Geosynthetics and yReinforced Soil Structures

Geosynthetic Reinforced Pile Platforms

Dr. K. Rajagopalf f CProfessor of Civil Engineering

IIT Madras, Chennai, Indiae-mail: gopalkr@iitm.ac.in

Construction on Soft Foundation SoilProblems

(a) Slope instability (b) Unacceptable vertical settlements (From Lawson,2012)

(From Lawson,2012)

2(c) Localised differential settlements at

embankment surface(d) Difficulty to move the construction

equipment( Concept- Lawson,2012)

Methods of Ground Improvement

Soil Replacement Preloading Preloading Light Weight Fill

Preloading ith Vertical Drain Preloading with Vertical Drain Vacuum Preloading Stone Column-OSC,ESC Piled Raft Basal Reinforcement Piled Embankment Geosynthetic Reinforced Pile Supported

Embankment3

Geosynthetic Reinforced yPiled Embankments

Rail/Road embankmentRail/Road embankment

Soft clay

Pil I li dPiles Inclined Piles

Firm stratum

Advantages of Geosynthetic Reinforced Piled g yEmbankments

Faster construction-Loading rate not dependent on the rate of consolidation of soil

Eliminates differential settlements especially for large height embankments

Slope stability

Relatively small pile caps and no need for raking piles

Low long term maintenance costs

5

Embankment Piling

CFA (Continuous Flight Auger) piles

Load Transfer Platform at Second Severn Crossing13

Load Transfer Platform at Second Severn Crossing(Tensar, UK brochure)

Measured data fromMeasured data from Second Severn, UK(Tensar, UK brochures)

14

( )

Application areas

Bridge abutment approach roads (Buchanan 1984)(Buchanan,1984)

Airport runways (Hossain and Rao, 2005) Subgrade improvement (Han, 1975) Minimize differential settlements under storage

tanks (Alzamora et al. 2000) Segmental retaining wall (Alzamora et al. 2000) Widening of the existing roadway embankment

(Han and Gabr 2002) To construct confined embankment structures

(Lawson 2012)

15

Construction Sequence

Installing piles with certain grid formation in the soft soil up toa certain depth.

Geosynthetic material is laid on top of a thin layer (0.1 m) ofl t i lgranular material.

After placing the geosyntheticp g g ylayer, the embankment fill isconstructed to the requiredheight in stages.

Fi ll th t ti h Finally the construction such asrailway or road pavement isbuilt on top of the embankment

16

built on top of the embankment Geosynthetic Reinforced Piled Embankment System

Plan Layout of the Piles

L t ( ) S d (b) T i l(a) (b)

Layout (a) Square and (b) Triangular

Geosynthetic Layout

(a) (b)

17Optimal geosynthetic layout (a) direction of placing the layers and (b) direction of load

(Lawson,2012)

Load Transfer Mechanism

(b) Membrane action of geosynthetic(a) Soil Arching (b) Membrane action of geosynthetic(Russell and Pierpoint,1997)

(a) Soil Arching

( ) C t ti f t d th il d t th(c) Concentration of stresses around the pile due to thestiffness difference between the soft foundation soil and the rigid pile

18

soil and the rigid pile

Design Methods

(a) British Standard-BS8006:1995

This is the most widely used method and is very conservative This is the most widely used method and is very conservative.

Based on Marston’s (1913) formula for positive projectingased o a s o s ( 9 3) o u a o pos ve p ojec gconduits, Jones et al.(1990) developed an empirical relationshipfor the ratio of average vertical stress acting on the pile caps tothe average vertical stress acting across the base of theembankment .

where c cv

v

p C a HH

pc=Arched vertical stress on top of the pileσv=Average vertical stress on top of the pile

19

Cc= Arching Coefficient (Marston 1913)a = size of pile caps Positive Projecting Conduit

(Marston,1913)

BS8006 adopted Jones et al.(1990) for the design of piled b k tembankments.

