An Innovative Composite Liner System for Coal Combustion Residual Containment Projects

Transcription

1 2013 World of Coal Ash (WOCA) Conference - April 22-25, 2013 in Lexington, KY An Innovative Composite Liner System for Coal Combustion Residual Containment Projects Ed Zimmel 1, Dhani Narejo 1 and Jimmy Youngblood 1 1 GSE Environmental LLC, Gundle Road, Houston, TX KEYWORDS: geomembrane, geosynthetic clay liner, drainage geocomposite, composite liner system, coal combustion residual, leak location liner INTRODUCTION A composite liner refers to a geomembrane that is underlain by a layer of either compacted clay or geosynthetic clay. One or more filters and drainage materials are almost always used in association with liners. These can be of a particulate type (such as sand, gravel, or select waste) or, alternatively, a geosynthetic drainage layer (GDL) is often used. The geosynthetic clay liner (GCL) alternative to clay and the GDL alternative to sand or gravel represent the current state of practice for liner systems in municipal solid waste projects, though this is not yet the case in CCR containment projects. Equivalency beyond doubt and proven performance are often necessary in order for geosynthetic alternatives to become acceptable substitutes for traditional materials. While this is generally accepted to be the case for many types of waste containment projects, CCR projects are relatively new, and performance data is still being generated. It is necessary for the properties of existing geosynthetics to be investigated in terms of the specific environments of CCR projects, and new products are typically developed when existing ones prove to be inadequate. This paper presents data for the GDL, geomembrane, and GCL used in CCR containment composite liner systems. Figure 1 shows a cross-section of part of a CCR containment cell in which the liner system consists of a GDL, a geomembrane, and a GCL. The GDL consists of a polymeric core with a geotextile bonded on both sides of it. Of primary interest in this paper is the retention performance of the top geotextile of the GDL when placed against very fine CCRs. This top geotextile, which is marked in red in Figure 1, will be discussed in this paper. A GCL is comprised of bentonite clay sandwiched between two geotextiles, or between a geotextile and a geomembrane. For a GCL, it is the chemical compatibility of the clay component with ion-rich CCR leachate that is our principal interest here. Therefore, the clay component, which is represented by broken red lines in Figure 1, will be discussed in detail in this paper. The last component of the liner system, but probably the most important, is the geomembrane. This component, which is also highlighted in the figure as red, offers us the opportunity to confirm beyond doubt that an as-constructed composite liner system is leak-free. The purpose of this paper is to describe the engineering performance of these three components of the liner system as related to CCR projects. Here, we address only those properties that are significantly different from many other types of waste containment projects. The design of a specific

2 project will involve many properties such as interface shear strength, puncture protection, hydraulics, ultra-violet protection and so on that are not addressed here, since these are common to all waste containment projects. Figure 1 Conceptual Layout of a Single Composite Liner System, with Materials of Interest Marked in Red. A GDL AS THE PRIMARY LEACHATE COLLECTION LAYER As a primary leachate collection and removal layer, a GDL maybe in a direct contact with the overlying fine CCR, which contains fly ash and FGD (flue gas desulfurization) gypsum. One of the several requirements that the GDL must meet is the retention of the fine CCR particles by the top geotextile. The challenge in this regard is obvious if we consider the particle size range of to mm for fly ash and FGD gypsum, as shown in Table 1. By comparison, the opening size of many commercially available geotextiles is in the range of 0.1 to 0.3 mm, with 0.2 mm being a value that is quite commonly used in design. The permeability range of 10-4 to 10-7 cm/sec shown in Table 1 is several orders of magnitude less than the available values in geotextiles. Therefore, there is a certain window between the particle size and permeability of fine CCRs that can be targeted in the development a new geotextile that has a much smaller opening size than those of commercially available geotextiles, but that still meets the hydraulic conductivity requirements of a project. Table 1 - Range of Engineering Characteristics of Fine CCRs 1 Typical Characteristics Fly Ash FGD Gypsum Wet Dry Particle size (mm) Dry density (kg/m 3 ) Permeability (cm/sec)

