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

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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)

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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

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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°

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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

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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?

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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

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Coastal RevetmentsCoastal revetments will have factors of safety higher than those for infinite slopes because of:• Finite height• Toe

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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

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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

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