BS8006 gives empirical equations for arching coefficient as follows

cCas follows

cEnd bearing piles,C 1.95 0.18Ha

cFriction piles,C 1.5 0.07

aHa

a

BS8006 considers two cases

1. Embankment height is below thecritical height of 1.4(s-a):

Arching is not fullydeveloped

Partial arching

20

Partial archingHere A= Load acting on the piles due to arching, B= Load taken by the geosynthetic andC= Load acting on the soft subsoil

For 0.7 1.4 ,s a H s a

2 22 2Load on the geosynthetic, fs q s c

Tv

s f H f w pW s as a

1Geosynthetic Tension T 1

v

TW s a

rGeosynthetic Tension, T 1

2 6where is the geosynthetic strain

f are the partial fact

a

f

ors used in the designfs f , are the partial factqf ors used in the design

b k h i h i b2. Embankment height is abovethe critical height of 1.4(s-a):

Full arching is developed

21 Full arching

Height of embankment above arching height plays no Height of embankment above arching height plays norole in the tension developed on the geosynthetic.

Same is the case with surcharge

For H>1 4 s a

For H>1.4 ,

1 4

s a

sf s a

2 22 2

1.4 fs cT

v

sf s a pW s as a

r

1Geosynthetic Tension, T 12 6

TW s a

2 6a

22

Horizontal force at the slope

,,

Horizontal force at the embankment slope after BS8006(Satibi,2009)

Geosynthetic tensile load needed to resist the horizontal f f th b k t i T

0.5 ( 2 )

hrs a fs qT K f H f q H

force of the embankment is Trs

a

where K Active lateral earth pressure coefficient

, = partial factors used in the designfs qf f

23

, p gfs qf f

(b) Hewlett and Randolph Method(1988)

This theory is based on limit state of soil inhemispherical domed region over piles.p g p

The stability of arch at the crown and at the pile top ofh h i h i l d d d i h ithe hemispherical dome formed defines the entire

stability.

24Hemispherical domes (Hewlett & Randolph, 1997)

Stress Reduction Ratio ( S3D ) defined as the ratio of the Stress Reduction Ratio ( S3D ) defined as the ratio of theaverage vertical stress acting on the reinforcement tothe overburden pressure due to the embankment fill was

1

used to check the stability.

3 1 2

2

1 at the crown of the arch2

1 1 1 11

pD k

pp

Sk a a a ak

k

2 1k

21 ppk s s s s

2 1

3

2 1 2 1 at the pile top 1 1

2 2 3 2 2 3

pkp p

Dp p

s k s a kaSs H k H k

- Largest value is the critical S3D

25

(c) The new German Method (EBGEO 2004)

In the old German approach the arching modeldeveloped by Hewlett and Randolph (1988) was used tocalculate the stresses generated due to arching.

EBGEO 2004 adopts the m lti shell arching theor EBGEO 2004 adopts the multi-shell arching theorybased on the work of Zaeske (2001).

26Multi shell arching theory adopted in New German Method

(Kempfert,2004)

3-dimensional soil element is considered and theequilibrium of forces about the radial direction is usedto calculate the vertical stress coming onto the soil,zo k

2

22 2, 1 1 2 1 1 24

gzo k g g g

hp h h h hh

, 4g g gh

where

2 2

221 21

2

where1 1 2 ( ), K=tan 45 , ,

2 8 8crit ka K s a s as a

s

In the second step the vertical stress acting on the top of,zo k

p g pthe subsoil is used to calculate the vertical load Fk onthe geosynthetic.

27

Load distribution on the geosynthetic for rectangular pile layout (Kempfert,2004)

2

x,k ,1 , F2 2 180

yLx x y Lx zo k

x

saA s s atn As

2

y,k ,1 , F2 2 180

xLy x y Ly zo k

y

saA s s atn As

28

y

The maximum strain k is obtained from thedimensionless design graphs (EBGEO 2004)

kJwLErsb

dimensionless design graphs (EBGEO, 2004).