3 Table 2 summarizes the specifications for a hybrid geotextile consisting of a woven monofilament geotextile and a nonwoven needlepunched geotextile. The two geotextiles are bonded together in a needlepunching process in order to obtain a single product, whose properties are presented in Table 2. To manufacture a GDL, the woven side of the hybrid geotextile is bonded to a geonet. In addition to the hybrid geotextile meeting the filter requirements, the woven geotextile forms a bridge over the geonet strands, ensuring that there are clean channels for the liquid flow. Table 2 - Specifications for the Hybrid Geotextile Property Test Method Average Value Mass ASTM D grams/m 2 Apparent Opening Size (AOS) ASTM D mm Filter Opening Size, FOS (O 95 ) CAN/CGSB mm Permittivity ASTM D sec -1 Grab Strength ASTM D N Grab Elongation ASTM D % Puncture Strength ASTM D N Mullen Burst Strength ASTM D The geotextile, and the associated GDL, were developed for use in CCR projects specifically to meet the requirements to filter out very fine CCRs. However, the claim that such a material is indeed suitable for such an application must be based on an extensive testing program. The testing program undertaken, and the results obtained are summarized in Table 3. The testing included accelerated, performance, and field tests that were performed by independent organizations. The results obtained show that the geotextile is consistently able to retain fine CCR particles and develop a filter cake within the passage of just a few pore volumes. Based on the test data, as well as research and testing performed by other experts on fine base soils and CCRs, a retention criterion is proposed for use with fine particles that has been used by other experts in the past: O 95 d 85 where O 95 = the filter opening size (FOS) obtained from hydrodynamic sieving, and d 85 is a particle size corresponding to an 85% value on a gradation curve of a CCR. A specific CCR must be sampled and tested for its gradation in order to obtain the entire particle size distribution curve. The above equation then needs to be used in order to determine the required opening size for the geotextile for a given design project. Once the geotextile is selected based on the above equation, additional tests, if necessary, can be performed to ensure that the geotextile is indeed compatible with the CCR material against which it is being considered for use. Several types of test methods are available for this purpose, including index, performance, and field tests. The authors experience has shown that tests performed on a slurry offer the most consistent index of a geotextile s retention behavior. Such tests are conservative, and they also avoid the

4 Table 3 - Summary of Accelerated, Performance and Field Testing Performed on the Hybrid Geotextile or Drainage Geocomposite (Reports available upon request) Test Type Test Method Organization Conducting Tests Accelerated Filter press API TRI/Environmental, RP 131, ASTM D Austin, TX 5891 Performance Gradient ratio ASTM D 5101 Performance Hydraulic conductivity ratio ASTM D 5567 TRI/Environmental, Austin, TX TRI/Environmental, Austin, TX Ohio State University, Columbus, OH Field Not Applicable Ohio State University, Columbus, OH Field Not Applicable AMEC E&I, Inc., Louisville, KY Summary of Tests Seven tests performed on fly ash from one plant with slurry at 500%, in which the piping of ash was measured. An average retention efficiency of 99.6% was obtained, with little or no loss of fines through the geotextile. Three tests performed on fly ash samples from the same plant. Initial gradient ratio ranged from 0.7 to 3.0. Final gradient ratio obtained was 0.5 to 1.0 under system gradients of 1, 2 and 6, showing no clogging of the geotextile. Seven fly ash and FGD gypsum samples from five different sites were tested to measure geotextile compatibility and soil loss. Percent solids in the effluent was less than 0.5%. Final hydraulic conductivity ratio ranged from 0.5 to 1, and it stabilized within two pore volumes. Four intermediate-scale test basins in which the geocomposite was tested under fly ash and FGD samples from four different plants. Effluent properties were measured to determine the retention performance of the geotextile. Total suspended solids in the effluent were less than 20 mg/l, showing the geotextile s effective retention property. Three large test basins were constructed in which the drainage geocomposite was used in two, and a selected ash in the third as a control. Both test basins with the geocomposite performed very well with no piping, while the select ash basin showed a significant soil loss in the effluent.

