Effectiveness of compacted soil liner as a gas barrier layer in the landfill final cover system

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1 Available online at Waste Management 28 (2008) Effectiveness of compacted soil liner as a gas barrier layer in the landfill final cover system Seheum Moon a, Kyoungphile Nam b, Jae Young Kim b, *, Shim Kyu Hwan b, Moonkyung Chung c a Institute of Technology, Samsung Engineering and Construction, Seohyun-Dong, Bundang-Gu, Sungnam-Si, Gyonggi-Do , Republic of Korea b School of Civil, Urban and Geosystem Engineering, College of Engineering, Seoul National University, Gwanak-Ku, Seoul , Republic of Korea c Principal Research Engineer, Geotechnical Engineering Research Division, Korea Institute of Construction Technology, Kyungki, Republic of Korea Accepted 22 August 2007 Available online 25 October 2007 Abstract A compacted soil liner (CSL) has been widely used as a single barrier layer or a part of composite barrier layer in the landfill final cover system to prevent water infiltration into solid wastes for its acceptable hydraulic permeability. This study was conducted to test whether the CSL was also effective in prohibiting landfill gas emissions. For this purpose, three different compaction methods (i.e., reduced, standard, and modified Proctor methods) were used to prepare the soil specimens, with nitrogen as gas, and with water and heptane as liquid permeants. Measured gas permeability ranged from to cm 2, which was a magnitude of two or three orders greater than hydraulic permeability ( to cm 2 ). The difference between gas and hydraulic permeabilities can be explained by gas slippage, which makes gas more permeable, and by soil water interaction, which impedes water flow and then makes water less permeable. This explanation was also supported by the result that a liquid permeability measured with heptane as a non-polar liquid was similar to the intrinsic gas permeability. The data demonstrate that hydraulic requirement for the CSL is not enough to control the gas emissions from a landfill. Ó 2007 Elsevier Ltd. All rights reserved. 1. Introduction Traditionally, landfill management strategies have almost exclusively addressed the problem of groundwater contamination prevention and reduction of leachate generation (Izadi and Stephenson, 1992). Thus the requirements for the barrier layer in a landfill final cover system have been mainly focused on its hydraulic performance. Biodegradation of wastes within landfills produces various gases that consist of primarily methane and carbon dioxide. This biogenerated gas, i.e., LFG (landfill gas) will increase the pressure within landfills and thereby escape from landfills to the atmosphere. Where methane is not controlled, fire and explosions could occur. Concentrations * Corresponding author. Tel.: ; fax: address: jaeykim@snu.ac.kr (J.Y. Kim). above the lower explosive limit of methane have been reported at a distance up to 300 m off-site (Izadi and Stephenson, 1992). The Loscoe, UK and Masserano, Italy incidents, which resulted in extensive property damage and loss of lives, show the importance of controlling migration of gas from landfills (Didier et al., 2000; Vangpaisal and Bouazza, 2004). In recent years, landfills have been implicated in greenhouse warming scenarios as significant sources of atmospheric CH 4. In the United States, landfills are the largest source of anthropogenic CH 4 emissions, which have been estimated to be Tg CO 2. This is about 25% of the total US CH 4 emissions (US EPA, 2006). Therefore, the barrier layer in cover systems should have the property of not only preventing the infiltration of precipitation but also of stopping the biogas migration into the atmosphere or the areas surrounding the landfills. A compacted soil liner (CSL) has been widely used as a single barrier layer or as a part of a composite barrier layer X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi: /j.wasman

