Earthen barriers technology for waste containment

WASTE MANAGEMENT, Vol. 10, pp. 147-153, 1990 0956-053X/90 $3.00 + .00 Printed in the USA. All rights reserved. Copyright © 1990 Pergamon Press plc

EARTHEN BARRIERS TECHNOLOGY FOR WASTE CONTAINMENT

T. B. Edil* and P. M. Berthouex University of Wisconsin-Madison, College of Engineering, Department of Civil and Environmental Engineering, Rm. 2226, Engineering Building, 1415 Johnson Drive, Madison, Wisconsin 53706 U.S.A.

ABSTRACT. Performance data on several earthen barriers (liners) that can be used in containing municipal and industrial waste waters and the leachate generated in sanitary landfills are presented. The principal requirements fO r such liners currently accepted in practice are described. The difficulties common to the laboratory tests used in det#rmination of the permeability of such liners are discussed. The performance of fly ash-sand and bentonite-sand mixtures iin laboratory permeability tests is provided. The critical factors regarding the laboratory measurement of liner permeability[ are discussed and recommendation for obtaining a low-permeability barrier are presented based on a review of the current practice and the authors' experience.

INTRODUCTION

Earthen liners are commonly used in the construction of low-permeability barriers to contain waste liquids in wastewater lagoons and sanitary landfills (1). The use of earthen materials is especially desirable where the waste disposal sites are remote or need to be constructed using normal earth moving equipment. Furthermore, earthen liners can be constructed uti- lizing relatively low level technology. These condi- t ions may be found more of ten in deve lop ing countries making the earthen liners a particularly appropriate means of waste containment. Earthen hydraulic barriers include natural clay, bentonite-soil mixtures, fly ash, fly ash-soil mixtures, soil cement, and soil-asphalt mixtures. Of these, fly ash and fly ash-soil mixtures are particularly attractive options since fly ash is basically a waste material itself and using it as a liner would generate significant econ- omies.

Inasmuch as the earthen liners may appear appropriate and attractive, unless carefully designed and constructed, they may easily fail in providing the required performance (2,3). Such problems are be- coming quite common. Most construction problems,

RECEIVED 26 APRIL 1989; ACCEPTED 4 APRIL 1990. *To whom correspondence may be addressed. Acknowledgements- Wisconsin Power and Light Company

and the Graduate School of the University of Wisconsin-Madison provided financial assistance for the fly ash studies. A number of former graduate students were involved in various phases of the earthen liner research including Allan E. Erickson, Kevin Ves- perman, Carl J. Burkhalter, Stacy A. House, and Linda K. Sand- strom. Their efforts and care are appreciated.

147

however, can be eliminated by using lal~or-intensive procedures and in this respect the earthen liners provide a more attractive choice in developing countries. In recent years there has been a greSter scrutiny especially with regards to the long-term !durability of earthen liners in contact with various Waste liquids, in particular, the organic liquids (4). Ih the design of a hydraulic barrier, the maximum allowable hydraulic conductivity must be specified! for the material being used. Prior to construction, I the material that is proposed for use must be tested tO ensure that it has a hydraulic conductivity at or below the specified level. Accurate measurement of hydraulic conductivity for such low-permeabili ty barriers has proven to be a difficult task (5). Problems are present within the procedural steps and the equipment used to measure the hydraulic conductivity. Furthermore, there is laboratory and field evidencej that certain chemicals, in particular some organic Solvents, can affect the structural integrity of certain soil liners adversely (4-7). Therefore, there is a great interest in recent years with regards to the methods of de- termining hydraulic conductivity and 10ng-term durability of earthen liners.

This paper provides performance dala on selected earthen liner materials including fly ash! fly ash-sand, and bentonite-sand mixtures. The diffigulty and critical factors regarding the laboratory measurement of liner permeability are presented along with recom- mendations for obtaining a low-permeability barrier. The emphasis is on certain liners suitable for use in sanitary landfills and municipal and non-hazardous industrial wastewater ponds.

