Technical Note by G.L. Sivakumar Babu, H. Sporer, H. Zanzinger, and E. Gartung SELF-HEALING PROPERTIES OF GEOSYNTHETIC CLAY LINERS

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1 Technical Note by G.L. Sivakumar Babu, H. Sporer, H. Zanzinger, and E. Gartung SELF-HEALING PROPERTIES OF GEOSYNTHETIC CLAY LINERS ABSTRACT: The sealing effect and containment of moisture in landfill covers and liners containing geosynthetic clay liners (GCLs) are largely influenced by moisture retention, swelling, and the self-healing behaviour of bentonite used in GCLs. The method of binding of GCLs also significantly affects the behaviour. To examine these aspects, percent swell and swell pressure tests under different effective pressures were conducted on two sodium-based GCLs. Permittivity tests were performed on GCL specimens with different hole diameters and also on specimens dried at 20 and 60 o C. The test results are presented and discussed. The study shows that the GCLs considered have a good self-healing capacity for the case of desiccation cracks or punctures. In addition, the method of binding the GCL components, i.e., stitch-bonded or needlepunched, has a significant influence. KEYWORDS: Geosynthetic clay liner, Self healing, Swell, Bentonite, Permittivity test. AUTHORS: G.L. Sivakumar Babu, Assistant Professor, Department of Civil Engineering, Indian Institute of Science, Bangalore, India, , Telephone: 91/ , Telefax: 91/ , gls@civil.iisc.ernet.in; and H. Sporer, Telephone: 49/ , Telefax: 49/ , gbsp@lga.de, H. Zanzinger, Telephone: 49/ ; Telefax: 49/ , helmut.zanzinger@lga.de, and E. Gartung, Telephone: 49/ ; Telefax: 49/ , gbgg@lga.de, LGA (Grundbauinstitut), Tillystr. 2, D Nuremberg, Germany. PUBLICATION: Geosynthetics International is published by the Industrial Fabrics Association International, 1801 County Road B West, Roseville, Minnesota , USA, Telephone: 1/ , Telefax: 1/ Geosynthetics International is registered under ISSN DATE: Original manuscript submitted 25 September 2000, revised version received 26 August 2001, and accepted 6 September Discussion open until 1 June REFERENCE: Sivakumar Babu, G.L., Sporer, H., Zanzinger, H., and Gartung, E., 2001, Self-Healing Properties of Geosynthetic Clay Liners, Geosynthetics International, Vol. 8, No. 5, pp INTRODUCTION Geosynthetic clay liners (GCLs) are being used for many sealing applications such as landfills and surface impoundments. For landfill cover and liner systems, sufficient capacity for containment of moisture and the self-healing properties enabling closure GEOSYNTHETICS INTERNATIONAL 2001, VOL. 8, NO

2 of punctures or desiccation cracks due to temperature differentials are necessary. Hydration or swelling behavior of bentonite used in GCLs largely influences containment as well as the sealing effect of GCLs. The capacity for containment of moisture is reflected in terms of moisture absorption capacity under load. For example, Daniel et. al (1993) performed hydration tests on a GCL; sand layer combinations under a 14 kpa effective pressure showed that bentonite is effective for water uptake, and water content was in the range of 193% from an initially dry state (wilting point). Bonaparte et al. (1996) performed hydration tests on three different GCL products in contact with compacted clay and reported that after 75 days, the water content of the GCLs was in the range of 90%. Melchior (1997), using field studies, observed high leakage rates in a final cover test site containing a GCL and reported that reduced swell capacity of the GCL was insufficient to seal preferential flow paths formed due to desiccation. The swelling behavior, apart from mineralogical and physico-chemical characteristics, is also a function of the binding method of GCL components. The objective of the current technical note is to examine and address the following issues: 1. How can the self-healing capacity of GCLs be assessed in quantitative terms? 2. Is there a link between swelling behavior and self-healing properties of GCLs that indicates to what extent cracks close? 3. How does the method of binding GCL components influence self healing? To assess the self-healing capacity of GCLs, percent swell tests or direct measurements of hydraulic conductivity or permittivity are helpful. Shan and Daniel (1991) examined the first two issues to some extent and Rowe and Lake (1999) contributed to some understanding of the third issue. For example, Shan and Daniel (1991) examined the self-healing properties of bentonite by making three holes with different diameters (12, 25, and 75 mm) in 152 mm-diameter specimens and allowed the specimens to rehydrate under a confining stress of 14 kpa. The measured hydraulic conductivity values indicated: (i) the bentonite rehydrated to fill the 12 and 25 mm-diameter holes; (ii) there was no significant change in hydraulic conductivity; and (iii) the hydraulic conductivity values were in the range of m/s. However, for the 75 mm-diameter hole, hydraulic conductivity increased to m/s. Rowe and Lake (1999) showed that the final void ratios in a needle-punched GCL are much less than the corresponding values in a similar GCL without needle punching at the same effective stress. Temperature ranges in a landfill significantly affect the performance of landfills. Rowe (1998) presented a review of typical temperature variations in different locations of the landfill under various operating conditions and indicated that temperatures ranging from 14 to 70 o C were reported in landfills in Europe and North America. Temperatures close to 20 o C were prevalent at the upper levels and 60 to 70 o C near gas venting systems and at lower levels. In view of the increased use of GCLs in landfills, additional information with regard to the self-healing capacity of GCLs, in the case of punctures or desiccation cracks due to temperature differentials, is desirable and is addressed in the current technical note. For the current study, percent swell and swell pressure tests under different effective pressures on two sodium-based GCLs were conducted. Permittivity tests were performed on GCL specimens with different hole 462 GEOSYNTHETICS INTERNATIONAL 2001, VOL. 8, NO. 5

