Geosynthetic Embankment Considering Strain

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Geosynthetic Embankment Stability on Soft Ground Considering Reinforcement Strain

ABSTRACT

Kwang Yeol Lee

Dept. of Civil Engineering, Dongseo University

Busan, South Korea

Chin Gyo Chung

Dept. of Civil Engineering, Busan Technical College

Busan, South Korea

Jae Hong Hwang


Busan, South Korea

Jin Won Hong


Busan, South Korea

Yong Soo Ahn


Busan, South Korea

The existing ways of designing embankment using geosynthetic have

been focusing on soil strain rather than reinforcement strain. With

regard to destruction to embankment using geosynthetic reinforcement,

the behaviors of geosynthetic reinforcement and soil are the same at

the initial stress phase, whereas they make a gap in strain as stress

increases, This issue may have a big impact on reinforcement as a

critical factor of geosynthetic reinforcement design in earth structures.

This study shows reinforcement stress and soil stress in embankment

reinforced by PET Mat on soft ground through the quantitative

analysis on strain behavior. As the result, reinforcement strain greatly

depends on the tensile strength of reinforcement, regarding destruction.

The maximum stress on reinforcement by external loads will not

exceed the yield tensile strength and it will be ideal when

reinforcement stress is higher than the stress in soil of embankment. Inaddition. the safety factor with shear destruction of embankment will

increase together with the yield tensile strength of reinforcement,

though the factor will be unchanged after reinforcement strain matches

soil strain.

KEY WORDS : Embankment; Soft ground; Reinforcement;

PET Mat; Stress; Geosynthetics.

KNTRODUCTION

Embankment on soft ground will have shear stress from vertical load. If

the ground does not have enough shear strength, shear failure may occur

within or in the lower part of embankment, Soft ground improve method

is, therefore, selected for bearing-capacity against failure and

embankment stability, such as replacement method, consolidation method,

pile bearing method, light-weight embankment method, reinforcement

method, etc. Especially, the method of reinforcing sotI ground with

highly polymerized geosynthetic is considered more efficient and cost-

effective (1999, Korean Geotechnical Society).

In case of embankment upon geosynthetic installed on soft ground,

reinforcement will improve stability and bearing-capacity again lower-

part shear failure and prevent shear failure of embankment.

Reinforcement may also decrease horizontal and vertical displacement of

lower-part ground, diminishing differential settlements.In general, analytic studies on geosynthetic embankment have been based

on Critical Equilibrium theory, providing some problems of applying

Critical Equilibrium theory to complicated soil-geosynthetic system

analysis. The main issue is that the strain effect from interaction between

embankment soil and geosynthetic is not considered. Therefore, the

design of stabilizing soft ground with geosynthetic requires soil-

geosynthetic strain to be considered.

This study will compare and analyze the difference in behavior and stress

No: 2003~SSK-03 Lee Page: I of 6

Proceedings of The Thirteenth (2003) International Offshore and Polar Engineering Conference

Honolulu, Hawaii, USA, May 25 – 30, 2003

Copyright © 2003 by The International Society of Offshore and Polar Engineers

ISBN 1 –880653-60 – 5 (Set); ISSN 1098 –6189 (Set)

573



from reinforcement strength gap, with an effort to complement the

existing reinforcement (geosynthetic) design on embankment.

In addition, Finite Difference Method (FDM) will be applied to

geosynthetic behavior which was analyzed as elastic body, measuring

strength and plastic behavior in embankment and suggesting design

method focusing on strain difference between soil and reinforcement

at the reinforcement installed place.

Mechanical Behavior of Geosynthetic Reinforcement in

Embankment

Geosynthetic reinforcement is quite different from the existing

reinforcement systems mentioned above when it comes to the large

reinforcement strain. This provides a great flexibility from low inertia

moment and relatively low modulus of elasticity. Therefore, only

tensile behavior is recognized for geosynthetic. The reason is that

bending moment and compressive strength are quite low compared to

tensile stress, For all passive reinforcements, geosynthetic

reinforcement will require displacement threshold at the soil-inclusion

interface to get reinforcement optimized. Fig. 3 shows geosynthetic

strain in embankment subjected to loads. Strain is much higher in the

middle zone of discontinuity, decreasing at the ends and displacement

further increased.

