geotextiles in coastal revetments

8/12/2019 Geotextiles in Coastal Revetments

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Geotextiles in Coastal RevetmentsGeotechnical Considerations

Lex Nielsen

September 2009


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Section HeaderSub-heading

Stockton Surf Club sandbag revetment

Stockton Beach rock revetment

STORM 9THMarch 2001


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Stockton Surf Club Sandbag Seawall

Soil Filters

IEAust Maritime Panel - Geotextiles in Coastal Revetments


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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 ()



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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 layers 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 (sg)

GTX/GTX (gg)

1. Manufacturers Recommendations

2. Soil Filters testing (Terrafax 1200R)

3. Literature review



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Sand/GTX (sg)Manufacturers Recommendations

Exxon:

tan sg= CI tans

CI = 0.70.8

tan sg= 0.7 to 0.8 tans

For s = 3235

sg= 2429



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Sand/GTX (sg)Geofabrics-Elco



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Geofabrics-ElcoSand/Terrafix 1200R

sg= 31


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Sand/GTX (sg)

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 ofSlippage 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 strengthby torsional ring shear tests, Geotextiles and Geomembranes16 pp161-174.



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Sand/NWNPGTX Friction Angle

Dixon, 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 n=10kPa):

sg= 27= 34.5

Low Confining Stress Data

= 27.1



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Koerner, GR & D Narejo (2005)



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Sand/NWNPGTX Friction AngleMartin Koerner & Whitty (1984)

Present two test results:

30for Ottawa sand

26for concrete sand



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Sand/NWNPGTX Friction AngleWilliams & 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 s = 3235

sg = 2932



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Tan, SA, SH Chew & WK Wong (1998)

Peak = 33

Residual = 26



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Summary sgSource Typical Result

Manufacturer Exxon 2429

Geofabrics Elco 31

Dixon, Jones & Fowmes 27

Koerner & Narejo 23

Martin Koerner & Whitty 2630

Williams & Houlihan 2932

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 = 3.1

Residual: 1.0/tan20 = 2.7 1.5/tan20 = 4.1



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From Coghlan, Carley, Cox

Blacka, Mariani, Restall,

Hornsey & Sheldrick, Coasts &

Ports 2009


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From Coghlan, Carley, Cox, Blacka, Mariani, Restall, Hornsey & Sheldrick, Coasts & Ports 2009


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Section Header

Sub-heading

Sandbag/Geotextile Friction?



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35

3020

1 : 1.5

1 : 2

1 : 3

terrafix vs Sand

Woven vs Sand

Standard Slope

Typical Angles

Infinite slopes - For Factor of Safety = 1.0

i.e. slope = 1/tan

For sand, = 33

1/tan= 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 sg = 29, FoS = 2 tan29 = 1.1

Rock 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 toe

Sensitivity of FoS to Slope and adopted sgSlope 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:

sg = 27

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



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Section Header

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geotextiles in coastal revetments

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