Here,J t il tiff f thJk= tensile stiffness of the

geosynthetic (kN/m)Lw= (s-a)= pile clear spacingbErs= width of support

29(EBGEO,2004)

Horizontal force at the slope

The additional horizontal force in the reinforcement The additional horizontal force in the reinforcementbeneath the embankment slope is given by

1E E h k P h k ,

ah

2where K Active earth pressure coefficient

k ah k k k ahE E h k P h z k

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(d) The Dutch Method (CUR 226)

Introduced in 2009.

Adopts major parts of the German EBGEO 2004.

Flat terrain-thin embankments are constructed andtherefore the EBGEO method was modified to suit thetherefore the EBGEO method was modified to suit therequirements. (Eekelen et al.2010)

Main difference from EBGEO-Different set of load-and-resistance factors were adopted in the DutchG id liGuideline.

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(e) Guido Method

Guido et al. (1987) observed that the inclusion of stiffbiaxial geogrid within a granular fill improved thebearing capacity of the foundation soilbearing capacity of the foundation soil.

Concluded that the angle of load spread through a Concluded that the angle of load spread through agranular fill reinforced with geogrid would be at anangle of 45 degrees.

The approach is mainly for asingle layer of geosynthetic atthe base of the embankmentfill.

3Stress Reduction Ratio= D

s aS

32

3Stress Reduction Ratio3 2DS

H

(f) The Swedish Method

Carlsson (1987) considered a wedge of soil with aninternal angle at the apex of the wedge equal to 30º.g p g q

Valid in two-dimensional model.

Carlsson adopted a critical height of 1.87(s-a).

Miriam and George (2003) presented the expression for p pS3D for this model as per Hewlett & Randolph (1997)

3

26 tan15D

s a s aS

H

33

6 tan15s a HTwo dimensional model by Carlsson,1987

Rogbeck et al. (1998) modified this model into a 3D form which is an inverted truncated pyramid

h di i l d l b b k l 1998

Modified form of this 3D arching model was adopted by

Three dimensional model by Rogbeck et al. ,1998(Lawson,2012)g p y

Nordic authorities (Svanø et al.2000).

I N di d i th hi l id d t i l d34

In Nordic design the arching angle was widened to includean angle of arching between 68º-75º.

A i i U i C ll (R ll d Pi i 1997 H

Numerical Analyses-Different approaches Axisymmetric Unit Cell (Russell and Pierpoint 1997, Han

and Gabr 2002, Yoo and Kim 2009)

3D Column (Yoo and Kim 2009, Jenck et al. 2009)

Full three dimensional analyses (Huang et al.2005,Liu etal.2007)

3DColumnPile

Full Embankment

35 Axisymmetric unit cell

Major Numerical Work-3D Column Russell and Pierpoint (1997) carried out a numerical study using Russell and Pierpoint (1997) carried out a numerical study using

FLAC3D to compare the different analytical methods.-Terzaghi (1943), Hewlett and Randolph (1988) and BS 8006g ( ), p ( )

Two cases were considered-The A13 piled embankment(heavily reinforced) and the Second Severn Crossingembankment (minimal reinforcement).

Design methods predicted differently for different embankmentgeometriesgeometries

Tension force calculated by different design methods Design Methods A13 Embankment

(Reinforcement Tension,

kN/ )

Second Crossing

(Reinforcement Tension,

kN/ )kN/m) kN/m)

BS8006 73 491

Ter aghi 104 297

36

Terzaghi 104 297

Hewlett & Randolph 104 280

Han and Gabr (2002)Major Numerical Work-Axisymmetric unit cell

Han and Gabr (2002)investigated the influenceof the tensile stiffness ofthe geosynthetic, the heightof the fill, and the elasticmodulus of the pilematerial.

One layer of geosyntheticwas used and a full bondwas assumed between thegeosynthetic and the soil.

Major findings are givenbelow

Pile Layout and the axisymmetric model considered for the analysis (Han and Gabr,2002)

37

below.