148 T . B . EDIL AND P. M. BERTHOUEX

LINER SELECTION A N D DESIGN

The principal requirement for a hydraulic barrier is low permeability to water and waste fluids and little or no interaction with the waste that might increase permeability. Absorptive/adsorptive capacity for pollutants, and strength initially and after contact with waste fluids, are secondary requirements. When earthen materials are properly selected and compacted, they can be made extremely impermeable to the flow of liquids but there will always be some seepage. To achieve an adequate seal in wastewater lagoon/pond or sanitary landfill systems using earthen materials, the following two related criteria are suggested, based on a review of the current practice in the United States (8,9) and the requirements of regulatory agencies of North Central states (10), to be satisfied individually (11).

1. The percolation rate of lagoon/pond water specified shall not exceed 9.36 for municipal waste and 4.68 for industrial facilities and sludge lagoons in m3/ha.d (exfiltration shall be prevented by leachate collection in sanitary landfills), and

2. The coefficient of permeability (hydraulic conductivity), k in mm/s specified for this seal shall not exceed 10 -6 with a minimum seal thickness of 0.3 m for wastewater lagoons and 1.5 m for sanitary landfills.

Such a very low percolation rate is obtainable by present day technology and is essential to ensure protection of groundwater. In the selection and design process the first step is to determine if this low level of permeability is achievable using the available and economically feasible liner materials by performing laboratory permeability tests on samples of compacted earthen liner materials. The second step is to determine if the permeabilities measured in the laboratory tests are likely to remain unchanged in the field as a result of environmental and physico-chemical effects, i.e., the long-term durability of the liner. There are certain difficulties with respect to both of these two steps. Accurate measurement of the hydraulic conductivity has proven to be a difficult task with regard to low-permeability earthen materials. Problems are present within the procedural steps and the equipment used to measure the hydraulic conductivity. Presently there are no consensus standards available for the hydraulic conductivity tests on low- permeability barriers (American Society for Testing and Materials is in the process of developing one such standard). The second consideration relating to the durability of liners is recently receiving consid- erable attention. This aspect is even less developed than the permeability testing. However, it ultimately controls the long-term performance of an earthen liner. A number of possible failure mechanisms for

earthen liners have been suggested (12). These mechanisms result in an increase in the permeability of the liner materials as a result of changes in the physical and chemical environment of the liner subse- quent to construction. Therefore, even if the liner has the proper impermeability at the end of construction, it still may be affected subsequently by these factors.

The affect of the failure mechanisms on the earthen liners take place basically in two ways: 1. a change in density, and 2. a change in effective pore- size distribution (12,13). The physical factors include wetting/drying, freezing/thawing, temperature changes, and stresses. These factors may result in cracking and loosening of earthen liners when, and if, they are exposed to such environmental conditions. Waste fluids may interact chemically with the liner material in a variety of ways. Either organic or inorganic acids and bases may solubilize portions of the clay structure resulting in increased permeability. Pore fluid substitution, i.e., changes in cation type and concentration, dielectric constant, etc., affect the force field around clay particles and the inter- particle forces. Consequently, the size of the pores and the resistance to pore fluid movement are affected. These chemical factors result in a variety of mechanisms including volume change (swelling or shrinkage) and fissuring, due to the replacement of interlayer water in clay mineralogical structure by organic chemicals. While it is clear that the permeability is invariably influenced when permeation is by an organic chemical in its pure or concentrated form, compounds having low solubility such as hy- drocarbons and water soluble organics at concentra- tions less than about 75 to 80 percent have no effect on permeability (4).

The purpose of the liner is not only to act as a hydraulic barrier, i.e., control wastewater or leachate seepage but also restrict the escape of hazardous substances such as heavy metals and organic chemicals. The importance of diffusion as a transport mechanism for pollutant migration through earthen barriers is being recognized more and more as the technology of constructing hydraulic barriers with little or no fluid flow has improved (14). This subject currently is being investigated.

