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GEOSYNTHETICS ASIA 2012 5th Asian Regional Conference on Geosynthetics 13 to 16 December 2012 | Bangkok, Thailand
AIR PERMEABILITY OF COMPACTED CLAYS AND GEOSYTHETIC CLAY LINERS
Hsin-Yu Shan 1, Jenn-Tien Yao 2
1 Department of Civil Engineering, National Chiao Tung University, Taiwan; Tel: +886-35131562; Fax:
+886-35716257; Email: [email protected],
2 Fareastern General Construction, Inc., Taiwan; Tel: +886936292783; Email: [email protected] ABSTRACT
Two of most important functions of a landfill cover are to minimize the infiltration of water and to control the emission of landfill gas. Compacted clay liner (CCL), geosynthetic clay liner (GCL), and geomembrane (GM) are the three major types of hydraulic barrier materials used in the bottom lining systems as well as cover systems of landfills. However, it is well recognized that clay liners crack upon desiccation. The cracked liner may enhance the capacity to conduct fluid, which not only increase the infiltration of the downward moving water but also the emission of upward moving gas. Laboratory air permeability tests were conducted to quantify the rate of air passing through desiccated clay liner specimens. In addition, the equilibrium water content of clay liners in the field condition is also studied. The results show that desiccated clay liners may allow considerable amount of landfill gas to pass through. In addition, the air permeability of desiccated GCLs are much higher than that of desiccated CCLs. Accordingly, it is suggested that geomembranes should be used to contain landfill gas for landfills located in areas where landfill gas emission are to be controlled effectively.
Keywords: landfill, clay liner, air permeability, landfill gas emission
INTRODUCTION
In recent years, landfill gas emission has raised considerable concerns since methane is a major greenhouse gas. As a result, for closed landfills, the effectiveness of the cover system to control the emission rate of methane and non-methane organic compounds (NMOCs) needs to be assured. Gas passes through the landfill cover system by means of advection and molecular diffusion. In MSW landfills where large amount of gas is produced, the internal pressure is usually greater than atmospheric pressure such that landfill gas will be released not only by diffusion but also by pressure-driven advection. In the meantime, the natural fluctuation of atmospheric pressure can also cause gas to flow into or out of the landfill. Furthermore, a change in leachate/water table or difference of temperature across the cover system may also lead to gas migration. In many cases, the temperature within the landfill reaches higher than 40°C due to the heat generated by the anaerobic degradation process (Tchobanoglous et al.
1993).
Gas movement by diffusion is driven by gradient of concentration. When a gas is more concentrated in one region of a mixture more than another, it will diffuse into the less concentrated region. Thus the molecules move in response to a partial pressure gradient or concentration gradient of the gas. The present paper will focus only on advective transport.
Figueroa and Stegmann (1991) performed several field tests on a 0.6 m-thick soil cover
(SC-SM) at a German landfill. They found that the landfill gas flow rates ranged from 5.2 x 10-6 to 9.6 x 10-5 m3/m2/s. They suggested that the dominant gas transport mechanism was advection.
In most of the modern landfills, compacted clay liner (CCL), geosynthetic clay liner (GCL), and geomembrane (GM) are the three major types of hydraulic barrier materials used in the bottom lining systems and cover systems.
Many landfills have used compacted clays as hydraulic barrier in the cover system since the hydraulic conductivity of well-constructed CCL can be as low as 1 x 10-10 m/s and can meet the regulatory requirements. However, the major disadvantage of compacted clay liner is that they will crack as a result of desiccation, freeze-thaw cycles, and differential settlement (Koerner and Daniel 1992, Daniel and Koerner 1993). For clay minerals with high swelling potential, the cracks may heal upon rehydration. Kraus et al. (1997), McBrayer et al. (1997), and Day (1998) have looked into the phenomenon of crack-healing of compacted clay. Furthermore, Day (1998) suggested that an important factor in the healing of cracks upon wetting is the type of clay mineral. He stated that for montmorillonite the desiccation cracks are completely healed upon wetting. The hydraulic conductivity of cracked Otay Mesa natural clay specimen decreased from 7 x 10-7 m/s to 3 x 10-10 m/s as a result of healing of cracks.
Geosynthetic clay liners have not only been used in bottom liners for landfills and surface
GEOSYNTHETICS ASIA 2012 5th Asian Regional Conference on Geosynthetics 13 to 16 December 2012 | Bangkok, Thailand
12 impoundments (Schubert 1987; Daniel and Koerner 1991; Trauger 1991, 1992; Clem 1992), but also in final covers for landfills and remediation projects as well (Koerner and Daniel 1992, Daniel and Richardson 1995; Woodward and Well 1995). The main advantages and disadvantages of GCLs have been discussed by Boardman (1993) and Manassero et al. (2000) amongst many others.