2 1910 S. Moon et al. / Waste Management 28 (2008) in the final cover system due to its low hydraulic permeability. However, it is questionable whether the compacted soil is also effective in prohibiting gas emissions from landfill sites. In this study, gas and liquid permeability tests were conducted on the clayey soil in order to assess the effectiveness of compacted soil as a gas barrier in the final cover system. 2. Gas flow through porous media The experimentally derived equation of Darcy s law (Eq. (1)) is for the one-dimensional flow of a homogeneous incompressible fluid (Bear, 1979) Q ¼ KA dh ð1þ dl where Q is volumetric flowrate (L 3 T 1 ), K is hydraulic conductivity (L T 1 ), A is cross-section area (L 2 ) and dh/ dl is the head gradient causing the flow. The hydraulic conductivity, K, varies with properties of medium and permeant. A more useful concept is that of intrinsic permeability, k, defined as a constant for a given medium material or pore structure regardless of fluid chemistry (Bloomfield and Williams, 1995). The relationship between K and k is K ¼ kqg ð2þ l where k is intrinsic permeability (L 2 ), q is density of the fluid (M L 3 ), g is acceleration due to gravity (L T 2 ) and l is dynamic viscosity of the liquid (M L 1 T 1 ). Combining Eqs. (1) and (2), the following generalized equation can be obtained: Q ¼ k l A dp ð3þ dl where dp/dl is pressure gradient. The volumetric flowrate, Q, is not usually constant in the porous media due to the compressibility of gas and the pressure gradient. Therefore, Eq. (3) is valid only under constant pressure. Eq. (4) can be derived from Eq. (3) as follows: PQdl ¼ k AP dp l ð4þ At constant T (temperature), Eq. (5) can be obtained by integrating Eq. (4), assuming that P A is inlet pressure, P B is outlet pressure, and L is length of sample P B Q B ¼ k A P 2 A P 2 B ð5þ l 2L For liquid, the flow velocity at bounding solid walls is taken to be zero. When the permeating fluid is a gas, however, the gas flow velocity at solid walls cannot in general be taken to be zero. This phenomenon is known as gas slippage (Klinkenberg, 1941). Because of gas slippage, measured gas permeability of porous media is always greater than its intrinsic gas permeability. Klinkenberg (1941) considered porous media to be represented by a bundle of capillaries, and he obtained the following relationships between gas permeability and intrinsic permeability k g ¼ k i;g 1 þ 4ck ð6þ r where k g is gas permeability (slip enhanced gas permeability), k i,g is intrinsic gas permeability (slip corrected gas permeability), c is constant (approximately 1), k is gas mean free path, and r is capillary radius. 3. Materials and methods 3.1. Soil properties Soil used in this study was obtained from the Kimpo landfill site, Korea. The soil was classified as CL according to the Unified Soil Classification System (USCS), and its geotechnical properties were summarized in Table 1. The obtained soil was air dried and pulverized. The crushed soils were then passed through a No. 4 sieve (4.75 mm in diameter) and mixed with distilled water to the desired moisture content. The moistened soils were sealed in plastic bags and allowed to hydrate for 1 day prior to compaction. Compaction tests were conducted with soil specimens by using modified (ASTM D 1557), standard (ASTM D 698), and reduced Proctor methods. The reduced Proctor method was applied following procedures described in ASTM D 698, except that 15 blows were applied per layer instead of 25 blows (Daniel and Benson, 1990) Gas permeability test Prior to conducting the gas permeability test, the compacted specimens were dried at room temperature. Once the weight of the specimen ceased changing, it was ovendried at 55 C for approximately 1 day. The residual moisture contents of the oven-dried specimens were less than 2%. The gas permeability tests were performed with nitrogen gas on the oven-dried soil specimens to obtain gas-saturated permeabilities. An apparatus for the determination of gas permeability is similar to a flexible wall permeameter and is illustrated in Fig. 1. While nitrogen gas was driven through a confined soil specimen, the gas flowrate and pressure difference were measured. By varying flowrate and measuring corresponding pressure difference ranging from 0.5 to 5 kpa, gas permeability can be determined using Eq. (5). Table 1 Geotechnical properties of the tested soil Property Value Specific gravity (g/cm 3 ) 2.67 Liquid limit (%) 39.4 Plasticity index (%) 14.4 Grain size (%) Percent sand ( mm) 12.0 Percent silt ( mm) 77.8 Percent clay (<0.005 mm) 10.2

3 S. Moon et al. / Waste Management 28 (2008) Cell pressure a Inflow Mariotte bottle Soil Gas regulator Constant driving force Influent Outflow Mariotte bottle Manometer Flow controller Flow meter Soil Fig. 1. Schematic diagram of a gas permeameter Rigid wall hydraulic permeability test b Effluent For each compaction method, two specimens were compacted at the desired molding moisture content. One specimen was dried to measure its gas permeability and the other specimen was primarily permeated with water in a rigid-wall, compaction-mold permeameter. The specimen with the mold was placed in between the end plates of a compaction mold permeameter. The top plate included a port for venting gases and another port for