(a) (b)Effect of (a) pile modulus and (b) geosynthetic stiffness on the maximum settlements

(Han and Gabr,2002)

The influence of geosynthetic tensile stiffness becomes lessimportant when the stiffness exceeds 4,000 kN/m.

( , )

For a pile of elastic modulus of 30,000 MPa, the maximumttl t f th i f d d d b 20% f

38

settlement for the reinforced case was reduced by 20% fromthat for the unreinforced case.

(a) (b)Effect of geosynthetic (a) Stress Concentration Ratio(b) Tensile force distribution

(Han and Gabr,2002)

(a) (b)

The inclusion of geosynthetic reinforcement enhances thestress transfer from the soil to the piles.

Tension is not uniform along the geosynthetic and themaximum tension occurs at the edge of the pile.

39

maximum tension occurs at the edge of the pile.

Major Numerical Work-Full three dimensional

Geogrid Reinforced Pile supported highway embankment Geogrid Reinforced Pile supported highway embankmentlocated in Shanghai China-Liu et al. (2007)

Case history back analyzed by 3D fully coupled finite-element analysis.

Instrumented cross section of the embankment40

Instrumented cross section of the embankment (Liu et al.,2007)

Full three dimensional model developed (Li l 2007)(Liu et al.,2007)

Significant load transfer from the soil to the piles due to soilg parching-contact pressure acting on the pile was 14 timeshigher than that acting on the soil located between the piles.

Lateral displacements considerably reduced- stability of theembankment increased significantly

41

embankment increased significantly.

Design of Geosynthetic Reinforced Piled Embankment - Example

Pulverized fly ash filled embankment

9 m = 14kN/m3Pile caps

(1.1 m square)( q )

Soft clay(Without piles

settlement = 700 mm)

4 m

)

Embankment Details

Reinforcement detailsReinforcement details

Low creep reinforcement Tensile safety factor = 3.0 Peak extension at failure = 12%

Geotextiles Longitudinal Strength (kN/m)

Transverse Strength (kN/m)g ( ) g ( )

A 1000 50A 1000 50

Circular arc Deformation analysis

Aa = 4-1.1 = 2.9 m

A

2Geosynthetic

RG

Assuming

b = 0.2 0.7= 0.14 m

TT

b 0.2 0.7 0.14 m

From the geometryb TTTT

212

b tana

a11 03

2

.

a R sin

2

7 58

G

G

a R sin

R . m

1

2T R bGWeight of the fill , W

52 08W . kN mT

Considering the reaction force asg

0 15 18.9 kN mBW . h

The tension in the geosynthetic,

251.5 kN/m

Consider a single layer of geosynthetic (Optimal)

T G T BT R W W

Consider a single layer of geosynthetic (Optimal),

total strength = 1050 kN/m

The strain in the ge G251 5otextile, 12 2 871050

. % . %

GFrom the geometry 90 0 6GR a . %

As εG < the predictedG p

Try with b = 0.19 m

14 93º = 14.93º

RG = 5.63 m

WT = 38.08 kN/m

TT = 108 kN/mT

For this the strain εG, from the load deformation data = 1.23%

F h 1 2%From the geometry, εG = 1.2%

As these two are compatible the tension in the geosynthetic TT = 108 kN/m.

εG = 1.2 %G

Catenary Deformation analysis

From the Equation of the catenary, the tension in the geosynthetic is given byg y g y

21 1 aT WT WB

2

2 2

1 1 162

1 16 4 161 1 1

TaT WT WB a b

b a b bl

2 21 16 4 161 1 182G e

b a b blogb aa a

1 69Loading coefficient 0 12cc

. hC .B c

1D Arching: Pressure ratio = C B /h1D Arching: Pressure ratio = Cc Bc/h

2D Arching: Pressure ratio = (Cc B /h)22D Arching: Pressure ratio = (Cc Bc/h)2

Loading Coefficient,1 69 0 12 13.71c

c

. hC .B

Pressure ratio – (1D) = CcBc/h = 1.676Pressure ratio – (2D) = (1.676)2 = 2.809In any 4 square piles,

o Pile area = 1.21 m2

T l 16 2o Total area = 16 m2

o Soil area = 14.79 m2

Total load = 16149 = 2016 kN Total load = 16149 = 2016 kN Load on the pile = 1.211492.809 = 428 kN Load on soil = 2016 428 = 1588 kN = 107 4 kN/m2 Load on soil = 2016-428 = 1588 kN = 107.4 kN/m2

