Geosynthetic Durability• Changes in mechanical behavior over service life (“How long will my geosynthetic last?”) ⇒ a key question• Causes? – Installation damage – Time-dependant mechanisms (biological, UV, hydrolysis or other chemical)• Investigation? – Standardized tests for effects of damage or degradation – Studies of exhumed geosynthetics
Laboratory Testing Program• Main goal: characterize combined effects of installation damage and degradation on strength and stress-strain behavior• Wide width tensile tests (ASTM D 4595)• Compare exhumed (~30 yr in-service) to virgin
Laboratory Testing Program• Exhumed geotextile sample: 1.5-m by 1.0-m• Free of surficial soil; some embedded particles• Selvage present• Relatively intact; several large tears Exhumed sample, 2001 (installed 1972)
Laboratory Testing Program• No WWT in 1972• Virgin geotextile sample• Same multifilament woven polyester, obtained and stored since 1974 Virgin sample, 1974• Selvage present
Laboratory Testing Program• Tests performed in machine direction• Curtis “Sure-Grip” hydraulic clamps• Jaws padded (tongue- depressors and 0.3-mm latex)• Strain rate: 10% / minute• Strains measured globally (LVDTs on clamps)
Laboratory Test Results• 18 total virgin tests• 9 exhumed tests (limited material)• No evidence of slippage in the jaws• No exhumed specimens ruptured at the jaws• One virgin specimen ruptured at jaws (omitted)
Laboratory Test Results Wide width tensile test results Mean 10% Mean yield Mean Mean offset secant tensile elongation at tensile tensile strength rupture modulusSample modulus (lb/in.) (%) (lb/in.) (lb/in.) [C.V.] [C.V.] [C.V.] [C.V.] (%) (%) (%) (%) 1972 334 21.9 2060 1230 Virgin [3.7] [3.8] [13.0] [3.8](17 tests) 2001 149 15.4 1390 1010Exhumed [17.9] [13.7] [16.9] [7.1](9 tests) >50% reduction ~33% reduction
Laboratory Test Results Wide width tensile test results Mean 10% Mean yield Mean Mean offset secant tensile elongation at tensile tensile strength rupture modulusSample modulus (lb/in.) (%) (lb/in.) (lb/in.) [C.V.] [C.V.] [C.V.] [C.V.] (%) (%) (%) (%) 1972 334 21.9 2060 1230 Virgin [3.7] [3.8] [13.0] [3.8](17 tests) 2001 149 15.4 1390 1010Exhumed [17.9] [13.7] [16.9] [7.1](9 tests)
Conclusions: Tensile Strength• Mean yield tensile strength reduced by >50% over the service life• No sample retrieved after installation ⇒ effects of installation damage and degradation cannot be separated. (View as a cumulative effect)
Conclusions: Tensile Strength• Long-term geotextile tensile strength typically reduced in practice (Elias et al. 2001): Tult Tal = RFCR ⋅ RFD ⋅ RFID where : Tal = allowable long - term tensile strength Tult = yield tensile strength RFCR = creep reduction (1.6 ~ 2.5 for PET) RFD = durability reduction (1.1 ~ 2.0) RFID = installation reduction (1.05 ~ 3.0)• Durability and installation damage reductions warranted in this case
Conclusions: Stress-Strain• Reduction of almost 33% in offset tensile modulus (represents initial elastic modulus)• Installation damage and degradation cannot be separated, must view as a cumulative effect• Significant softening of the reinforcement
Conclusions: Variability• Behavior of exhumed sample much more variable than the uninstalled sample• Evidenced by the computed coefficients of variation• Likely due to inherent spatial variability in the damage and/or degradation processes
Acknowledgements• Bob Holtz• Per Riise, geotechnical engineer with Jacobsson & Widmark for retrieving the sample• Rainier Massarch, geotechnical consultant, for transporting the sample• Gunnar Löngårdh of AB Fodervävnader, manufacturer of the geotextile; supplied the virgin sample in 1974
References and Related Mat’lNol Embankment Case History• Harney, M.D., 2006, “Mechanical properties of geotextile reinforcement, 30 years after installation,” Proceedings of the 8th International Conference on Geosynthetics, Yokohama, 18-22 September 2006.• Holtz, R.D., and Massarch, K.R., 1976, “Improvement of the Stability of an Embankment by Piling and Reinforced Earth,” Proceedings of the Sixth European Conference on Soil Mechanics and Foundation Engineering, Vienna, Vol. 1.2, pp. 473-478.• Holtz, R.D., and Massarch, K.R., 1993, “Geotextile and Relief Piles for Deep Foundation Improvement Embankment Near Göteborg, Sweden,” Geosynthetics Case Histories, International Society for Soil Mechanics and Foundation Engineering, pp. 168-169.
References and Related Mat’lReports, etc• Elias, V., 2001, “Long-term Durability of Geosynthetics Based on Exhumed Samples from Construction Projects,” Report No. FHWA- RD-00-157, U.S. Department of Transportation, Federal Highway Administration.• Elias, V., Christopher, B.R., and Berg, R.R., 2001, “Mechanically- Stabilized Earth Walls and Reinforced Soil Slopes Design and Construction Guidelines,” Report No. FHWA-NHI-00-043, U.S. Department of Transportation, Federal Highway Administration.
Questions? The Godfather?• Michael Harney, University of Washington, email@example.com• Ditto, Shannon & Wilson, firstname.lastname@example.org