Active zone

Zone of

discontinuity

Passive zone

Fig. I Deformed geosynthetic sheet in a slope

Tensile behavior

In general, geosynthetic tensile stress has been modeled based on

quazilinear elasticity principle and elastic-plastic model from brittle

failure. Fig. 2 shows typical curve of Tension force-Strain fromgeosynthetic tensile stress test. Three parameters can be deduced here,

which are critical factors of geosynthetic reinforcement behavior. The

stiffness modulus of linear part before destruction is the stiffness

modulus J (kN/m) of geosynthetic tensile strength test and the tensile

strength at the time of destruction is the one of this geosynthetic. And

its strain means destruction strain.

Tensi n force T (kN/m)

t Tr

Strain a

Fig. 2 Model of tensile test on geosynthetic sheet

No: 2003~SSK-03 Lee

Anchorage behavior

As Fig. 3 explains, anchorage behavior is composed of tangent force

recovered through soil-reinforcement (geosynthetic) friction. This

mechanism is the fundamental to reinforcement system. Equation (I)

shows that surface friction resistance of reinforcement element r is

equated with opposite tensile strength, dT, allowing reinforcement in

embankment, not pull-out (Bourdeau et al. 1994, Gotteland 1991).

dT=2 r ds

T+ T+dT

-7

--

0”ds

(1)

Fig. 3 Anchorage equilibrium of a geosynthetic element

According Fig. 4, the behavior at the soil-inclusion interface was defined

by the perfect elastic-plastic model. In this model, working state stress is

associated with the stress based on mohr-coulomb failure criterion, @g

and Cg is soil-reinforcement friction and adhesion respectively. Cg is

usually ignored. In general, soil strength decreasing factor( p ) is defined

out of unique features of soil, providing optimal friction at soli-inclusion

interface at the range of O.S(non-woven)-l(woven or geogrid).

Geosynthetic for embankment reinforcement has this factor p at the

range of 0.8-I.

U (mm)

t

‘c nt WW

TP @?4

cg

~

j 0”

Fig. 4 Characteristics of the soil-inclusion interface

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Conditions & Materials

The earth structures chosen for this study is the embankment using

geosynthetic upon soft ground and the soft ground has been improved

via SCP (Sand Compaction Pile) method, which allowed the

underneath soft ground to be strengthened in the center and left of

embankment. Zone l(subground), zone Z(subground of slop of the

embankment) and zone 3(subground of top of the embankment), as

shown Table 1 and Fig. 5. In this embankment, bi-direction PET Mat

was installed 40.lm in parallel with the embankment after the

underneath ground was improved, and banked to 8.5m above, when

the reinforcement tensile strength is 50, 100, 130, 170, 320kNim.

Fig.5 describes its analytic section and the underneath ground has

different soil integers per interval. In the stability analysis on

embankment, design load was train load SOkNim. The materials of

embankment and underneath ground integers are as follows in Table 1

and the result of reinforcement (PET Mat) stress-strain test is provided

in Fig. 6.

Table 1 Soil Properties of embankment and subsurface ground

Fig. 5 Analytic Section Dimension

500

400

-g 300

!3,-2 200gVI

100

0

0 5 10

Strain(%)

Fig. 6 Stress-strain behavior of woven PET Mat

(Tensile strength 320kN/m)

Bi-direction woven PET Mat has been selected for reinforcement and

analyzed by changing its tensile strength as 50, 100, 130, 170, 320kN/m,

strain and stress behaviors of soil-reinforcement (PET Mat) being

investigated at the reinforcement location. It is also analyzed how the

safety factor has been changed against embankment shear failure as

reinforcement (PET Mat) tensile strength increased.This study makes use of ITASCA FLAC ver.4.0, FDM program. Ver. 4.0

has analysis time shortened and is known to provide almost real value

from stability analysis on slope embankment, compared to other

programs.

Analysis Results

When change was made to the tensile strength of PET Mat as 50, 100,

130, 170, 320kN/m, high-strength reinforcement installed-base

experienced slope or toe failure and low-strength reinforcement

experienced base failure. In order to identify when failure phase is

changed, tensile strength has been changed per IOkNim interval. The

result is that IO-120kN/m reinforcement experienced base failure and

130-320kNim reinforcement experienced slope or toe failure. As

reinforcement (PET Mat) strength was increased, in-embankment shear

strain has been diminished. The safety factor against embankment shear

failure was 1.32 prior to PET Mat reinforcement installation, maybe

resulting from ground bearing-capacity achieved through lower part

improvement. When reinforcement of various tensile strengths is installed,

strain and stress of soil-reinforcement (PET Mat) and its safety factor

change will be measured for optimized reinforcement design. Figs. 7-8

present the embankment shear strains with IOOkNim and 320kNim

reinforcement used for each.