Although GCLs are usually installed to limit advection of liquids (e.g., water through a cover system) they may also serve an important role in covers as a gas barrier. Theoretically, hydrated GCLs as well as wet compacted soils should hardly allow any gas to pass through (Daniel, 1991).
Nevertheless, Trauger and Lucas (1995) did measure the rate of methane gas migrating through GCLs via diffusion. Their results show that the rate of gas transport through GCL was very low as long as its water content was greater than 90%. The permeance is about 2 x 10-6 m/s for GCL sample with a water content of around 50% and drops below 1 x 10-9 m/s when water content reached above 90%. This suggests that the gas permeability of GCLs is dependent on the water content.
GCLs are known to have a phenomenal ability to retain moisture such that they might have the potential to be effective barrier to gas migration.
Research on GCLs buried in sands showed that they were able to absorb water from the environment very quickly why buried dry or hardly lose any water when buried saturated (Geoservice, 1989;
Daniel et al., 1993).
With GCLs being increasingly used as part of the capping, their gas performance has come under a growing scrutiny. Recent work has shown that the gas permeability of GCLs is affected by the manufacturing process and the form of bentonite (Didier et al., 2000; Bouazza and Vangpaisal, 2000;
Shan and Yao, 2000; Aubertin et al. 2000;
Vangpaisal and Bouazza, 2001, 2003).
This paper presents a test method developed speci.cally to assess the gas permeability of GCLs. It is based on the method developed by Matyas (1967) for the measurement of air permeability in soils. The testing apparatus has been designed to accommodate the GCL sample and gas. Flowmeters are used to monitor gas outflow from the device. The test method offers the possibility of carrying out gas permeability tests at different pressure gradients and confining stresses.
EXPERIMENTAL PROGRAM
The air permeability of soil depends on factors such as the size and number of cracks, the air porosity (na), and the degree of saturation. Many of these factors are dependent on each other or on some other factors. For example, the degree of saturation
depends on both the water content and the void ratio.
Among these factors, the air permeability of soils is most sensitive to the variation of degree of saturation. On the other hand, the void ratio of a clay liner in the cover system will be almost constant, since effective stress is kept unchanged throughout the service period. Therefore, the degree of saturation solely depends on water content.
Furthermore, the cracking of clay liners is also closely related to the water content. Therefore, in a cover system, the water content is the single most important factor that affects its air permeability of a clay liner. As a result, this research focused on determining the effect of water contents on the air permeability of the clay liners.
Materials
The three clays selected to represent the compacted liners were kaolinite, Hsin-chu clay, and Chung-li clay. The properties of the clays are listed in Table 1.
Table 1 Properties of soils samples.
Soil Finer Note:*Portion Finer than #40 Sieve s
The CCL samples were compacted with 48% of the energy produced by standard Proctor compaction test. The results of the compaction tests are listed in Table 2 and the compaction curves are shown in Fig.
1. The shrinkage limits of the bentonite in the clays were determined according to standard test method ASTM D427-92 and are listed in Table 3. The results of hydraulic conductivity of the CCL specimens are listed in Table 4.
Table 2 Results of compaction tests.
Soil d, max
Note: Specimens were compacted with 48% of Standard Proctor compaction energy.
GEOSYNTHETICS ASIA 2012 5th Asian Regional Conference on Geosynthetics 13 to 16 December 2012 | Bangkok, Thailand
Fig. 1 Compaction curves of clays tested.
Table 3 Shrinkage Limit of Soils.
Soil Shrinkage limit (%)
Table 4 Results of fixed-wall hydraulic conductivity tests.
Soil k (m/s)
Hsin-chu Clay 4.6 × 10-9 Chung-li Clay 1.2 × 10-9 Kaolinite 2.8 × 10-9 Note: Hydraulic gradient = 100.
The two GCLs tested in this study were a needle-punched GCL and a stitch-bond GCL, which will be designated as GCL-A and GCL-B, respectively. GCL-A is comprised of a nonwoven needlepunched geotextile that is needle punched again through a layer of bentonite into a woven slit-film geotextile. The bentonite content is 3.6 kg/m2. The water content of the bentonite in dry GCL-A is about 10 - 12%. In GCL-B, 3.6 kg/m2 of bentonite is sandwiched between woven geotextile on the top and open weave geotextile at the bottom.
