Geotextiles in Coastal Revetments Geotechnical Considerations Lex Nielsen September 2009 - PowerPoint PPT Presentation

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Geotextiles in Coastal Revetments Geotechnical Considerations Lex Nielsen September 2009

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  1. Geotextiles in Coastal RevetmentsGeotechnical Considerations Lex NielsenSeptember 2009

  2. Stockton Surf Club sandbag revetment Stockton Beach rock revetment Section HeaderSub-heading STORM 9TH March 2001

  3. IEAust Maritime Panel - Geotextiles in Coastal Revetments Stockton Surf Club Sandbag Seawall Soil Filters

  4. IEAust Maritime Panel - Geotextiles in Coastal Revetments Stockton Seawall- PWD Design

  5. IEAust Maritime Panel - Geotextiles in Coastal Revetments Soil Mechanics 101Slope Stability Rock @ 35° Sand @ 28°

  6. IEAust Maritime Panel - Geotextiles in Coastal Revetments Soil Mechanics 101 Internal Friction (Φ)

  7. IEAust Maritime Panel - Geotextiles in Coastal Revetments Soil Mechanics 101Shear strength

  8. IEAust Maritime Panel - Geotextiles in Coastal Revetments Soil Mechanics 101Typical shear strengths / stable infinite slopes

  9. IEAust Maritime Panel - Geotextiles in Coastal Revetments Revetment Structure The revetment structures comprise several layers • Armour layer • Rock, Concrete units, sandbags • Underlayer • Rock • Sandbag • Separation layer • Rock filters • Geotextile • Base layer • Rock core • Dune sand • Soil embankment The slope stability of each layer’s interface needs to be examined in respect of inter-facial frictional strength

  10. IEAust Maritime Panel - Geotextiles in Coastal Revetments Geotextile Interface Frictional PropertiesSand/GTX (Φsg) GTX/GTX (Φgg) Manufacturers Recommendations Soil Filters testing (Terrafax 1200R) Literature review

  11. IEAust Maritime Panel - Geotextiles in Coastal Revetments Sand/GTX (Φsg) Manufacturers Recommendations • Exxon: • tan Φsg = CI × tanΦs • CI = 0.7 – 0.8 • tan Φsg = 0.7 to 0.8 × tanΦs • For Φs = 32° – 35° • Φsg = 24° – 29°

  12. IEAust Maritime Panel - Geotextiles in Coastal Revetments Sand/GTX (Φsg) Geofabrics-Elco

  13. Geofabrics-ElcoSand/Terrafix 1200R Φsg = 31°

  14. IEAust Maritime Panel - Geotextiles in Coastal Revetments Sand/GTX (Φsg) Literature review Dixon, N., DRV Jones & GJ Fowmes (2006). “Interface shear strength variability and its use in reliability-based landfill stability analysis”, Geosynthetics International, vol. 13, no. 1, pp 1-14.  Koerner, GR & D Narejo (2005). Direct Shear Database of Geosynthetic-to-Geosynthetic and Geosynthetic-to-Soil Interfaces, Geosynthetic Research Institute, GRI Report No. 30, June.  Martin JP, RM Koerner & JE Whitty (1984). “Experimental friction Evaluation of Slippage between Geomembranes, Geotextiles and Soils”, Proc. Int. Conf. Geomembranes, Denver, USA pp 191-196.  Williams, ND & MF Houlihan (1987). “Evaluation of Interface Friction Properties between Geosynthetics and Soils”, Proc. of Geosynthetics ’87 Conference, New Orleans, USA, February, pp 616-627. Tan, SA, SH Chew & WK Wong (1998). “Sand-geotextile interface shear strength by torsional ring shear tests”, Geotextiles and Geomembranes 16 pp161-174.

