 sat

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

Vol. 13, Bund. H 9

0,01

σ (25 kPa) 1E-3 σ (50 kPa) σ (100 kPa) 1E-4 σ (200 kPa) σ (400 kPa) σ (800 kPa) 1E-5

1E-6

1E-7

1E-8 hydraulic conductivity k(cm/s) 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 <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 other study, 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. Vol. 13, Bund. H 10

100

80 Dune Sand Algeria Dune Sand Inde

40 Passing proportions weight (%) by

0 100 10 1 0,1 0,01 1E-3 1E-4 Particle size (mm) Figure 8: Grain size distribution of dune sand (Algeria and India)

0,01 12% B + 88 % S (Algeria) 1E-3 10% B + 90 % S (Algeria) 6% B + 94 % S (Inde) 1E-4 8% B + 92 % S (Inde) σ = 400kPa 1E-5 -6 1E-6 k=10 cm/s

1E-7

hydraulic conductivity, k (cm/s) -8 1E-8 k=10 cm/s

1E-9

1E-10 0 200 400 600 800 1000

normal stress σ (kPa) Figure 9: Hydraulic conductivity vs Bentonite content of S/B mixtures (Algeria, 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: Vol. 13, Bund. H 11

- 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: σ1 =100 kPa, σ2 =200 kPa, σ3 =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:

τ = c + σtgϕ...... (4)

τ : Shear strength (kPa) σ : Normal stress (kPa) ϕ : 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 sand bentonite additions.

300 0 % B+100 % S 300 0 % B+100 % S 3 % B+97 % S 3 % B+97 % S 5 % B+95 % S 5 % B+95 % S 250 10 % B+90 % S 250 10 % B+90 % S 12 % B+88 % S 12 % B+88 % S (kPa) (kPa) τ τ 200 15 % B+85 % S 200 15 % B+85 % S

150 150

100 100 shear strenght shear strenght

50 50

0 0 0 100 200 300 0 100 200 300 normal stress σ (kPa) normal stress σ (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 Vol. 13, Bund. H 12

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

3 γd (kN/m 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 ϕ 43 39 35 31 28 25 (degree) Unsaturated mixture

C (kPa) 13 21 31 48 60 71

ϕ (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 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 µm). 0,01 10

k (saturated) 1E-3 G % 8 1E-4

1E-5 hydraulic Conductivity 6 1E-6

swell G(%) 4 1E-7

hydraulic conductivity k(cm/s) 1E-8 free swell 2 1E-9

1E-10 0

0 2 4 6 8 10 12 14 16 bentonite content B%

Figure 11: Hydraulic conductivity and free swell of S/B mixtures Vol. 13, Bund. H 13

45 1E-10

φ° (unsaturated case) 40 φ° (saturated case) 1E-9 k (cm/s)

(degré) 1E-8 φ 35 friction angle 1E-7 30 1E-6

friction angle 1E-5

20 1E-4

15 hydraulic conductivity k(cm/s) hydraulic conductivity 1E-3

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

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 variations 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 granular soil (dune sand). If additional bentonite is more than 10%, particles of sand would not be any more 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°.

Vol. 13, Bund. H 14

CONCLUSIONS

Several conclusions can be drawn: • The common requirement for compacted soil liners that its hydraulic conductivity should -6 -8 be less than (10 -10 ) 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.

REFERENCES

1. Alston, C., Daniel, D. E. and Devroy, D. J (1997) “Design and construction of sand- Bentonite liner for effluent treatment lagoon, Marathon, Ontario “, Canadian Geotechnical Journal., No. 34, pp. 841-852.

2. Ameta, N.K and Abhay Shivaji Wayal (2008) « Effect of bentonite on permeability of dune sand » EJGE. Vol 13 , Bund. A.

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 Hydraulic Containment Applications”, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 123(5): 438-445.

5. Mitchell, J.K. (1993). “Fundamentals of soil behaviour”, 2ed, John Wiley & Sons, New York.

6. Olson, R.E. and D.E. Daniel (1981) “Measurement of the hydraulic conductivity of fine grained soils.” ASTM special technical publication 746: 18-64.

7. Olson, R.E. (1986) “State of art: Consolidation theory,” Consolidation of soils: testing and evaluation by R. N. Yang and F.C. Townsend (Eds) ASTM, STP 892, 7-70.

8. Parker, R.J., Bateman S., and Williams D. (1993) “Design and management of landfills” Geotechnical management of waste and contamination”, Fell, Phillips & Gerard (eds), Balkema, Rotterdam, pp.209-252. Vol. 13, Bund. H 15

9. Pascal Thériault (2000) “Etude de l'influence des métaux lourds sur la conductivité hydraulique de couches sable/bentonite”. Mémoire pour l'obtention du grade de maître ès sciences (M.Sc.). Canada. Mai 2000. 119p.

10. Seed, H.B., Woodward R.J., and Lundgren R. (1962) - “Prediction of swelling potential for compacted clays”. Journal of the soil Mechanics and foundations division. ASCE, vol 88, N° SM4, 1962, pp.107-131.

11. Serratrice, J.F & B.Soyez, (1996). “ Les essais de gonflements”. Bulletin des laboratoires des ponts et chaussées. 204. Juillet –Août 1996, réf .4082, pp 65-85.

12. Wu, J. Y. et Khera, R. P., (1990),” Properties of a Treated-Bentonite /Sand Mix in Contaminant Environment. Physic-Chemical Aspects of Soil and Related Materials”, ASTM STP 1095, K. B. Hoddinott and R. O. Lamb, Eds., American Society for Testing and Materials, pp. 47-59.