inflow. The bottom plate had a centrally located port for outflow. Both inflow and outflow lines were connected to Mariotte bottles to maintain a constant hydraulic gradient of 25 as shown in Fig. 2a. Rigid-wall hydraulic permeability (k w,r ) tests were continued until the hydraulic permeability was steady Flexible wall permeability test Four specimens were compacted by standard Proctor method at each desired molding moisture contents in flexible-wall permeability tests. Fig. 2b shows the schematic diagram of a flexible wall permeameter. One specimen was used to obtain rigid wall hydraulic permeability. Another specimen was extruded from a compaction mold just after compaction and its flexible wall hydraulic permeability (k w,f ) was measured. A comparison of rigid and flexible wall hydraulic permeabilities was investigated to ensure that side wall leakage was not a problem. The other two specimens were dried to determine their gas permeabilities. After determination of gas permeabilities, the oven-dried specimens were resaturated with water and heptane as polar and non-polar permeants under vacuum. Hydraulic and heptane permeability ðk w;f and k H;f Þ tests were performed on the oven-dried specimens to investigate the effect of soil permeant interaction on permeability. 4. Results and discussion 4.1. Effect of compaction condition on gas and hydraulic permeabilities The result of the compaction test is shown in Fig. 3. The optimum moisture content (OMC) of each compaction Tail water reservoir Head water reservoir Soil Cell pressure water Vent Driving pressure Fig. 2. Schematic diagrams of liquid permeameters: (a) a rigid-wall permeameter and (b) a flexible-wall permeameter. Dry unit weight, r d (kn/m 3 ) Reduced Standard Modified Moisture content (%) Fig. 3. Compaction curves of Kimpo clay with modified, standard, reduced Proctor methods. method was 20.8%, 18.5%, and 16.3%, and the dry unit weight of each compaction method was 16.31, and kn/m 3, respectively. Measured gas and rigid-wall hydraulic permeabilities (k g and k w,r ) are illustrated in Fig. 4. In this study, hydraulic permeability varied about one order of magnitude from dry to wet of optimum moisture content for all compactive efforts. Previous research studies reported two or more

4 1912 S. Moon et al. / Waste Management 28 (2008) Gas permeability (cm 2 ) Hydraulic permeability (cm 2 ) 10-7 Modified Proctor Standard Proctor Reduced Proctor Optimum moisture content Molding moisture content (%) Fig. 4. Gas and rigid-wall hydraulic permeabilities of CSL specimens at different MMC (for water at 20 C, cm 2 in hydraulic permeability = 1 cm/s in hydraulic conductivity). orders of magnitude differences in hydraulic permeability over a similar range of moisture contents (Mitchell et al., 1965; Benson and Daniel, 1990; Daniel and Benson, 1990; Wang and Benson, 1995). Gas permeability also varied about one order of magnitude from dry to wet of optimum moisture content as the hydraulic permeability did. For a given compaction method, the gas permeability of dry compaction soil was higher than that of wet compaction soil. Furthermore, lower hydraulic and gas permeabilities were obtained with higher compactive effort. The gas and hydraulic permeabilities of the wet compaction specimens were smaller than the dry compaction specimens even at the same porosity, as shown in Fig. 5. These results could be explained by the soil structure. It is well known that the soil compacted wet of optimum has a dispersed particle structure and smaller pores than that compacted dry of optimum (Mitchell et al., 1965; Benson and Daniel, 1990; Benson et al., 1994) Difference between hydraulic and gas permeabilities The results in this study indicate that gas permeability was about two or three orders of magnitude greater than hydraulic permeability, as shown in Fig. 4. In many research studies, it has been reported that the gas slippage could enhance gas flow through the porous media, and thereby measured gas permeability was greater than liquid permeability (e.g., Klinkenberg, 1941; Heid et al., 1950; Macary, 1999; Bloomfield and Williams, 1995). In general, Hydraulic conductivity (cm/sec) Permeability (cm 2 ) k w,r dry of optimum k g, dry of optimum k w,r wet of optimum k g, wet of optimum Total porosity under optimum Total porosity Fig. 5. The relationship between porosity and permeability of specimens compacted by standard Proctor method. the discrepancies between gas and liquid peremabilities were considerable for low permeable media (Klinkenberg, 1941; Bloomfield and Williams, 1995). In order to assess the effect of gas slippage on gas