  15. IEAust Maritime Panel - Geotextiles in Coastal Revetments Sand/NWNPGTX Friction AngleDixon, Jones & Fowmes (2006)

  16. IEAust Maritime Panel - Geotextiles in Coastal Revetments Sand/NWNPGTX Friction AngleDixon, Jones & Fowmes (2006) Low Confining Stress Data For low confining stress data (10kPa-30kPa), adopting a characteristic value being the mean minus 1.0  SD, ensuring about 90% of the data lie above the value, results in (for σn=10kPa): sg = 27 = 34.5 = 27.1

  17. IEAust Maritime Panel - Geotextiles in Coastal Revetments Sand/NWNPGTX Friction Angle Koerner, GR & D Narejo (2005)

  18. IEAust Maritime Panel - Geotextiles in Coastal Revetments Sand/NWNPGTX Friction Angle Martin Koerner & Whitty (1984) • Present two test results: • 30 for Ottawa sand • 26 for concrete sand

  19. IEAust Maritime Panel - Geotextiles in Coastal Revetments Sand/NWNPGTX Friction Angle Williams & Houlihan (1987) • Advocate the use of a large shear box (305 mm  305 mm). • The CI was around 0.9 for needle-punched geotextile with clean sands. • For Φs = 32° – 35° • Φsg = 29° – 32°

  20. IEAust Maritime Panel - Geotextiles in Coastal Revetments Sand/NWNPGTX Friction Angle Tan, SA, SH Chew & WK Wong (1998) Peak = 33° Residual = 26°

  21. IEAust Maritime Panel - Geotextiles in Coastal Revetments Summary Φsg 1/ tan 27.5° = 1.9; 1.5/tan 27.5° = 2.9

  22. IEAust Maritime Panel - Geotextiles in Coastal Revetments NWNPGTX/NWNPGTX Friction Angle Geofabrics-Elco Peak: 1.0/tan26° = 2.0 1.5/tan26° = 3.1 Residual: 1.0/tan20° = 2.7 1.5/tan20° = 4.1

  23. NWNPGTX/NWNPGTX Friction Angle From Coghlan, Carley, Cox Blacka, Mariani, Restall, Hornsey & Sheldrick, Coasts & Ports 2009

  24. NWNPGTX/NWNPGTX Friction Angle From Coghlan, Carley, Cox, Blacka, Mariani, Restall, Hornsey & Sheldrick, Coasts & Ports 2009

  25. IEAust Maritime Panel - Geotextiles in Coastal Revetments Section HeaderSub-heading Sandbag/Geotextile Friction?

  26. IEAust Maritime Panel - Geotextiles in Coastal Revetments Infinite slopes - For Factor of Safety = 1.0 i.e. slope = 1/tanΦ For sand, Φ = 33° 1/tanΦ = 1.5 Sand slope 1:1.5 NWNPGTX on sand 1:2.0 NWNPGTX on NWNPGTX 1:2.7 33° 27°

  27. IEAust Maritime Panel - Geotextiles in Coastal Revetments Coastal Revetments • Coastal revetments will have factors of safety higher than those for infinite slopes because of: • Finite height • Toe

  28. IEAust Maritime Panel - Geotextiles in Coastal Revetments

  29. IEAust Maritime Panel - Geotextiles in Coastal Revetments On an Infinite 2:1 slope for Φsg = 29°, FoS = 2 × tan29° = 1.1 Rock toe improves FoS from 1.1 to 1.5 Toe is critical for stability

  30. IEAust Maritime Panel - Geotextiles in Coastal Revetments Small Rock Revetment on Sand 4m High with toeSensitivity of FoS to Slope and adopted Φsg Slope 1:2.5 Slope 1:2.0

  31. IEAust Maritime Panel - Geotextiles in Coastal Revetments Small Rock Revetment on Sand 4.0m High with toeAlternate (preferred) design

  32. IEAust Maritime Panel - Geotextiles in Coastal Revetments Conclusions • For preliminary text-book design of revetments using non-woven needle-punched geotextiles, recommend adopting: • Φsg = 27°Φgg = 20° • For the use of geotextile underneath a rock armoured revetment, GTX/sand interface slope must be flatter than 1:2.5 for an adequate factor of safety for overall geotechnical stability • For overall geotechnical stability of sandbag armoured revetment slopes in sand, slopes must be flatter than 1:3.0

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