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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
Dept. of Civil Engineering, Dongseo University
Busan, South Korea
Jin Won Hong
Dept. of Civil Engineering, Dongseo University
Busan, South Korea
Yong Soo Ahn
Dept. of Civil Engineering, Dongseo University
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
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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)
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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
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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
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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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