3 diameters and dried at 20 and 60 o C. All of the test results are presented and discussed. 2 EXPERIMENTAL PROGRAMME 2.1 Basic Properties of GCLs The index properties of GCLs, i.e., mass per unit area, swelling volume, water adsorption, and montmorillonite content, were determined following ASTM D 5993, ASTM D 5890, ASTM D 5891, and methylene blue adsorption tests (Egloffstein 1995; Gartung and Zanzinger 1998), respectively. The properties of the tested GCLs are given in Table Swell Pressure Tests Type 1 and Type 2 sodium bentonite were used in stitch-bonded GCL specimens and Type 3 sodium bentonite was used in needle-punched GCL specimens. Swell pressure tests under different effective stresses were conducted on GCL specimens in oedometers in accordance with ASTM D 4546 to determine the magnitude of swell under known vertical pressures. Specimens were set up under a seating pressure of 2 kpa, deionised water (ph of 7) was used, and the corresponding swell pressures that developed were monitored. The amount of percent swell at different effective stresses was measured. The results of the tests are presented in Figure 1. Swell pressure tests were also conducted on GCL specimens, whereby specimens were allowed to swell completely and loads were subsequently applied. Tests were performed on 100 mm-diameter specimens, cut from the GCL materials. Care was taken to ensure that no bentonite was lost in trimming and specimen handling. Specimens were set up under a seating pressure of 2 kpa and deionised water (ph of 7) was allowed and the corresponding swell pressures that developed were monitored. Each specimen was loaded gradually over time and the specimen was brought back to the original thickness. The corresponding pressure for each case was obtained. Experiments were conducted on stitch-bonded GCLs (with and without stitches) and needlepunched GCLs (with and without fibers). The results are presented in Figures 2 and 3. Table 1. Physical properties of the tested GCLs. Type of GCL Bulk density (g/cm³) Swell volume (ml) Water absorption (%) Montmorillonite content (%) Stitch bonded Sodium bentonite, Type1 Sodium bentonite, Type Needle punched Sodium bentonite, Type GEOSYNTHETICS INTERNATIONAL 2001, VOL. 8, NO

4 (a) Final water content (%) Water absorption test under load Sodium bentonite Type 1 Sodium bentonite Type (b) Effective stress (kpa) 110 Percent swell (%) Percent swell under load Sodium bentonite Type 1 Sodium bentonite Type Effective stress (kpa) Figure 1. Results of swell pressure tests: (a) final water content versus effective stress; (b) percent swell recorded under different effective stresses. 464 GEOSYNTHETICS INTERNATIONAL 2001, VOL. 8, NO. 5

5 GCL needle-punched, sodium bentonite 2 kpa, thickness = 8.62 mm, GCL threads removed 2 kpa, thickness = 8.75 mm, GCL threads intact 20 kpa, thickness = 7.46 mm, GCL threads removed 20 kpa, thickness = 7.35 mm, GCL threads intact Thickness (mm) % 61.8% 44.0% 38.2% Figure Time (hours) Variation of thickness with time for a needle-punched sodium bentonite GCL % 68.34% Thickness (mm) GCL stitch-bonded, sodium bentonite 2 kpa, thickness = 7.96 mm, GCL without stitches 2 kpa, thickness = 8.10 mm, GCL with stitches 20 kpa, thickness = 6.66 mm, GCL without stitches 20 kpa, thickness = 6.65 mm, GCL with stitches 24.77% 22.25% Time (hours) Figure 3. Variation of thickness with time for a stitch-bonded sodium bentonite GCL. GEOSYNTHETICS INTERNATIONAL 2001, VOL. 8, NO