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Fig. 7 Embankment Shear Strain (PET Mat - 1OOkN/m)

Fig. 8 Embankment Shear Strain (PET Mat - 320kNim)

250i --tSOll

~ *PET (320KNhl)

I + PET (I 70uh)

*PET (I 30KN/m)

+ PET (I OOKNlm)

20 30

PET Mat Distance (m)

Fig. 9 Soil-PET Mat Stress Distribution at the location of PET Mat

As shown by Fig. 9, the soil of PET Mat installed-base will have the

shear stress at most 164kN/m2 in the center. In case of 50, 100 and

13OkN/m reinforcements being installed, the stress will be as high as

tensile strength at the location of PET Mat, not soil shear stress, which

will tend to be failured. PET Mat with tensile strength higher than

17OkN/m is considered good as it is higher than soil shear stress. And

320kNim PET Mat will be over-design because PET Mat only stays at

227kNim in the embankment. Therefore, considering in-embankment soil

shear stress and cost-effectiveness, the reinforcement tensile strength

should be designed to be higher than 17OkNlm.

W PET Mat (SOkN/m)

+So~l(lOOkN/m)

--O- PET Mat (I OOkN/m)

+Sod (I3OkNim)

+ PET Ma, (I 30kNh)

+Sod (I7OkNim)

-H-PETMat(I7OkNim)

---+-Ssod (32OkNim)

+ PET Mat (320kN/m)

10 20 30 40 50

PET Mat Scope (m)

Fig. 10 Comparison of Soil-PET Mat Displacement at the location of

PET Mat (horizontal)

2

+ PET Mat (SOkN/m)

+ Sod (I OOkNim)

+ PET Mat (I OOkNim)

+ Sod (I 30kNim)

*PET Mat (I3OkNim)

b Sod (I 7OkNim)

-B-PET eat (I7okNh)

W Sod (320kNim)

+ PET Mat (320kNim)

0 10 20 30 40 50

PET Mat Scope (m)

Fig. 11 Comparison of Soil-PET Mat Displacement at the location of

PET Mat (vertical)

As explained in Figs, 10-11, change in PET Mat tensile strength will

make the soil-PET Mat displacement altered. The biggest displacement

will happen in the shear strain failure zone mentioned before. The soil-

PET Mat displacement was much bigger in low-strength rather than high-

strength reinforcement installed-base, such as when the reinforcement

lower than the stress analysis result, 17OkN/m was installed, the

displacement became magnified to 4.5-10.5 cm. This may result from the

fact that PET Mat tensile strength did not reach soil shear stress, causing

576



reinforcement tensile destruction. So if the tensile strength of

displacement that is unchanged even after reinforcement being

strengthened is called the optimal strain tensile strength (Tos), Tos

will be 170kN/m, with soil-PET Mat displacement considered.

0

0 100 200 300 400

Tensile strength of PET Mat (kN/m)

Fig. 12 Horizontal Strain of two phases

Tensile strength of PET Mat (kN/m)

Fig. 13 Vertical Strain of two phases

As the embankment of low-strength reinforcement (PET Mat) has a

big soil-reinforcement displacement, this reinforcement (PET Mat)

cannot have the same behavior as soil. So it is anticipated that slope

failure will occur out of stress concentration due to embankment

differential settlements. With PET Mat of tensile strength higher than

soil shear stress, the displacement may be less than 2 cm and strength

increase will not have little effect on soil-PET Mat displacement.

Figs. 12-13 show the displacement gap at the soil-inclusion interface.

The gap based on reinforcement strength is linear and almost the same

at the PET Mat strength higher than 17OkN/m. In addition, in vertical

directions, PET Mat higher than 17OkNim had the displacement gapincreased by 0.04cm. Considering displacement gap unchanged even

after reinforcement strengthened, the optimal strain tensile strength

was also 170kN/m.

The reason of displacement gap at the same location is that each

material has different modulus of elasticity. And the displacement gap

may depend on adhesion, friction and strength of in-embankment soil

and PET Mat.