W 107 4 kN/WT = 107.4 kN/m

WB = 0.15 h = 18.9 kN/m

As per the equations shown earlier

T = 309 8 kN/mTT = 309.8 kN/m

From load-extension data εG = (309.8/1050)12 = 3.5 %

Using the equation for ‘1+εG’ as shown earlier, εG = 3.4 %

As the two values are in close agreement further iteration is notAs the two values are in close agreement further iteration is not necessary.

BS 8006-1995 Method

According to BS8006, the minimum height of embankmentrequired is 0.7 (s-a) and for full arching to develop the height of theembankment should be greater than 1.4 (s-a)

In the present case, 0.7(4 – 1.1) = 2.03 m < 9 m and1.4(4-1.1)=4.06 m < 9 m- Full arching develops in this case

The Arching coefficient (considering end bearing pile).1 95 0 18. HCc .a

The vertical stress on the pile cap = 15.77

2 215 77 1 1C a 215 77 1 114 9 468 1 kN/m9c

c vC a . .p .H

For H > 1.4(s-a), The distributed load carried by the geosynthetic reinforcement

1 4s s a 2 22 2

1 4

176 85 kN/

cT

v

. s s a pW s as a

(Serviceability condition, partial factors in the equations are given a value of 1) = 176.85 kN/m

Tension in the reinforcement (BS8006-Design strain is 5%) 11 486 2 kN/mTW s a

T

Tension due to lateral thrust,

1 486.2 kN/m2 6rT a

0 5 170 1 kN/mLT . Ka H .

Total tension = 656.3 kN/m

7 /L

Results of Design

By Circular arc methodTT = 108 kN/m; εG = 1.2 %; WT = 38.08 kN/mT ; G ; T

By Catenary deformation methody yTT = 310 kN/m; εG = 3.4 %; WT = 107.4 kN/m

By BS 8006 1995 methodTT = 656.3 kN/m; εG = 5 %; WT = 176.85 kN/mT 656.3 N/ ; εG 5 %; WT 76.85 N/

54

References1. Alzamora, D., M. H. Wayne and J. Han (2000) Performance of SRW supported by geogrids and jet

grout columns Proc., ASCE Specialty Conf. on Performance Confirmation of Constructed Geotechnical Facilities, Geotechnical Special Publication, 94, 456–466.

2 British Standards BS8006: 1995 Code of practice for strengthened/Reinforced soilsand other fills2. British Standards BS8006: 1995 Code of practice for strengthened/Reinforced soilsand other fills. Section 8.3.3 British Standard Institution.

3. Carlsson, B. Reinforced soil, principles for calculation, Terratema AB, Linköping (in Swedish), 1987.4. CUR 226 2010(2010) Dutch CUR design guideline for piled embankments. ISBN 978 –90–376-0518-1.( ) g g f p5. EBGEO (2004): Bewehrte ErdkÖrper auf punkt - und linienfÖrmigen Traggliedern, Entwurf Kapitel

6.9, 05/16/2004 version.6. Guido, V.A., J.D. Knueppel, and M.A .Sweeny (1987) “Plate loading tests on geogrid - reinforced

earth slabs” Proceedings of Geosynthetics 87 Conference, New Orleans, 216-225.7. Han, R. (1975) Piled Embankment Supported by Single Pile Caps. Proceedings of the Conference on

Soil Mechanics and Foundation Engineering, Istanbul, 1,283-290.8 Han J and M A Gabr (2002) Numerical analysis of geosynthetic reinforced and pile supported earth8. Han, J. and M.A. Gabr (2002) Numerical analysis of geosynthetic-reinforced and pile-supported earth

platforms over soft soil. Journal of Geotechnical and Geoenvironmental Engineering, ASCE,128(1), 44-53.