permeability, pore sizes of tested specimens compacted by standard Proctor method were measured by mercury intrusion capillary porosimetry. The pore sizes of specimens tested ranged from 0.12 to 7.1 lm, with the most abundant pore sizes of 0.6 to 1.5 lm (Fig. 6). Using the actual radius, the mean free path of nitrogen (k, m; Bloomfield and Williams, 1995), and for c equal to 1 (Klinkenberg, 1941) relationship between measured and intrinsic gas permeability was simplified from Eq. (6) as follows: 1:07 k i;g 6 k g 6 4:93 k i;g ð7þ When measured gas permeabilities (k g ) range from to 10 8 cm 2, for example, the estimated intrinsic gas permeabilities (k i,g ) should fall into between and cm 2, and the intrinsic gas permeabilities were about times of magnitude larger than the hydraulic permeabilities measured in this study (Fig. 7). The difference between intrinsic gas permeability and hydraulic permeability is presumably due to the interaction between water molecules and soil matrix (Loll et al., 1999). It is obvious that if a fluid reacts with some constituent of the Specific porosity (cm 3 /cm 3 ) Pore diameter (μm) Fig. 6. Pore diameter distribution obtained from mercury intrusion of specimens compacted by standard Proctor method.

5 S. Moon et al. / Waste Management 28 (2008) Hydraulic permeability (cm 2 ) 1:1 ratio line Range of intrinsic gas permeability Gas permeability (cm 2 ) Liquid permeability (cm 2 ) 1:1 ratio line Range of intrinsic gas permeability water heptane Gas permeability (cm 2 ) Fig. 7. Difference between gas and rigid-wall hydraulic permeabilities of specimens compacted by standard Proctor method. Fig. 8. Comparison between hydraulic, heptane, gas permeability of specimens compacted by standard Proctor method. soil matrix, e.g., if water causes clay to swell, then differences between the permeabilities for different liquids and air can be expected (Klinkenberg, 1941) Flexible-wall permeability test Results of the flexible-wall permeability tests are summarized in Table 2. Prior to flexible-wall permeability tests, rigid-wall hydraulic permeability (k r ) tests were performed in order to assess the side wall leakage problem. Flexiblewall hydraulic permeability (k w,f and k w;f ) tests were conducted on both the non-dried and oven-dried specimens. Table 2 shows that rigid-wall hydraulic permeability (k r ) ranged from to cm 2 ( to cm/s) and flexible-wall hydraulic permeability (k w,f ) ranged from to cm 2 ( to cm/s). The ratio of k w,r to k w,f ranged from 0.68 to Measured hydraulic, heptane and gas permeabilities (k w;f, k H;f and k g ) of oven-dried specimens are illustrated in Fig. 8. The range of k w;f was from to cm 2 ( to cm/s) and was about two or three orders of magnitude smaller than k g. The k w;f was also smaller than k H;f ranging from to cm 2 ( to cm/s) over the similar range of gas permeability. The difference between k w;f and k H;f is presumably due to change in electrical double layer formed by negative surface charge of clay particles. There is a broad agreement that the hydraulic permeabilities of clayey soil are affected by electrical double layer (Broderick and Daniel, 1990). When liquid flows through CSL, the effective pore spaces for a polar fluid like water are presumably less than those for a non-polar fluid like heptane because clay particles having swelling property contain hydrophilic surfaces which may impede water flow. It should be briefly noted that the crack structures induced by desiccation could affect the gas and liquid permeability. In this study, however, hydraulic permeabilities ðk w;fþ of oven-dried specimens were similar to hydraulic permeabilities (k w,f ) of non-dried specimens and significant crack structures were not observed in oven-dried samples. It seemed that significant cracking was not developed during desiccation or that cracking which had been induced by desiccation healed on rewetting. However, even though the significant cracking was not observed on the oven-dried specimen, the micro-cracks can exist. The cracks may affect the gas and heptane permeabilities because they did not heal in contact with gas and heptane. Authors thought that the healing of cracks on rewetting with water was also explained by soil water interaction such as swelling and expanding of diffuse double layer. Table 2 Results of permeability tests on specimens compacted by standard Proctor method using water, heptane and gas as permeants MMC (%) Permeability (cm 2 ) Rigid-wall Flexible-wall S1 S2 S3 S4 k w,r k w,f k g k w;f k g =k w;f k g k H;f k g =k H;f ND 1.28E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E ND: Not determined.