5 effects of sample placement, which can significantly affect the test results. A geotextile showing a satisfactory retention behavior in an accelerated slurry test will always exhibit acceptable behavior in a performance test (such as a gradient ratio or hydraulic conductivity ratio test), while the reverse is not necessarily true. A GCL COMPATIBLE TO CCR LEACHATE GCLs are factory-made composites of clay and geosynthetics that weigh approximately one pound per square foot. The clay is mainly sodium bentonite, while the geosynthetics can be woven or nonwoven geotextile or geomembrane. Sodium montmorillonite mineral is the primary component of bentonite, and it controls the hydraulic conductivity of the GCL. A weak inter-layer bond allows the montmorillonite crystal layers to separate during hydration, as water molecules enter the inter-layer space. Consequently, cations on the inter-layer surfaces become exchangeable, which renders the physical properties of the sodium montmorillonite susceptible to interactions with the permeant liquid. The degree of this interaction depends on the valence, relative abundance, and size of the cations. Generally, cations of a greater valence replace cations of a lower valence. Sodium bentonite is prone to cation exchange when permeated with solutions containing divalent or trivalent ions. The initiation and rate of cation exchange depends on many factors, including the initial moisture content of the bentonite, the initial hydration liquid, the pressure, the gradient, and the clay particle size. Further discussion of cation exchange as related to GCLs can be found in several publications on this topic 2, 3, 4. The Resource Conservation and Recovery Act (RCRA) is the primary law governing the disposal of solid and hazardous waste in the U.S. Within the RCRA, subtitle D addresses the requirements for solid waste disposal, including landfills, waste combustors, and transfer stations. Although GCLs were not included in subtitle D s prescriptive liner systems, the regulations did allow the use of an alternative liner system if its equivalency to the prescriptive liner systems could be demonstrated. The equivalence to CCLs has been the main basis of approval of GCLs in an overwhelming majority of the projects in which these materials have been used. The equivalence calculations consider hydraulic, physical/mechanical, and construction-related aspects of the use of a GCL in a project. In the consideration of hydraulics, the most important criteria are the steady state flux, the breakthrough time, and the adsorption capacity. These calculations are easy to perform if the hydraulic conductivity of a GCL is less than or equal to 10-9 cm/sec, but they can become complex if the hydraulic conductivity increases with time due to cation exchange. A review of the leachate chemistry data from a number CCR disposal sites developed by EPRI shows that these leachates can be rich in cations and anions that can negatively influence the hydraulic conductivity of GCLs 5. Cation exchange in sodium bentonite can be modified in several ways, including prehydration, agglomeration, and polymeric adsorption. The latter method is used as the basis of the development of polymer-modified bentonites in a number of different industries. Although the exact formulation of bentonite-polymeric compounds is

6 proprietary, as a general principle, one or more poly-cations are adsorbed on bentonite particles, which may then change the physical structure of clay particles by forming agglomerates, as well as change their chemical nature by bonding with charged sites. The end result, in the case of the modified clay used with GCLs, is a lesser affinity towards divalent and trivalent ions that are known to have a negative effect on the hydraulic conductivity of GCLs. Based on this principle, preferential clays can be developed for use in specific environments such as salt water, brine, oils, and organic compounds. GCL products based on this principle are available currently in the U.S. from multiple manufacturers. A polymer-modified bentonite is used by GSE Environmental in one of its GCL products that has already been used in several projects in the U.S. It should be noted that many polymers that have historically been used over the years are actually thickening agents that have molecular weights many, many times in excess of those used by GSE, and thus do not act in the same manner. The polymers used by GSE are not biological in nature, i.e. they are not bi-polymers as are thickening agents such as guar and xanthan gum. Both of these are complex polysaccharides and thus are subject to biological degradation, i.e. they will be consumed as a food source by indigenous microorganisms. The polymers used by GSE are currently in use in NSF certified applications, so they are neither toxic in nature nor do they promote bacteria growth. They meet ANSI Standard 60 for drinking water additives. The hydraulic conductivity of this GCL was tested against leachate drawn from three different CCR projects according to ASTM test method D A standard GCL was also tested against the same leachate under exactly the same test conditions. In order to keep the test durations to a reasonable length of time and minimize the number of variables, the tests were performed on factory-received GCL without any pre-hydration. The pre-hydration sequence in the field is unknown, and these tests are conservative, since pre-hydration would increase the time required for the onset of cation exchange. As shown in Figure 2, hydraulic conductivity of 1x10-9 cm/sec or less was obtained for the polymer-modified GCL, while value ranging from 10-8 to 10-7 cm/sec were obtained for the standard GCL. We would also note that it takes from 50 to 200 days before the first the effects of cation exchange on the hydraulic conductivity of a standard GCL become apparent. The polymer-modified GCL generally shows a decreasing trend of hydraulic conductivity, which is necessary for the equivalence calculations. Table 4 Ionic Composition (mg/l) of Leachate from Three CCR Sites Used in Tests Ion Site 1 Gypsum Leachate Site 2 Fly Ash/FGD Leachate Site 3 Fly/Bottom Ash/Gypsum Leachate Calcium Magnesium Potassium Sodium ,200 Chloride 250 1, Sulfate 2,200 1,600 7,600