1.6

F.S

1.4

(1.3

0 100 200 300 400

Reinforcement Tensile Strength (kN/m)

Fig. 14 F.S of Design Tensile Strength

Safety factors have been analyzed for various reinforcements from

SOkNim low-strength reinforcement (usually used for isolation) to

320kNim high-strength reinforcement. No-reinforcement installed-base

was 1.32 and SOkN/m low-strength reinforcement was 1.47, both of

which show high reinforcement effects. This may result from the fact that

bearing-capacity has been achieved through soft ground improvement

method. Safety factors tended to increase linearly with the reinforcement

strength increase. When the strength was higher than 17OkN/m, however,

the safety factor kept unchanged as 1.57. This meets the minimum

requirement of 1.5 from USEPA recommendation.

Considering that soil-reinforcement displacement is less than 2 cm and

displacement gaps vertical and horizontal have the minimum value

0.119-0.145cm with the reinforcement higher than the optimal strain

tensile strength, reinforcement with optimal strain tensile strength will be

the best choice as far as structural stability and cost-effectiveness are

concerned.

CONCLUSION

1. In the comparison of soil-PET Mat stress distribution in embankment,

PET Mat with tensile strength lower than the optimal strain tensile

strength (Tos) will have little reinforcement effects for that its tensile

strength is lower than soil shear stress.

2. There was embankment failure due to reinforcement strength gap and

soil-PET Mat displacement gap at the location of reinforcement. When

reinforcement lower than 13OkN/m is installed, base failure occured and

in the case of being higher than I3OkN/m, slop or toe failure occured.

And with the reinforcement lower than the optimal strain tensile strength

(Tos), displacement of soil-PET Mat became larger than 3 cm, both

vertically and horizontally. All of them give cause to differential

settlements and shear failure due to geosynthetic zone of discontinuity

enlarged.

3. With a optimal strain tensile strength higher than that of PET Mat, soil-PET Mat displacement gap at the location of PET Mat became wider. The

reason may be that PET Mat having got tensile stress in the case of

embankment shear failure results in tensile strain which leads to

geosythetic creep strain and plastic behavior.

4. According to comparative analysis on the stress, displacement,

displacement gap and safety factor of soil-PET Mat at the location of PET

Mat, the optimal strain tensile strength (Tos) is the best suitable for the

No: 2003~SSK-03 Lee Page: 5 of 6

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design tensile strength.

In designing the in-embankment PET Mat, it is necessary to measure

stress, displacement, displacement gap and safety factor of soil-PET

Mat at the location of PET Mat and also consider the PET Mat

installation location.

A further study upon reinforcement (PET Mat) design considering

cost-effectiveness could be carried out, as if differential settlements of

very soft ground is expected, or if multiple layers of isolation

reinforcement (PET Mat) are installed, etc.

REFERENCES

C.Beneito, Ph. Gotteland (2001). “Three-dimensional numerical

modeling of geosynthetics” FLAC and Numerical Modeling in

Geomechanics, A. A. Balkema Publishers, pp 19 l- 192

Chen, R.H., and Chameau, J.L., (1982) “The Three Dimensional

Limit Equation Analysis of Slopes”, Geotechnique, Vol. 32, No. I

Giroud, J.P., and Beech, J.F., (1989) “Stability of Soil Layers on

Geosynthetic Lining Systems”, in Geosynthetics ‘89, IFAI, San

Diego, CA

Guglielmetti, J.L.. Koerner, G.R. and Battino, F.S.(1996), “Geotextile

reinforcement of soft landfill process sludge to facilitate final

closure: An instrumented case history”, Proc. GRI-9 conference on

Geosynthetics in Infrastructure Enhancement and Remediation, GII,Philadephia, pp 195 - 211

Koerner, R.M.(1996), “The state of the practice regarding in-situ

monitoring of geosynthetics”, Proc. I”’ European Geosynthetics

Conference, Netherlands

Lee, K.Y. et. al., (1997) “Sorption Capacity of Marine Clay and

Weathered Soil under Kimpo Metropolitan Landfill to Heavy Metals

and Inorganic Contaminants” International Symposium on

Environmental Engineering, ISEE’ Conference, 1997. 9, pp 58

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Geosynthetic Embankment Considering Strain