PERMEABILITY TESTING OF LOW PERMEABILITY BARRIERS

There are certain problems common to all of the tests suggested for the determination of the hydraulic conductivity of low-permeability earthen materials. These problems can be placed in one of two cate- gories, equipment or specimen. In some cases, so- lutions to these problems are readily available; in other cases, more research is required. In measuring

EARTHEN BARRIERS FOR WASTE CONTAINMENT 149

the hydraulic conductivity of a low-permeability material, the most common equipment-related problem is system leaks. With such low-permeability barriers, even a small leak could substantially increase the measured inflow rate or decrease the measured outflow rate. Therefore, a method of checking for leaks is needed such as a water budget analysis. Equip- ment-related flow imbalances could also be caused by evaporation and the presence of air bubbles in the outflow burette (5). There are also specimen- related problems that affect the measured flow rates. The moisture condition of the specimens tested seems to affect how the permeant passes through them. For instance, specimens of sand-bentonite mixtures compacted near optimum moisture content appear to continue to hydrate as permeability tests progress. As bentonite absorbs more in-flowing permeant, zones of differential hydration may develop within the specimen (5). In the case of fly ash and fly ash-soil mixtures, moisture is needed to complete the hydration reactions that control the extent of cementation as well as reduction in permeability (15). Another specimen-related problem in permeability testing is the growth of bacteria on the earthen material within the permeameters. This activity tends to reduce the measured hydraulic conductivity by clogging the pores.

Application of back pressure has been promoted as an effective procedure for improving the degree of saturation of a specimen during permeability tests (16,17). Elevating the pressure at the inflow and outflow burettes, by the application of back-pressure, forces air in the burrettes, lines, porous stones, and specimen into the permeant, thus saturating the whole system.

Permeability measurements of the earthen barriers can be a long process because of the low flow rates and long-time periods required to reach steady- state conditions. In an attempt to reduce the length of time required to finish a permeability test, hydraulic gradients in excess of 100 have been used and investigated (18). There is a controversy about the possible adverse affects of application of such high gradients on the permeability test results (16).

Another source of controversy in permeability testing is the type of permeameter, i.e., flexible-wall versus rigid-wall permeameters. Flexible wall permeameters have been described as the best type of equipment for performing permeability tests (16). They model in-situ pressure conditions and provide a better seal along the specimen edges. Certain clay specimens may shrink when exposed to certain types of hazardous waste permeants creating a gap or a channel between the specimen and the rigid walls of the permeameter. In conclusion, it seems as though adequate equipment has been developed to perform permeability tests on barrier soils, but the exact pro-

cedures have not been defined that will produce accurate results each time. However, careful testing consistent with the best available geotechnical test procedures can produce reliable laboratory permeability test results.

At certain sites there may not be a sufficient amount of natural clay material to construct a barrier of sufficient thickness for waste containment. In such cases, the properties of existing natural coarse- grained soils can be modified by the addition of certain finer grained mixtures. Two such materials are described in the following sections.

SOIL-BENTONITE LINERS

Bentonite has been used widely as a soil modifier by blending it with existing soils. Bentonite is a com- mercially available earthen product based in highly water-swelling clay minerals. Swelling soils are en- countered in many localities around the world. Iden- tification, mining, and marketing of soils high in water-swelling clay minerals locally in developing countries is highly desirable if it has not been already done. In areas where soils suitable for line~ materials are not readily available, mixing with al bentonite clay could enhance sealing characteristicS in a cost- effective manner. At the same time, however, one has to recognize the relatively higher selasitivity of swelling clay minerals to different chemical constit- uents of the waste (19). This aspect, perhaps, is more critical for the sanitary landfills and certain industrial waste lagoons than municipal wastewate~ lagoons.

The permeability test results using a mixture of 90% Ottawa sand (passing 4~20 sieve an~l retained on #30 sieve) and 10% bentonite clay a(e summa- rized in Table 1 (5). Water was used to prepare the samples and as the permeant. Specimens! were prepared by compaction at moisture contents! exceeding the optimum moisture content using th e standard Proctor method. Both rigid and flexible wall permeameters were used in a test setup as sh6wn in Fig.