7 Figure 2 Response of a Polymer-Modified Bentonite GCL to Three Leachates from CCR Projects. Figure 3 Response of a Standard Bentonite GCL to the Same Leachate as in Figure 2.

8 CONDUCTIVE GEOMEMBRANE FOR EFFECTIVE LEAK LOCATION SURVEY Geomembranes are scanned continuously during the manufacturing process by in-line scanners to such a degree that even a pin-hole sets off a loud alarm. With such stateof-the-art technology, it is generally accepted that geomembranes are defect-free when they are delivered to a jobsite. The installation of geomembranes in the field, though, is a completely different matter. Every geomembrane installation is considered to have a few holes, though with the aid of the leak location technology, it is possible to install leak-free liners. The average density of leaks in a geomembrane installed under a strict quality control and quality assurance program but without a leak-location survey is 4 leaks per hectare for an exposed installation, and 0.4 leaks per hectare for a covered installation, as of the early 2000s 6. The number of leaks is much higher without a stateof-the-practice CQA (construction quality assurance) program. Leak location surveys, however, are not perfect, and the accuracy of a survey depends to a great extent on the nature of the materials underlying and overlying the geomembrane. In general, a conductive medium improves the accuracy of a survey, while a non-conductive medium reduces its accuracy. The electrical liner integrity surveys (ELIS) are currently the most effective and practical means of locating leaks in installed geomembranes 7,8. These types of surveys include ASTM methods D7002, D7007, D7240 and D7703. All of these methods require a good conductive medium underneath an isolating geomembrane. The High Density Polyethylene (HDPE) and Linear Low Density Polyethylene (LLDPE) geomembranes are not normally electrically conductive, and traditionally the subgrade soil acts as the conductive medium below the geomembrane when the application allows it. The technology functions along the principal that polyethylene geomembranes are electrically isolative and by applying an electrical potential above and below the geomembrane, an electrical current will flow only if a hole is present in the geomembrane. Although, there can be some challenges with these techniques due to conductivity of the soil, insulating materials like geonets, and/or wrinkles in the geomembrane. GSE has improved the efficiencies of these tests by incorporating a conductive layer on the bottom surface of the Leak Location Liner product. The bottom conductive layer typically consists of a thin layer of highly concentrated special conductive carbon black to make this layer electrically conductive. The recently improved Leak Location Liner conductive backing carries sufficient current for a ELIS, acting as the conductive medium below the geomembrane. The intimate contact of this conductive layer provides many advantages for the different ELIS. Since the conductive layer is coextruded, it is an integral part of the base liner and cannot be removed thus preventing proper performance for duration of the lifetime of the geomembrane. GSE s Leak Location Liner (L 3 ) is a co-extruded geomembrane with a thin conductive layer built into the geomembrane. This conductive layer makes it possible for the current to flow through a defect in the geomembrane regardless of the nature of the underlying

9 medium. Thus, the accuracy of a survey is greatly improved, and even those leaks are detected that could otherwise have been missed due to less-than-optimum site conditions. The use of the L 3 is explained graphically in Figure 4, which highlights how a survey is made possible when the underlying medium is a geonet or dry soil. Figure 4 An Explanation of the Use of a Conductive Geomembrane in Leak Location Surveys CASE HISTORY OF THE USE OF CONDUCTIVE GEOMEMBRANE IN LARGE TEST CELLS Water Dipole Tests: Figure 5 shows a photo of a test of L 3 that is currently in progress at GSE s Houston facility. The cross-section of the liner for the pond is L 3 / geonet / Figure 5 A Photo of Water Puddle Tests on L 3 at GSE s Houston Facility. geomembrane under about a foot of water. Several 1/10 th -inch size holes were intentionally placed in the L 3 with the technician kept unaware of their location. The technician then waded through the liquid with a probe in hand, unaware of the hole locations. He was able to successfully locate all the holes with little effort. The