9. Hewlett, W.J. and M.F. Randolph (1988) Analysis of piled embankments. Ground Engineering, 21(3), 12-18.

10. Hossain, S. and K.N. Rao (2006) Performance Evaluation and Numerical Modeling of Embankment over Soft Clayey Soil Improved with Chemico-Pile. Transportation research record, USA, Issue Number: 1952, 80-89.,267–274.

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Number: 1952, 80 89.,267 274.

References

11. Huang, J., J.G. Collin, and J. Han (2005) “3D Numerical Modelling of a Geosynthetic –Reinforced Pile-Supported Embankment- Stress and Displacement Analysis”16th International Conference on Soil Mechanics and Geotechnical Engineering, Osaka, Japan, 12-16.Confe ence on Soil echanics and Geotechnical nginee ing, Osa a, Japa , 6.

12. Kempfert, H.G., C.Gobel,D.Alexiew and C. Heitz (2004) German Recommendations for Reinforced Embankments on Pile-Similar Elements. Proceedings of the EuroGeo3,MunichDGGT,279-284

13. Jones, C.J.F.P., C .R. Lawson, and D.J. Ayres Geotextile reinforced piled embankments. , pp 155-160. In Den Hoed (eds.) International Conf. Geotextiles, Geomembranes and related products, Balkema, Rotterdam, 1990.

14. Lawson, C.R. (2012) “Role of Modelling in the Development of Design Methods for Basal R i f d Pil d E b k ” b bli h d i h P di f E F 2012 D lf hReinforced Piled Embankments”, to be published in the Proceedings of EuroFuge 2012, Delft, the Netherland.

15. Liu, H.L., W. W. Charles, and K. Fei (2007) Performance of a geogrid-reinforced and pile-supported highway embankment over soft clays-Case study. Journal of Geotechnical and G i t l E i i ASCE 133(12) 1483 1493Geoenvironmental Engineering, ASCE, 133(12), 1483-1493.

16. Marston, A. and A.O. Anderson (1913) The theory of loads on pipes in ditches and tests of cement and clay drain tile and sewer pipe. Engineering experiment station, Bulletin No.31.

17 Miriam E S and M F George (2003) Influence of Clay Compressibility on Geosynthetic Loads17. Miriam, E.S., and M.F. George (2003) Influence of Clay Compressibility on Geosynthetic Loads in Bridging Layers for Column-Supported Embankments.Geotechnical Special Publication, no 130-142, 447-460.

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References

18. Reid, W. M. and N. W. Buchanan(1984)Bridge approach support piling. Piling and Ground Treatment, Thomas Telford Ltd., LondonTreatment, Thomas Telford Ltd., London

19. Rogbeck, Y., S. Gustavsson, I. Sodergren and D. Lindquist(1998) Reinforced Piled Embankments in Sweden-Design Aspects. Proceedings of the Sixth International Conference on Geosynthetics, 2, 755-762.

20. Russell, D. and N. Pierpoint (1997) An assessment of design methods for piled embankments. Ground Engineering, 30(11), 39-44.

21. Satibi, S. (2009) Numerical analysis and design criteria of embankments on floating piles. A h h b d h f S S GPhD thesis submitted to the Universität of Stuttgart, Stuttgart, Germany.

22. Yoo, C. and S.B. Kim (2009) Numerical modeling of geosynthetic-encased stone column-reinforced ground. Geosynthetics International, 16(3), 116-126.

23 Z k D (2001) Z Wi k i b h t d23. Zaeske, D. (2001). ZurWirkungsweise von unbewehrten und bewehrtenmineralischenTragschichtenu berpfahlartigenGrundungsetementen. SchriftenreiheGeotechnik,University of Kassel, Germany, Heft 10, February.

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THANK YOU !THANK YOU !

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