TABLE 1 Hydraulic Conductivity (mm/s) x l r

Hydraulic Conductivity

Leg 1, Leg 2, Leg 3, Leg 4, Test No. 1st Low i 1st High i 2nd Low i 2nd High i

RW1 NM 1.3 0.5 RW2 4.3 4.2 3.0 RB1 7.2 380.0 RB2 2750.0 5100.0 5490.0 FW1 2.4 1.6 2.9 FB1 4.1 1.7 FB2 3.3 1.4 3.4

3.1

1.5

NM: not measurable; R: rigid wall B: back pressure; i: hydraulic gradient; F: flexible wall W: without back pressure~

150 T . B . EDIL AND P. M. BERTHOUEX

SOURCE ~ l l

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a l

E I

TRIAXIAL CELL PERMEAMETER IFLEXlBLE WALL)

FIGURE 1. Schematic of permeameters.

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t-

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FLOW MEASUREMENT EQUliIMENT

1. A pressure of 380 kPa was used in the test in- volving back-pressure. Low and high hydraulic gradients were nominally 29 and 290, respectively. The test results shown in Table 1 indicate that coefficients of permeability lower than 10-6 mm/s was achievable by adding 10% bentonite to otherwise highly perme- able sand soil. The only exception involves the specimens that were tested in a rigid wall permeameter using back-pressure. In this case, channeling and drastic increase in permeability occurred. Therefore, back-pressuring, while results in improvement of saturation in flexible-wall permeameters, may produce adverse effects in rigid-wall permeameters. The re- suits also indicate that rigid or flexible-wall permeameters, when carefully used, may product similar results unless the specimen is shrinking due to chemical reactions.

FLY A S H AS A L I N E R

A large percent of the electric power generated in the world is produced from coal combustion. This process produces large quantities of coal ash per year. Fly ash, the lighter, smaller particles carried in the flue gas, is frequently handled dry for disposal or utilization. This dry storage maintains the pozzolanic capabilities of the fly ash. Fly ash-stabilized

soil may have potential for use as a liner material at fly ash and/or scrubber sludge landfill sites, nonhazardous waste lagoons, e.g., manure pits, wastewater treatment lagoons, and landfills, either alone or in combination with geomembranes.

The potential for each of these applications requires, as an initial first step, an understanding of the permeability of the fly ash and fly ash-soil mixtures and what factors affect permeability. In an investigation aimed at this, fly ash-soil mixtures were prepared with different fly ash contents by compaction and were tested for their permeabilities (15). The soil used in preparing the fly ash-soil mixtures was a commercial fine quartz sand known as Portage sand. The specimens were prepared and stored for seven days in a humidity controlled environment at room temperature after compaction. Subsequently, specimens were trimmed and placed into the permeameters. Permeability tests utilized a flexible-wall permeameter in a falling head permeability test. De- aired water was used as the permeant and the saturation of the specimens was improved by applying a nominal hydraulic gradient for 24 hours prior to permeability testing. A back-pressure of 380 kPa was then established in both inflow and outflow burrettes to remove any trapped air. A gradient of 20 for the low permeability materials, (k < 10 -6 mm/s) and 10


for the high permeability materials (k > 10 -6 mm/ s) was then established. Permeability testing was continued either until approximately one pore volume of permeant was passed through the specimen or, in the lower permeability specimen, at least one- half of the pore volume of permeant was passed through the specimen.

Permeability test results indicate that the 100% and the 40% Belle Ayr fly ash-sand specimens ex- hibit essentially identical permeability as shown in Fig. 2 (in the range of 10 -7 t o 10 -8 mm/s). However, with less than 40% fly ash the measured, permeabilities were 100 to 1,000 folds higher (k > 10 -5 mm/ s). The Belle Ayr fly ash is a highly pozzolanic fly ash with significant self-cementitious properties. An- other fly ash from a different coal source, however, exhibited permeabilities between 5 × 10 -4 and 1 × 10 -5 mm/s. This latter fly ash while self-cementitious was not as pozzolanic as the Belle Ayr fly ash. This suggests that there is another major factor affecting permeability that involves the pozzolanic reaction products.