6 2.3 Permittivity Tests Percent swell values are useful to assess the self-healing capacity of GCLs indirectly. At the same time, direct measurements of permeability variations in terms of permittivity are useful to directly assess to what extent punctures/holes formed, or desiccation cracks, as a result of temperature differentials, self heal. With regard to evaluation of permittivity, Gartung and Zanzinger (1998) suggest that a rigid-wall permeameter be employed (following DIN ) for determination of permittivity of GCLs and, in particular, when they are likely to have minor cracks, fissures, or macropores. Hence, in the current study, permittivity tests on GCLs were conducted in rigid wall permeameters. The tests were conducted on needle-punched GCL specimens with circular holes 6, 15, 30, and 55 mm in diameter. The diameter of the holes represent the extent of damage in the form of punctures or tears in field conditions. Tests were also conducted on GCL specimens dried at 20 and 60 o C, which represent the variation of temperatures in the landfill body. A rigid-wall permeability cell was used to determine the permittivity of GCLs. The 145 mm-diameter GCL specimen was gently placed between two 10 mm-thick metallic filter plates installed into the permeability cell. A cover stress of 20 kpa was applied and the specimens were allowed to swell in deionised water. The flow rate and permittivity were measured at a 300 mm differential head. The ambient temperature was taken into account. The permittivity test results are presented in Figures 4 and 5. 3 TEST RESULTS AND DISCUSSION 3.1 Swell Pressure Tests Figure 1a shows the relationship between final water content and effective stress for sodium bentonite, stitch-bonded GCLs. It can be observed that at low effective pressures, the water retention is quite high, to the extent of 140% corresponding to 20 kpa, suggesting that the GCL specimens have a good moisture absorption capacity. Figure 1b shows percent swell recorded under different effective stresses. Increase in effective stress reduces the percent swelling. Corresponding to 20 kpa, the percent swell is in the range of 40 to 50%. The values obtained correspond to the one-dimensional condition. In the field, lateral swelling enables the closure of cracks that formed due to desiccation. In the absence of a proper relationship between one-dimensional vertical heave and the lateral heave as a function of overburden, percent swell values from onedimensional conditions can be considered an indication of the self-healing potential or capacity of the GCL under a known overburden pressure. Figure 2 shows test results for a sodium bentonite, needle-punched GCL for effective stresses of 2 and 20 kpa with fibers intact and fibers removed. The initial thickness of the specimens is also reported. At 48 hours (two days) and 2 kpa, the percent swell with respect to initial thickness is % (( )/8.62) with fibers removed and 61.8% (( )/8.75) with fibers intact. At 20 kpa, the. effect is less pronounced, 466 GEOSYNTHETICS INTERNATIONAL 2001, VOL. 8, NO. 5

7 10-6 Permittivity 10-7 Diameter of holes 6mm 15 mm 30 mm 55 mm 10-8 Figure Time (days) Results of permittivity tests corresponding to different hole diameters Permittivity 10-8 Drying temperature of sodium bentonite GCLs 20 C 60 C Time (days) Figure 5. Results of permittivity tests for sodium bentonite GCLs dried at 20 and 60 o C. GEOSYNTHETICS INTERNATIONAL 2001, VOL. 8, NO