The shrinkage limits of the bentonite in the GCLs are listed in Table 5. It is interesting to note that the shrinkage limits of the bentonite are very low comparing to the high water content of saturated bentonite.
Table 5 Results of shrinkage limit tests on bentonite in the GCLs.
GCL Shrinkage limit (%)
Volume change (%) GCL-A 35.2 87.86 GCL-B 29.9 88.15
Water Retention Test
Clay liner specimens were placed under 0.5 m of moist sand and loosely compacted moist clay in two 86-liter plastic buckets separately. The specimens had been allowed to absorb water under dead weights that imposed a vertical stress that is equivalent to 0.5 m of soil before they were put in the buckets. The test was performed over a 90-day period spanning from March to May. The monthly average temperatures were 17.2°C, 21.1°C, and 24.6°C, respectively. The average humidity during the test period was about 85%. The suction in the cover soils was monitored with tensiometers.
The soil water characteristics of the clay liner samples were determined with a 15-bar pressure cell.
The main drainage curves (MDC) of the compacted clay specimens are shown in Fig. 2. The specimens were soaked to enhance saturation before the test.
On the other hand, the specimens of water retention tests were placed in the surrounding soils immediately after they had been cut from the compacted samples. As a result, the water contents of the retention test specimens were a little less than those indicated by the MDC.
Fig. 2 Soil water characteristic curves of compacted clay samples.
Air Permeability Test
The diameter and the height of the CCL specimens were 101.6 mm and 19.5 mm, respectively. The clays were compacted at a water content 2% wet of optimum with 48% of standard Proctor compaction energy. In order for the cracks to develop more easily, it was decided to use shorter CCL specimens. Therefore, the specimens were compacted in a compaction mold of reduced size.
The compacted samples were trimmed and then retrieved from the compaction mold.
The GCL specimens were cut from the rolls supplied by the manufacturer to a diameter of 114.5 mm. The specimen was placed in an acrylic mold with an inner diameter of 114.5 mm. The specimen was then hydrated with tap water for 1 day.
Both CCL and GCL specimens were placed 1
Dry Density (x 103kg/m3)
Water Content (%) Hsin-chu Clay
Volumetric Water Content (%)
Hsin-chu clay Chung-li clay Kaolinite
GEOSYNTHETICS ASIA 2012 5th Asian Regional Conference on Geosynthetics 13 to 16 December 2012 | Bangkok, Thailand
14 inside an oven and heated under a temperature of around 35°C for a given period of time. During the desiccation period, a dead load weighing 4 kg was put on top of each specimen to provide a normal stress equivalent to that created by 300 mm of topsoil. In addition, the CCL specimens were put on top of a sheet of sand paper to prevent them from shrinking as a whole so that cracks could develop.
This process was repeated for different drying times to obtain specimens with various water contents.
The air permeability tests of CCL specimens were performed with flexible-wall permeameters (Fig. 3). During the tests, a low cell pressure of 3.5 kPa (0.5 psi) was applied to ensure good contact between the membrane and the specimen.
Influent Air Confining Pressure
Effluent Air to Flowmeter
Fig. 3 Schematic diagram of the compacted clay air permeability testing system.
Fig. 4 shows the schematic diagram air permeameter for testing the GCL specimens. GCL specimens were clamped between two ring-shape holders. Bentonite paste was placed along the edges of the specimens to prevent air leakage.
Fig. 4 Schematic diagram of the permeameter for measuring air permeability of GCL (Shan and Yao 2000).
For testing of both types of materials, the flow rate of air was adjusted by regulating influent air
pressure with the pressure control panel (Fig. 5).
Very low influent air pressure was used for the tests (less than 2 kPa). The range of flow rate was as high as 27 l/min for the more permeable specimens under larger gradients and as low as 0.5 l/min for less permeable specimens under smaller gradients. The head loss across the specimen was measured by U-tube manometer. For each specimen, head differences corresponding to 5 different flow rates were measured. The linear relationship between flow rate and gradient justified that the gas flow was in the laminar range. The test results of one of the compacted Chung-li clay specimen are shown in Fig.
6 as an example. After each test, the water content and the dimensions of specimen were measured.
Flowmeter
Air Pressure Control Panel
U-Tube Manometer
Fig. 5 Schematic diagram of the gcl air permeability testing system (Shan and Yao 2000).
Fig. 6 Relationship between flow rate and gradient of air permeability tests.
The air permeability of the specimens was computed with the following equation:
where htotal is the total head loss measured (mm);
hequipment is the head loss of system without specimen in it (mm); k is the air permeability (mm/s).
y = 8E-05x
GEOSYNTHETICS ASIA 2012 5th Asian Regional Conference on Geosynthetics 13 to 16 December 2012 | Bangkok, Thailand
The compressibility of air has been taken into account when computing the flow rates that passed through the specimens from the values measured with the flowmeter.