6 1914 S. Moon et al. / Waste Management 28 (2008) Conclusion A series of tests for gas and hydraulic permeabilities was performed with CSL specimens. The results showed that gas permeability was about two or three orders of magnitude greater than hydraulic permeability. The difference between gas and hydraulic permeabilities can be explained by gas slippage, which makes gas more permeable, and by clay water interaction, which impedes water flow and then makes water less permeable. The similarity between intrinsic gas and heptane permeabilities can support that soil water interaction is responsible for the difference between gas and hydraulic permeabilities of the CSL. In many countries, requirements for the barrier layer in a landfill final cover system have been focused on its hydraulic performance, and CSL is widely used as a barrier layer material due to its low hydraulic permeability. When it comes to gas, however, our data demonstrate that the permeability may not be low enough as found in water system. Even though the discrepancy between gas and hydraulic permeabilities was less severe in field conditions because landfill cover is rarely dry, it could be concluded that the hydraulic requirement for the CSL may not be sufficient to control the landfill gas emission. References Bear, J., Hydraulic of Groundwater. McGraw-Hill Inc., New York, NY. Benson, C.H., Daniel, D.E., Influence of clods on hydraulic conductivity of compacted clay. Journal of Geotechnical Engineering 116 (8), Benson, C.H., Zhai, H., Wang, X., Estimating hydraulic conductivity of compacted clay liners. Journal of Geotechnical Engineering 120 (2), Bloomfield, J.P., Williams, A.T., Empirical liquid permeability-gas permeability correlation for use in aquifer properties studies. Quarterly Journal of Engineering Geology 28, S143 S150. Broderick, G.P., Daniel, D.E., Stabilizing compacted clay against chemical attack. Journal of Geotechnical Engineering 116 (10), Daniel, D.E., Benson, C.H., Water content-density criteria for compacted clay. Journal of Geotechnical Engineering 116 (12), Didier, G., Bouazza, A., Cazaux, D., Gas permeability of geosynthetic clay liners. Geotextiles and Geomembranes 18 (2), Heid, J.G., McMahon, J.J., Nielsen, R.F., Yuster, S.T., Study of the permeability of rocks to homogeneous fluids. API Drilling and Production Practice, Izadi, M.T., Stephenson, R.W., Measurement of gas permeability through clay soils. Current Practices in Ground Water and Vadose zone Investigations, ASTM STP 1118, Philadelphia, pp Klinkenberg, L.J., The permeability of porous media to liquid and gases. Drilling Production Practice, Loll, P., Moldrup, P., Schjønning, P., Riely, H., Predicting saturated hydraulic conductivity from air permeability: application in stochastic water infiltration modeling. Water Resources Research 35 (8), Macary, S.M., Conversion of air permeability to liquid permeabilities extracts huge source of information for reservoir studies. Middle East Oil Show and Conference, Bahrain, Mitchell, J.K., Hopper, D.R., Campanella, R.G., Permeability of compacted clay. Journal of the Soil Mechanics and Foundations Division 91 (4), US EPA, Inventory of U.S. Greenhouse gas emissions and sinks: , EPA 430-R , Washington, DC. Vangpaisal, T., Bouazza, A., Gas permeability of partially hydrated geosynthetic clay liners. Journal of Geotechnical and Geoenvironmental Engineering 130 (1), Wang, S., Benson, C.H., Infiltration and saturated hydraulic conductivity of compacted clay. Journal of Geotechnical Engineering 121 (10),