In order to establish pozzolanic fly ash as an effective waste liner, its retention of initial physical characteristics in contact with waste water and leachate in the long-term (physical and chemical durability) and constructibility should be determined in addition to its hydraulic characteristics, i.e., low permeability. In an investigation of the interaction of an inorganic permeant with compacted fly ash, it is shown that the chemical quality of the effluent is quite favorable (20,21). The high pH (about 11.5) of the fly ash leachate helps keep the heavy metals and boron in the fly ash matrix. Many of the con- stituents found in fly ash such as aluminum, arsenic, chromium, selenium, silica, and strontium show a slight first flush phenomenon (Fig. 3). This occurs for a short period of time and at such low con- centrations that dilution effects may eliminate any problems. The elements that were present in the permeant such as boron, calcium, cadmium, sodium,

zinc, chloride, and sulfur display various types of release patterns (Fig. 4). While some reach equilib- rium at the fixed permeant concentration eventually (Fig. 4, Types 1, 3, and 4), there are others that are retained in the fly ash matrix such as the heavy metals (cadmium, chromium, and zinc) (Fig. 4, Type 2). The only exception to this pattern is calcium which breaks through at a concentration greater than that of the permeant (Fig. 4, Type 5) implying additional release from the fly ash matrix. Furthermore, there were no adverse effects on the permeability of the compacted fly ash due to long-term exposure to an inorganic permeant.

The physical durability studies of compacted fly ash show that exposure to wet/dry cycles!and freeze/ thaw cycles that are typical of northern climates does not affect the permeability in any sigfiificant way (20). Construction conditions such as moisture content, compactive effort, and, in particular, time between adding water and compacting the fly ash have a significant influence on the density arid permeability of fly ash/sand mixtures. Use of set retarders such as a lignin solution retard the setting and hard- ening of the fly ash/sand mixture from about 20-30 minutes to about one hour after introducing the water to the mixture (22). This provide~ a practical way of increasing the time available for field compaction.

These findings indicate that fly-ash stabilized soils present a significant alternative as a c0st-effective and resistant earthen barrier.

S U M M A R Y

Earthen liners are proposed as hydraulici barriers for the containment of sanitary landfill leachhte and municipal and nonhazardous industrial wastewaters. Earthen barriers offer appropriate options particularly in developing countries based on the availability of materials and technology. However, successful performance of such liners throughout tile life of the

10~3~ " .

r~ 10.5f

t f I

10-9/ , i

0 2O

Q

• FINAL

tx INITIAL

t I , I t i

40 60 80 PERCENTFLY ASH(%)

I

lOO 120

F I G U R E 2. Hydraulic conductivity of fly ash-sand mixtures.

152 T. B. EDIL AND P. M. BERTHOUEX

First Flush

=o o

Pore Volume

LaooedResDonse

Pore Volume

FIGURE 3. Typical effluent response curves of elements released from fly ash.

waste disposal system requires care in design, construction, and operation of such liners. This paper provides performance data on several earthen liner materials including fly ash, fly ash-sand, and bentonite-soil mixtures. There are difficulties and critical factors regarding the laboratory measurement of

~Permeant Level--

Pom Volume

Tt,am 2

i o o ¢.3

• -.~-Permeent Level

~..._Oetect ion,, . , Limit

Pore Volume

T ~ 3

~Permeant L ~.~.~.--

Pore Volume

~ v m 4

Permeant L e v e l ~

! Pore Volume

T ~ 5

JL/ Pore Volume

FIGURE 4. Typical effluent response curves of elements in inorganic permeant.

permeability of such barriers. These include permeameter type (rigid versus flexible wall permeameters), back-pressure application (in order to saturate the sample with water), and hydraulic gradient magnitude. Factors that control permeability include density, moisture content, soil modifier content (fly ash or bentonite content), and degree of cementation and hydration.

REFERENCES

1. Johnson, A. I., Frobel, R. K., Cavalli, N. J., and Petterson, C. B. Overview, Hydraulic Barriers in Soil and Rock. Amer. Soc. Test. & Mat'l., STP 874, 1-6; (1985).

2. Griffin, R. A., et al. Attenuation of pollutants in municipal landfill leachate by clay minerals: Part 1-Column leaching and field verification. Environ. Geol. Notes, Illinois State Geo- logical Survey (1976).

