Characterization of compacted sand-bentonite mixtures as landfill barriers in North Cyprus

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1 Characterization of compacted sand-bentonite mixtures as landfill barriers in North Cyprus A. Iravanian & H. Bilsel Eastern Mediterranean University, North Cyprus KEYWORDS: Compacted sand-bentonite, hydro-mechanical properties ABSTRACT: Na-bentonite from a bentonite mine in Yiğitler (Arsos) and uniform washed beach sand from Famagusta coast have been used to investigate the possibility of using the right proportions of these materials as compacted barrier layer to control water ingress and leachate migration in the landfill management in North Cyprus. Behaviour of 33%, 50% and 75% sandnatural bentonite content mixtures was studied including determination of soil-water characteristic curves, compressibility, hydraulic conductivity, and swelling- shrinkage during desiccation. This study concludes that 25% of Na-bentonite and uniform beach sand can be a feasible mixture of alternative materials for waste containment barriers which may be considered in future with the growing danger of sanitary and industrial waste on the island. 1 INTRODUCTION Nowadays with tremendous increase in number of industries which contaminate the environment, a suitable material to be applied in earth dams, landfills or other waste facilities as barrier to water and containment flow is essentially required. The material used as cover or liner should fulfill the requirements such as low hydraulic conductivity and adequate mechanical stability. In arid and semi arid climates, other factors such as tendency of soil to swell and shrink in hydration and dehydration respectively, would also play a perceptible role in choosing an appropriate material to be used as an engineered barrier. Using clayey compound with granular soils to be applied as hydraulic or evapo-transpirative barriers is a relatively new subject in waste management systems. Bentonite, as a fine particle sized soil with its specific properties seems to be an appropriate type of soil to be used as a matrix for granular particles of sand. It provides acceptable resistance to shear stresses, and exhibits less volume changes and is economic enough to be applied in great amounts. These are the ideal characteristics of an appropriate material for liners and covers (Marcial et al., 2006). In recent years sand-bentonite mixtures (also called bentonite enriched) have been used widely in construction of landfills and waste water ponds, which exhibited better results compared to clayey mixtures under the same conditions (Abichou et al., 2000). In this study, compressibility, hydraulic conductivity, swelling-shrinkage, and soil-water characteristic (SWCC) behaviours are presented for compacted samples of bentonite alone, and mixtures of bentonite and sand of different proportions. SWCC s were obtained by a chilled mirror psychrometer (dew point potentiameter, model WP4-T, Decagon Devices) measuring total suction between the ranges kpa with fairly high accuracy. Analysis of test results revealed that adding 25% natural Na- bentonite to uniform sand has beneficial effects on mixture by reducing and moderating the unwanted factors of non-treated bentonite. Although bentonite content of more than 25% was expected to lower the hydraulic conductivity, it yielded undesired effects such as, high 472

2 swell potentials, increase in susceptibility of shrinkage and hence formation of desiccation cracks. Therefore, this study concludes that 25% Na-bentonite and uniform beach sand on the island can be a feasible alternative material of waste containment barriers, which may be considered in future with the growing danger of sanitary and industrial waste on the island. 