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Geotextiles in Coastal Revetments Geotechnical Considerations Lex Nielsen September 2009.

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Presentation on theme: "Geotextiles in Coastal Revetments Geotechnical Considerations Lex Nielsen September 2009."— Presentation transcript:

1 Geotextiles in Coastal Revetments Geotechnical Considerations Lex Nielsen September 2009

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

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

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

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

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

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

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

9 Revetment Structure 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 The revetment structures comprise several layers IEAust Maritime Panel - Geotextiles in Coastal Revetments

10 Geotextile Interface Frictional Properties Sand/GTX (Φsg) GTX/GTX (Φgg) 1.Manufacturers Recommendations 2.Soil Filters testing (Terrafax 1200R) 3.Literature review IEAust Maritime Panel - Geotextiles in Coastal Revetments

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

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

13 Geofabrics-Elco Sand/Terrafix 1200R Φ sg = 31°

14 Sand/GTX (Φ sg ) Literature review 1.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 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. 3.Martin JP, RM Koerner & JE Whitty (1984). “Experimental friction Evaluation of Slippage between Geomembranes, Geotextiles and Soils”, Proc. Int. Conf. Geomembranes, Denver, USA pp Williams, ND & MF Houlihan (1987). “Evaluation of Interface Friction Properties between Geosynthetics and Soils”, Proc. of Geosynthetics ’87 Conference, New Orleans, USA, February, pp Tan, SA, SH Chew & WK Wong (1998). “Sand-geotextile interface shear strength by torsional ring shear tests”, Geotextiles and Geomembranes 16 pp IEAust Maritime Panel - Geotextiles in Coastal Revetments

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

16 Sand/NWNPGTX Friction Angle Dixon, Jones & Fowmes (2006) 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 σ n =10kPa):  sg = 27   = 34.5  Low Confining Stress Data  = 27.1  IEAust Maritime Panel - Geotextiles in Coastal Revetments

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

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

19 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° IEAust Maritime Panel - Geotextiles in Coastal Revetments

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

21 Summary Φ sg SourceTypical Result Manufacturer Exxon 24° – 29° Geofabrics Elco 31° Dixon, Jones & Fowmes 27° Koerner & Narejo 23° Martin Koerner & Whitty 26° – 30° Williams & Houlihan 29° – 32° Tan, Chew & Wong 26° Average27.5 1/ tan 27.5° = 1.9; 1.5/tan 27.5° = 2.9 IEAust Maritime Panel - Geotextiles in Coastal Revetments

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

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 Section Header Sub-heading Sandbag/Geotextile Friction? IEAust Maritime Panel - Geotextiles in Coastal Revetments

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

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

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29 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 IEAust Maritime Panel - Geotextiles in Coastal Revetments

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

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

32 Conclusions For preliminary text-book design of revetments using non-woven needle-punched geotextiles, recommend adopting: Φ sg = 27° Φ gg = 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 IEAust Maritime Panel - Geotextiles in Coastal Revetments

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34 Section Header Sub-heading IEAust Maritime Panel - Geotextiles in Coastal Revetments

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