Geotextiles in Coastal Revetments Geotechnical Considerations Lex Nielsen September 2009
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Transcript of Geotextiles in Coastal Revetments Geotechnical Considerations Lex Nielsen September 2009
Geotextiles in Coastal RevetmentsGeotechnical Considerations Lex NielsenSeptember 2009
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Stockton Surf Club sandbag revetment
Stockton Beach rock revetment
STORM 9TH March 2001
Stockton Surf Club Sandbag Seawall
Soil Filters
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Stockton Seawall- PWD Design
IEAust Maritime Panel - Geotextiles in Coastal Revetments
Soil Mechanics 101
Slope Stability
Sand @ 28°
Rock @ 35°
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Soil Mechanics 101Internal Friction (Φ)
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Soil Mechanics 101Shear strength
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Soil Mechanics 101Typical shear strengths / stable infinite slopes
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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
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Geotextile Interface Frictional PropertiesSand/GTX (Φsg)
GTX/GTX (Φgg)
1. Manufacturers Recommendations2. Soil Filters testing (Terrafax 1200R)3. Literature review
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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°
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Sand/GTX (Φsg)Geofabrics-Elco
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Geofabrics-ElcoSand/Terrafix 1200R
Φsg = 31°
Sand/GTX (Φsg)Literature review1. 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.
2. 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 191-196.
4. 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.
5. Tan, SA, SH Chew & WK Wong (1998). “Sand-geotextile interface shear strength
by torsional ring shear tests”, Geotextiles and Geomembranes 16 pp161-174.
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Sand/NWNPGTX Friction AngleDixon, Jones & Fowmes (2006)
IEAust Maritime Panel - Geotextiles in Coastal Revetments
Sand/NWNPGTX Friction AngleDixon, 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 σn=10kPa):
sg = 27
= 34.5
Low Confining Stress Data
= 27.1
IEAust Maritime Panel - Geotextiles in Coastal Revetments
Sand/NWNPGTX Friction Angle Koerner, GR & D Narejo (2005)
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Sand/NWNPGTX Friction Angle Martin Koerner & Whitty (1984)
Present two test results:
• 30 for Ottawa sand
• 26 for concrete sand
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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°
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Sand/NWNPGTX Friction Angle Tan, SA, SH Chew & WK Wong (1998)
Peak = 33°
Residual = 26°
IEAust Maritime Panel - Geotextiles in Coastal Revetments
Summary ΦsgSource Typical 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°
Average 27.5
1/ tan 27.5° = 1.9; 1.5/tan 27.5° = 2.9
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NWNPGTX/NWNPGTX Friction Angle Geofabrics-Elco
Peak: 1.0/tan26° = 2.0 1.5/tan26° = 3.1Residual: 1.0/tan20° = 2.7 1.5/tan20° = 4.1
IEAust Maritime Panel - Geotextiles in Coastal Revetments
NWNPGTX/NWNPGTX Friction Angle
From Coghlan, Carley, Cox Blacka, Mariani, Restall, Hornsey & Sheldrick, Coasts & Ports 2009
NWNPGTX/NWNPGTX Friction Angle
From Coghlan, Carley, Cox, Blacka, Mariani, Restall, Hornsey & Sheldrick, Coasts & Ports 2009
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Sandbag/Geotextile Friction?Sandbag/Geotextile Friction?
IEAust Maritime Panel - Geotextiles in Coastal Revetments
35°
30°20°
1 : 1.5
1 : 2
1 : 3
terrafix vs Sand
Woven vs Sand
Standard Slope
terrafix vs Sand
Woven vs Sand
Standard Slope
terrafix vs Sand
Woven vs Sand
Standard Slope
Typical Angles
High Elongation Geotextile Relies on interlock
Low Elongation Geotextile Relies on stacking
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
Coastal RevetmentsCoastal revetments will have factors of safety higher than those for infinite slopes because of:• Finite height• Toe
IEAust Maritime Panel - Geotextiles in Coastal Revetments
IEAust Maritime Panel - Geotextiles in Coastal Revetments
On an Infinite 2:1 slope for Φsg = 29°, FoS = 2 × tan29° = 1.1Rock toe improves FoS from 1.1 to 1.5 Toe is critical for stability
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Small Rock Revetment on Sand 4m High with toeSensitivity of FoS to Slope and adopted Φsg
Slope 1:2.5
Slope 1:2.0
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Small Rock Revetment on Sand 4.0m High with toeAlternate (preferred) design
IEAust Maritime Panel - Geotextiles in Coastal Revetments
ConclusionsFor 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
IEAust Maritime Panel - Geotextiles in Coastal Revetments
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