2 BACKGROUND In municipal waste landfills, compacted clay caps (CCC) are applied to protect the landfill from leakage of surface runoff into the landfill and compacted clay liners (CCL) are used to prevent seepage of contaminated fluid from landfill to the ground water below the landfill. Generally it is accepted that the soil liners should have a hydraulic conductivity of at least 10-9 m/s or less to fulfill the requirements of a hydraulic barrier (Kumar and Yong, 2002; Chalermyanont and Arrykul, 2005). Nevertheless application of clays has always involved the issue of cracking during the desiccation period, therefore instead of pure compacted clays, mixtures of clays with other types of soils are preferred which satisfactorily perform the strength properties and economical requirements (Ameta and Wayal, 2008). Sand and bentonite are completely different types of soils with respect to their grain size distribution, permeability, chemical activity and strength, but when mixed together at right proportions can form an excellent material to be used as engineering barrier against fluids seepage, by having low hydraulic conductivity and yet acceptable shear strength (Farajollahi and Wareham, 1998). Generally for the soils with wide range of grain size, the amount of bentonite used is normally less than 6% on a dry weight basis. However for the uniform-sized sands it varies between 10 to 15% (Kumar and Yong, 2002). Saturated Na-bentonite can preserve a gel form material up to 15 times more than its own volume by absorbing water up to 5 times of its mass (Ameta and Wayal, 2008). In sand-bentonite mixtures when gel forms around the soil particles, the effective size of soil particles increases, which causes increase in void volume, and thus decrease in dry unit weight (Kumar and Yong, 2002), therefore it can be said that the addition of bentonite to sand decreases the maximum dry density of mixture and increases the optimum water content. It also increases the liquid limit and plastic limit and reduces the specific gravity of the mixture. Higher the percentage of bentonite lower will be the hydraulic conductivity. However, as the cost of bentonite is high, to determine the minimum percentage of bentonite necessary to achieve the required properties should be the main task (Bilsel and Iravanian, 2008). 3 MATERIALS AND METHODS This paper presents experimental results attained on naturally recovered Na-smectite clay and its mixtures with 33%, 50% and 75% sand content by dry weight. The testing stage started with determination of the physical properties, and continued with soil suction measurements during desiccation, swelling-shrinkage behavior tests under 7 kpa vertical pressure, and measurement of compressibility characteristics of compacted samples in different proportions of bentonite and sand. SoilVision (1998) software, a knowledge-based system database, was used to fit the models and calculate the fitting parameters of soil-water characteristic curve (SWCC) and shrinkage curves. The materials used in this study are basically Na-smectite, obtained from the bentonite mine in Yiğitler, and poorly graded uniform sand from Silver Beach in North Cyprus. According to the Unified Soil Classification System, grain size data of the sand indicate mean diameter of D 50 = 0.20, uniformity coefficient C u = 1.53, coefficient of curvature of C c = 0.99 and effective diameter D 10 = DISCUSSIONS OF EXPERIMENTAL RESULTS 4.1 One-dimensional swell and compressibility 473

3 Characterization of compacted sand- bentonite proportions as landfill barriers in North Cyprus Iravanian, A., Bilsel, H. To investigate the swelling characteristics of natural Na-smectite clay and its mixtures with sand, one-dimensional swell tests were carried out using oedometers. Consolidation rings of 50 mm inner diameter and of height 14 mm were driven into the compacted sand and Na-smectite clay prepared at optimum water content, and two samples were obtained for each swell testing. The samples were inundated under a low surcharge of 7 kpa and full swell was measured. Specimens with varying sand-bentonite contents were allowed to swell until the increase in free swell with time became marginal. Figure 1 presents the free swell response with time for different mixtures of sandbentonite. The results depict a significant reduction in free swell with respect to the increasing sand content. The compacted mixtures were also subjected to consolidation upon completion of full swell., Natural Bentonite 33% Sand 50% Sand 75% Sand Swell (%) Time (min) Figure 1. Percent swell versus logarithm of time curves. Therefore, swell pressure and compression index could be determined for samples of varying sand and Na-smectite clay contents. Table 1 depicts a summary of the swell and compressibility properties of the tested mixtures, presenting a considerable decrease in the compression and rebound indices, as well as preconsolidation and swell pressures. The results indicate a marked reduction in swelling properties which might cause increase in permeability. Hence, reducing bentonite content