10 equipment detected the location of a hole up to 10 ft away from it. It would not be possible to perform this survey without L 3 unless the underlying geonet was flooded prior to the test. Ohio Pilot Project: The Ohio Pilot Project involved the testing of a 3.5 acre cell with a liner system that consisted of from top to bottom clay soil, geotextile, and L 3.The depth of the clay liner was two feet. The installer for the liner system was AEG, and the leak location survey was performed by I-Corp International. Four holes were detected in one acre by walking on the surface of the clay in a pre-planned pattern. Figure 6 shows one of the holes, which is roughly circular and quite small smaller than a penny. It is possible for such a small hole to be missed in a standard geomembrane, especially when the underlying ground is dry. With L 3, however, the accuracy of the survey and our ability to find defects is significantly improved. SKB Lansing Landfill Project: This project involved 4 acres of a landfill cell with a liner system consisting of - from bottom to top GCL, L 3, geocomposite, and 4 ft. of sand. The liner system was installed by GSI, and the leak location survey was performed by Leak Location Services of San Antonio, TX. This is a comparatively less-than-optimum leak location condition for a standard liner because of the use of a geocomposite which is an insulator below the geomembrane. Three holes were detected over the entire cell. One of the holes is a semi-circular cut about 1.5 inches as shown in Figure 7. This defect was found under 4 ft. of cover sand and drainage geocomposite. Figure 6 One of the Holes in the Liner Detected During the Survey.

11 CONCLUSIONS Figure 7 A Defect Detected in the Geomembrane during the Survey. This paper has described three components of a composite liner system for a CCR containment project: a geotextile filter, a GCL, and a geomembrane. In each case, the performance of each of these materials as related to a critical property was presented. For the geotextile, this property was the retention of the very fine particles in CCRs. For the GCL, it was the compatibility of the sodium bentonite with the CCR leachate. And for the geomembrane, it was the leak location characteristics. The engineering, equivalency, and CQA requirements for projects vary significantly. A composite liner system that includes the geosynthetics described in this paper, especially the geotextile and the GCL, can facilitate the key calculations an engineer must perform for any given project. This paper shows that it is possible for geosynthetics to meet design requirements that are different from those of solid waste landfills, in which these materials have already been in use for more than 30 years. REFERENCES [1] Butalia, T.S. CCP in Constructed Landfills: Characteristics, Beneficial Use, Disposal & Impact on Geocomposite Leachate Collection Systems, GSE Technical Seminar, 18 October 2012, Chantilly, VA. [2] Lin, L.C., and Benson, C.H. Effect of Wet-Dry Cycling on Swelling and Hydraulic Conductivity of GCLs, Journal of Geotechnical and Geoenvironmental Engineering January 2000, pp

12 [3] Jo, H.Y., Katsumi, T., Benson, C.H., and Edil, T.B. Hydraulic Conductivity and Swelling of Nonprehydrated GCLs permeated with Single-Species Salt Solutions, Journal of Geotechnical and Geoenvironmental Engineering, July 2001, pp [4] Kolstad, D.C., Benson, C.H., and Edil, T.B. Hydraulic Conductivity and Swell of Nonprehydrated Geosynthetic Clay Liners Permeated with Multispecies Inorganic Solutions, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 130, No. 12, December 2004, pp [5] EPRI. Characterization of Field Leachates at Coal Combustion Product Management Sites. Electric Power Research Institute [6] Forget, B. Rollin, A.L., and Jacquelin, T. Lessons Learned From 10 Years of Leak Detection Surveys on Geomembranes, [7] Laine, D.L. and Darilek, G.T. Locating Leaks in Geomembrane Liners of Landfills Covered with Protective Soil, Geosynthetics 97 Conference, 1997, pp [8] Laine, D.L and Mikas, M.P. Detection and Location of Leaks in Geomembrane Liners Using an Electrical Method: Case Histories, Proceedings of the 10 th National Conference, Superfund 89, 1989, pp