RESULTS AND DISCUSSION Water Retention Tests
Results of the water retention tests show that the hydrated GCLs did not have a strong ability to retain water. The variations of the water content of GCL specimens with time are shown in Fig. 7. The final water content of GCL-A and GCL-B buried in sand are 48% and 53%, respectively. The final water content of GCL-A and GCL-B buried in clay are 27% and 28% which are lower than the shrinkage limit of bentonite. The water contents of the specimens at the end of the tests were much lower than those reported by Geoservice (1989). On the other hand, the final water contents of the specimens are comparable to the results of absorption tests performed by Daniel et al. (1993).
Fig. 7 Variation of water content of CCL and GCL specimens with time.
The CCL specimens also lost considerable amount of water during the test period. The difference between final water contents of specimens buried in sand and clay is most significant for kaolinite. On the other hand, only a small difference in water content was measured for Chung-li clay. In addition, the final water contents of kaolinite and Hsin-chu clay were all lower than their shrinkage limits. By comparing the results with the index properties, it can be concluded that clays with higher plastic limit are capable of retaining more water.
Air Permeability Tests on CCLs
The relationship between air permeability and water content of CCLs are depicted in Fig. 8(a), Fig.
9 (a), and Fig. 10 (a). The air permeability of the compacted clay specimens shows a slight increase as the water content decreases. The trend is most obvious for kaolinite.
(a) Relationship between air permeability and water content
(b) Relationship between air permeability and volume change
(c) Relationship between volume change and water content
Fig. 8 Relationship between air permeability, water content, and shrinkage of compacted hsin-chu clay.
Among three CCLs, kaolinite has the lowest air permeability while Hsin-chu clay has the highest. It is interesting to note that the hydraulic conductivity of Hsin-chu clay is also the highest among the three clays (Table 4).
Gravimetric Water Content (%)
Time (day)
GEOSYNTHETICS ASIA 2012 5th Asian Regional Conference on Geosynthetics 13 to 16 December 2012 | Bangkok, Thailand
16 The increase of air permeability with decreasing water content did not seem to be solely related with the increased number of cracks. The cracks developed before the water content of the specimens fell below the shrinkage limit. The loss of water of clay particles along the cracks might actually widen the pathway for air.
(a) Relationship between air permeability and water content
(b) Relationship between air permeability and volume change
(c) Relationship between volume change and water content
Fig. 9 Relationship between air permeability, water content, and shrinkage of compacted chung-li clay.
(a) Relationship between air permeability and water content
(b) Relationship between air permeability and volume change
(c) Relationship between volume change and water content
Fig. 10 relationship between air permeability, water content, and shrinkage of compacted kaolinite.
The relationship between the air permeability and shrinkage of the clay specimens are shown in Figs. 8 (b), Fig. 9 (b), and Fig. 10 (b). Only the air permeability of compacted kaolinite decreased at the same time when it shrank (Fig. 10(b)). For Hsin-chu clay and Chung-li clay, there did not seem to be any relationship between air permeability and shrinkage (Figs. 8 (b) and 9 (b)). Again, the reason is that the air permeability of the CCLs is related to the widening of the cracks rather than the reduction of
y = 0.0009x-0.457 Shrinkage Limit = 40.5%
y = 12.099e-0.007x
0
Vol. Shrinkage (%)
w (%)
Shrinkage Limit = 40.5%
GEOSYNTHETICS ASIA 2012 5th Asian Regional Conference on Geosynthetics 13 to 16 December 2012 | Bangkok, Thailand
the total volume.
It is interesting to note that the optimum water content of Hsin-chu clay and Chung-li clay are higher than the shrinkage limits whereas the optimum water content of kaolinite is lower than the shrinkage limit. As a result, the volume change of kaolinite specimens was only about 1/3 of the volume change determined from shrinkage limit test, while this ratio was about 1/2 for the other two clays.
In addition, kaolinite has more fine particles and lowest plastic index value. These factors may contribute to the low air permeability of desiccated kaolinite.
On the other hand, the content of fines of Hsin-chu clay is slightly below 50% such that the sand particles may be in contact with each other.
Therefore, as the water content decreases, the clay in between the sand particles shrank and pathways of air formed. As a result, Hsin-chu clay had higher air permeability than the other two types of CCLs.
Theoretically, the volume of the clays does not change after the water content dropped below the
Theoretically, the volume of the clays does not change after the water content dropped below the