might create an ineffective barrier material. Saturated hydraulic conductivity is usually taken as a measure defining effectiveness of barriers. Therefore, consolidation testing program includes indirect determination of hydraulic conductivity values under varying confining pressures. Sand (%) Swell (%) Table 1. Swelling and compressibility parameters Compression Index (C c ) Preconsolidation Pressure (kpa) Swelling Pressure (kpa) Rebound Index (C s ) Barrier layers are expected to block the infiltration. However, in semi-arid and arid areas, macropores are formed upon desiccation, providing pathways for the infiltration of water. Therefore, it is of great importance to predict the hydraulic conductivity of barriers at initial stages of their design (Öberg-Högsta, 2002). It is universally accepted that the hydraulic conductivity of liners for hazardous waste should not exceed 10-9 m/s. In this study, saturated hydraulic conductivity was determined from the consolidation test results under increasing ranges of effective consolidation pressures. The estimated values are presented in Table 2. Based on these indirect measurements, the hydraulic conductivity reaches an acceptable value at a stress level higher than 200 kpa. 474

4 Stress range (kpa) Table 2. Saturated hydraulic conductivity values. Coefficient of saturated hydraulic conductivity (m/s) Natural 33 % 50 % 75 % bentonite Sand Sand Sand E E E E E E E E E E E E E E E E Shrinkage Test Sand-bentonite mixtures are less susceptible to damage by desiccation due to the rigid matrix formed by sand and swelling of bentonite to close the cracks formed. Bentonite reduces the hydraulic conductivity, while sand reduces the bentonite cracking under shrinkage. Samples compacted at optimum water content were saturated in one dimensional swell equipment and drained and allowed to desiccate at room temperature. The confining rings and surcharge loads were kept on the samples during the drying process in order to simulate the natural conditions and also to control the rate of moisture loss. Samples were weighed and average diameter and heights of selected samples were taken every day until they reached their residual water content. Volume change during shrinkage and suction measurements were carried out at varying time intervals to study the void ratio-water content (shrinkage curve) and soil-water characteristics relationships. The void ratio versus water content relationship is the shrinkage curves given in Figure

5 Characterization of compacted sand- bentonite proportions as landfill barriers in North Cyprus Iravanian, A., Bilsel, H., Figure 2. Shrinkage curves of (a) Natural Na-smectite, and (b) Na-smectite-33 % sand, (c) Na-smectite- 50 % sand, (d) Na-smectite-75 % sand. Increase of sand decreased the shrinkage limit, and hence the volume change. The shrinkage curve provides volumetric data for a soil as it dries and therefore allows calculations of volumetric properties. The shrinkage curve must be fit with the hyperbolic equation to allow these calculations to proceed. The model parameters of the hyperbolic fit to the shrinkage data are given in Table 2. The parameter a sh represents the minimum void ratio the dried specimens attained, and the b sh values are the minimum water content values at which volume change commenced. The latter is also referred to as shrinkage limit which decreases with increasing sand percentage. There is a significant decrease in volume change. No cracks are observed in the desiccated specimens of sand-bentonite with 75% sand content. Table 3. Shrinkage parameters of the specimens used. Sand percent a sh b sh c sh Shrinkage Limit Soil- water characteristic curve The compacted sand-bentonite barriers are frequently unsaturated in semi-arid areas. Therefore, soil suction is the key factor influencing hydraulic properties, volume change and strength. Hydraulic properties consist of soil water characteristic curve (SWCC), and hydraulic conductivity function. SWCC is a measure of water storage capacity of soil for a given soil suction. It describes the relationship between the volumetric water content,, or the gravimetric water content, w, and the matric suction, m (u a -u w ) or the total suction (that is matric plus osmotic suction), t. It has a similar role as the consolidation curve in saturated soil mechanics, and controls the behavior of hydraulic conductivity, shear strength and volume change at different suctions during wetting and drying processes. Therefore, SWCC can be considered as one of the most fundamental hydraulic characteristics of unsaturated soils. Water content of a soil decreases as suction increases following a drying path (desorption). On the other hand, water content increases when suction decreases following a wetting path (adsorption). For engineering practice, however, a single valued function, usually the desorption curve, is used in characterizing the hydraulic properties of unsaturated soils. The drying curve has a breaking point corresponding to the matric suction when the soil starts to desaturate, called the air-entry value 476