3. Haxo, Jr., N. E. Evaluation of selected liners when exposed to hazardous wastes. Proc. Hazardous Waste Research Symp., EPA 600-76-015 (1976).

4. Mitchell, J. K. and E T. Madsen. Chemical effects on clay hydraulic conductivity. Proc. ASCE Conf. on Geotechnical Practice for Waste Disposal '87, 87-116 (1987).

5. Edil, T. B., and Erickson, A. E., Laboratory permeability testing of a bentonite-sand liner material. Amer. Soc. Test. & Mat'l., STP, 874, 155-170 (1985).

6. Acar, Y. B., Hamidon, A. B., Field, S. D., and Scott L. The Effect of organic fluids on hydraulic conductivity of compacted kaolinite. Amer. Soc. Test. & Matl., STP 874, 171- 187 (1985).

7. Anderson, D. C., Crawley, W., and Zabcik, J. D. Effects of various liquids in clay soil; bentonite slurry mixtures. Amer. Soc. Test. & Matl., STP 874, 93-101 (1985).

8. Richardson, M., Acar, Y. B., and Edil, T. B., Geotechnical aspects of landfill lining regulations and hazardous waste regulations in the U.S.A., Proc. 2nd International Symp. on Environmental Geotechnology, Shanghai, P.R.C., 349-365 (1989).

9. Great Lakes-Upper Mississippi Board of State Sanitary En- gineers; Recommended Standards for Sewage Works (1978).

10. Wisconsin Administrative Code, Department of Natural Re- sources, Chapters NR 110, 1983; NR 213, 1984; NR 504 (1988).

11. Edil, T. B., and Didier, P. Sealing of lagoons and ponds: proposed quality requirements for soil liners. Proc. 4th Mad- ison Conf. Appl. Res. & Prc. Mun. & Ind. Waste, Madison, WI, 412-416 (1981).

12. Edil, T. B. Appropriate waste containment technology for developing countries. In: Appropriate Waste Management for Developing Countries, K. Curi, ed. Plenum Press, New York, 619-632 (1985).

13. Gray, D. H. Adequacy of landfill liner design criteria. A Report to the Toxic Substances Control Commission, State of Michigan, Lansing, Michigan (1984).

14. Quigley, R. M., Yanful, E. K., and Fernandez, E Ion transfer by diffusion through clayey barriers. Proc. ASCE Conf. on Geotechnical Practice for Waste Disposal '87, 137-158 (1987).

15. Vesperman, K., Edil, T. B., and Berthouex, P. M. Permea- bility of fly ash and fly ash-sand mixtures. Amer. Soc. Test. & Mat'l., STP 874, 289-298 (1985).

16. Zimmie, T. E Geotechnical test considerations in the determination of laboratory permeability for hazardous waste disposal siteing. Amer. Soc. Test. & Mat'l., STP 760, 293-304 (1981).

17. Black, D. K., and Lee, K. L. Saturating laboratory samples by back-pressure. Jour. Soil Mechanical & Foundation Div., ASCE, 99:SM1, 75-93 (1973).


18. Mitchell, J. K., Hooper, D. R., and Campanella, R. G. Permeability of compacted clay. Jour. of the Soil Mech. & Found. Div., ASCE, 21:SM4, 41-65 (1965).

19. Mitchell, J. K. Fundamentals of Soil Behavior. John Wiley & Sons, Inc., New York (1976).

20. Edii, T. B., Berthouex, P. M., and Vesperman, K. D. Proc. of the ASCE Conference on Geotechnical Practice for Waste Disposal '87, 447-461 (1987).

21. Sandstrom, L. K., Interaction of fly ash and soil with inorganic and organic leachate. M.S. Advanced Independent Study Re- port, Department of Civil and Environmental Engineering, University of Wisconsin-Madison (1989).

22. Burkhalter, C. Physical characteristics of fly ash as a waste liner. M. S. Advanced Independent Study Report, Depart- ment of Civil and Environmental Engineering, University of Wisconsin-Madison (1987).

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Earthen barriers technology for waste containment