8 although the effect of fiber removal is evident. The values are 44 and 38%, respectively, corresponding to specimens with fibers removed and those with fibers intact. Figure 3 shows the results for stitch-bonded sodium GCLs (Type 3 bentonite). It can be observed that the removal of stitches did not influence the change in thickness for both stresses. For example, for 48 hours and 2 kpa, the percent swell is in the range of 70.8% (with stitches) and 68.34% (without stitches). For 20 kpa, the corresponding values are 24.7% (without stitches) and 22.3% (with stitches). 3.2 Permittivity Tests The performance of the GCL seal can be directly quantified by its permittivity (Gartung and Zanzinger 1998). Figure 4 presents the results of permittivity tests corresponding to different hole diameters. It can be observed that for 6, 15, and 30 mmdiameter holes, the permittivity is in the range of to s -1 after three days of permeation. However, for 55 mm, the initial permittivity values are high at the end of one day and, ultimately, these values converge to values in the range of 10-8 s -1. The results suggest that 30 mm-diameter punctures, or less, may not pose significant problems and self healing is likely, but for large holes or tears, it is unlikely, which reinforces similar observations made by Shan and Daniel (1993). To examine the influence of temperature on the variation of percent swell, GCL specimens were initially dried at temperatures of 20 and 60 o C, which represent typical temperatures in the landfill body. After subjecting the specimens to drying at the required temperature, specimens were placed in a permeameter set-up and tests were conducted. Figure 5 shows the permittivity test results for sodium bentonite GCLs dried at 20 and 60 o C. The results show that for the specimen dried at 60 o C, the permittivity values are high up to, approximately, 4 days, but subsequently the values are less than 10-8 s CONCLUDING REMARKS The current technical note addresses the aspects of GCL self healing of holes or cracks under the influence of temperature and the method of GCL binding. Swell pressure, percent swell tests under simulated loading conditions in oedometer tests, and permittivity tests in a rigid wall permeameter were conducted. The results show that needlepunched GCLs perform better than stitch-bonded GCLs, and the tested GCLs have good self-healing properties. ACKNOWLEDGEMENTS The authors thank the Alexander von Humholdt Foundation, Bonn, Germany, for financial support and the authorities of LGA for providing the facilities to carry out the work. 468 GEOSYNTHETICS INTERNATIONAL 2001, VOL. 8, NO. 5

9 REFERENCES ASTM D 4546, Standard Test Methods for One-Dimensional Swell or Settlement Potential of Cohesive Soils, American Society for Testing and Materials, West Conshohocken, Pennsylvania, USA. ASTM D 5890, Standard Test Method for Swell Index of Clay Mineral Component of Geosynthetic Clay Liners, American Society for Testing and Materials, West Conshohocken, Pennsylvania, USA. ASTM D 5891, Standard Test Method for Fluid Loss of Clay Component of Geosynthetic Clay Liners, American Society for Testing and Materials, West Conshohocken, Pennsylvania, USA. ASTM D 5993, Standard Test Method for Measuring Mass Per Unit of Geosynthetic Clay Liners, American Society for Testing and Materials, West Conshohocken, Pennsylvania, USA. Bonaparte, R., Othman, M.A., Rad, N.S., Swan R.H., and vander Linde, D.L., 1996, Evaluation of Various Aspects of GCL Performance, Report of 1995 workshop on Geosynthetic Clay Liners, Daniel, D.E. and Scranton, H.B., Editors, pp. F1-F34. Daniel, D.E. Shan, H.Y., and Anderson, J.D., 1993, Effects of Partial Wetting on the Performance of the Bentonite Component of Geosynthetic Clay Liners, Proceedings of Geosynthetics 93, IFAI, Vol. 3, Vancouver, British Columbia, Canada, March 1993, pp Egloffstein, T., 1995, Properties and Test Methods to Assess Bentonite Used in Geosynthetic Clay Liners, Geosynthetic Clay Liners, Koerner, R.M., Gartung, E., and Zanzinger, H., Editors, Balkema, proceedings of an international symposium held in Nurnberg, Germany, April 1994, pp Gartung, E. and Zanzinger, H., 1998, Engineering Properties and Use of Geosynthetic Clay Liners, Geotechnical Engineering of Landfills, Thomas Telford, Dixon, N., Murray, E.J., and Jones, D.R.V., Editors, Proceedings of the symposium held at the Nottingham Trent University, Department of Civil and Structural Engineering on 24 September 1998, pp Melchior, S., 1997, In Situ Studies on the Performance of Landfill Caps, Proceedings of the International Containment Technical Conference US Dept. Of Energy, Germantown, Maryland, USA, pp Rowe, R.K., 1998, Geosynthetics and the Minimization of Contaminant Migration Through Barrier Systems Beneath Solid Waste, Proceedings of the Sixth International Conference on Geosynthetics, IFAI, Vol. 1, Atlanta, Georgia, USA, April 1998, pp Rowe, R.K and Lake, C.B., 1999, Geosynthetic Clay Liner Research, Design, and Applications, Proceedings of the Seventh International Waste Management and Landfill Symposium, Sardinia 99, Vol. 3, S. Margherita di Pula, Cagliari, Sardinia, Italy, October 1999, pp GEOSYNTHETICS INTERNATIONAL 2001, VOL. 8, NO

10 Shan, H.Y. and Daniel, D.E., 1991, Results of Laboratory Tests on Geotextile/Bentonite Liner Material, Proceedings of Geosynthetics 91, IFAI, Vol. 2, Atlanta, Georgia, USA, February 1991, pp GEOSYNTHETICS INTERNATIONAL 2001, VOL. 8, NO. 5