6 (AEV), and is identified as the suction at which air enters the largest pores of the soil (Fredlund and Rahardjo 1993, Rahardjo and Leong 1997). In order to predict the performance of sand-bentonite barriers, it is essential to determine the suction characteristics. Sand- bentonite mixtures develop very large suctions which cannot be tested by conventional methods, such as axis translation and osmotic techniques. In this study a chilled mirror potentiameter device was used to measure total soil suction. This equipment was chosen because of its practicality in giving quick response, and repeatability of the test results with high accuracy. Many other methods for measuring total suction are available such as filter paper and psychrometer methods but assessment made by Agus & Schanz (2005) showed that the chilledmirror potentiameter gives the most accurate results. This device which also has soil science and agricultural usage has the ability of measuring suctions between ranges kpa and higher within 10 minutes in fairly high accuracy. The dew point potentiometer (Model WP4 T, Decagon Devices, Inc., Pullman, W A USA) used in this research, determines total suction by measuring the dew point temperature of the head space above sample. It is done by cooling a mirror, the reflectance of which is carefully monitored by an optical sensor. As the mirror reaches the dew point it reflects changes and the device measures the temperature at which the first drop of dew was condensed on mirror. Using this temperature, the device calculates the suction of sample indirectly. Figure 3 depicts the soil-water characteristic curves of different mixtures of sand-bentonite with Fredlund and Xing (1994) models fitted by the use of SoilVision (1998) software. Figure 3. Soil-water characteristic curves for different sand-bentonite proportions. The hydraulic conductivity is not a constant in unsaturated soils but is a function of soil suction. Thus, it is essential to determine the SWCC and the hydraulic conductivity function for simulating transient seepage in unsaturated soils (Ng and Pang, 2000). Predictive models for the unsaturated hydraulic conductivity are used in this study to predict the unsaturated hydraulic conductivity of the material at degrees of saturation lower than one. The unsaturated hydraulic conductivity versus soil suction curves predicted from Fredlund and Xing (1994) model are presented in Figure 4. The 75% sand sample reaches the lowest hydraulic conductivity value faster during desiccation. 477

7 Characterization of compacted sand- bentonite proportions as landfill barriers in North Cyprus Iravanian, A., Bilsel, H., Unsaturated hydraulic conductivity, m/s 8.00E E E E E E E E-09 Natural Bentonite 33% Sand 50% Sand 75% Sand 0.00E Suction, kpa, Figure 4. Predicted unsaturated hydraulic conductivity by Fredlund and Xing (1994) model. Table 4 presents Fredlund and Xing (1994) model parameters of SWCC from SoilVision program. The air entry value obtained by Fredlund and Xing (1994) model decreases with the increasing percentage of sand in the mixtures, which is rather significant for 75% sand. As the percent sand in mixtures gets higher, the amount of water held in saturated condition, w s, reduces. The reduction in the slope of soil-water characteristic curves is due to reduction in the rate of drying, which is an indication of reduced unsaturated hydraulic conductivity. Residual water content, w r, is the maximum gravimetric water content, at which the water capacity (the rate of change of gravimetric water content with respect to matric suction) approaches zero and the unsaturated hydraulic conductivity becomes zero, which is observed to decease with increasing sand content. Table 4. Fredlund and Xing model parameters. Sand % Residual water content % AEV (kpa) CONCLUSIONS This study presents the findings of a research program to assess the most suitable barrier material for waste containment facilities in North Cyprus, where semi-arid climatic conditions prevail. Uniform sand and natural bentonite (Na-smectite) were chosen, which are local materials abundantly found. The results of the experimental program indicate that bentonite with 75 % sand significantly reduces the volume change upon drying, forming a uniform texture with no desiccation cracks. Studying the soil-water characteristic curves, it is observed that, while the air-entry value decreases with increasing sand content, the slope also reduces indicating a reduction in the unsaturated hydraulic conductivity function with respect to suction. Consolidation tests on saturated specimens of increasing sand content yielded reductions in compression and rebound indices as well as swell pressures. So no detrimental effects are expected to occur when mainly the liner of the barrier systems are loaded by waste, nor when both cover and liner swell due to wetting. Saturated hydraulic conductivity determined from consolidation test results yielded values in the order of 10-9 m/s for all mixtures within kpa stress range. However, the predicted unsaturated hydraulic conductivity curves show a reduction in unsaturated hydraulic conductivity as suction increases and 75% sand samples reaches the lowest hydraulic conductivity at a much lower suction value. Based on these results it is anticipated that the naturally recovered bentonite and the uniform beach sand can be efficiently utilized as a barrier material in a semi-arid climate. This study 478

8 concludes that 25% of Na-bentonite and uniform beach sand on the island can be a feasible alternative material of waste containment barriers, which may be considered in future with the growing danger of sanitary and industrial waste on the island. REFERENCES Abichou, T., Benson, C., and Edil, T Foundry Green Sands as Hydraulic Barriers: Field Study, J. of Geotech. and Geoenvironmental Eng., ASCE, Vol. 126(12), pp Agus, S.S., and Schanz, T Swelling pressure and total suction of compacted bentonite-sand mixtures, Proceedings of International Conference on Problematic Soils, May 2005.Eastern Mediterranean University, Famagusta, N. Cyprus. Vol. 1, pp Ameta, N.K., and Wayal, A.S Effect of Bentonite on Permeability of Dune Sand, EJGE, Vol. 13, Bund. A. Bilsel, H., Iravanian, A Hydro-mechanical properties of compacted sand-bentonite mixtures in a semi arid climate, Unsaturated Soils Advances in Geo-Engineering, proceedings of the First European Conference on Unsaturated soils, Durham, UK, pp Chalermyanont, T., Arrykul, S Compacted sand-bentonite mixtures for hydraulic containment liners, Songklanakarin J. Sci. Technol., 27(2), pp Das, B.M Principles of Geotechnical Engineering, second edition, PWS-KENT Publishing Company, pp Farajollahi, A., Wareham, D.G Predicting Hydraulic Conductivity of Bentonite-Sand Mixtures using a Minicompaction Apparatus. Retrieved June 2008 from Fredlund, D.G., and Rahardjo, H Soil Mechanics for Unsaturated Soils, New York: John Wiley and Sons, Inc. Fredlund, D.G., Xing, A Equations for the soil-water characteristic curve, Canadian Geotechnical Journal, Vol. 31(3), pp Kumar, S., Yong, W.L Effect of Bentonite on Compacted Clay Landfill Barriers, J. Soil and Sediment Contamination, 11(1): Leong, E.C., Tripathy, S. and Rahardjo, H Total suction measurement of unsaturated soils with a device using the chilled-mirror hygrometer technique, Geotechnique, 53(2), pp Marcial, D., Delage, P., Cui, Y.J A Laboratory Study of the Self Sealing Behavior of a Compacted Sand-Bentonite Mixture, J. Geomechanics and Geoengineering, Volume 1, Issue 1 March 2006, pp Ng, C.W.W., Pang, Y.W "Influence of stress state on soil-water characteristics and slope stability." J. Geotech. and Geoenviron. Eng.,Vol. 126(2), pp Rahardjo, H., Leong, E.C Soil-water Characteristic Curves and Flux Boundary Models, Unsaturated Soil Engineering practice: Geotechnical Special Publication No. 68, Geo Institute, ASCE, Utah. Sällfors, G., Öberg-Högsta, A.L Determination of hydraulic conductivity of sand-bentonite mixtures for engineering purposes, Geotechnical and Geological Engineering, Vol. 20, Number 1, pp (16). SoilVision Systems Ltd User s Guide- A Knowledge Based System for Soil Properties, Version 2.0, Saskatoon, Canada. 479