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# Valuate the effect of embankment height and pile spacing to the behavior of the Geosynthetic - Reinforced Piled mmbankment using FEM

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In this paper, a numerical model is based on the finite element method (FEM) is used to analyse the behaviour of piled embankments on soft soils reinforced with geosynthetic (GRPE). Both 2D and 3D numerical analyse (PLAXIS 2D and PLAXIS 3D Tunnel respectively) have been carried out to investigate the behaviour of piled embankments during and after construction. The influence embankment height (H) and pile spacing (n) on the performance of GRPE like vertical stress on soft soil, differential settlements, stress concentration ration has been introduced in this work. The results of the study are lessons learned for design engineers and researchers in designing and researching application of GRPE solutions for soft soil.
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Numerical Analysis in Geotechnics NAG2018, Ho Chi Minh City, Vietnam, 22nd March 2018
Keywords: pile; geosynthetic, embankment, settlement
ABSTRACT: In this paper, a numerical model is based on the finite element method (FEM) is used to
analyse the behaviour of piled embankments on soft soils reinforced with geosynthetic (GRPE). Both 2D
and 3D numerical analyse (PLAXIS 2D and PLAXIS 3D Tunnel respectively) have been carried out to
investigate the behaviour of piled embankments during and after construction. The influence embankment
height (H) and pile spacing (n) on the performance of GRPE like vertical stress on soft soil, differential
settlements, stress concentration ration has been introduced in this work. The results of the study are
lessons learned for design engineers and researchers in designing and researching application of GRPE
solutions for soft soil.
1. INTRODUCTION
The geosynthetic-reinforced piled embankment
(GRPE) structure consists of closely spaced piles
which penetrate the soft soil to reach a stiff bearing
substratum, the pile group is overlained by the
geosynthetic reinforced, upon which the
embankment is constructed, Fig. 1 Han and Gabr
(2002) [5] suggested the beneﬁts associated with
reinforced piled embankments are (1) single stage
construction without pro-longed waiting times; (2)
signiﬁcantly reduced diferential settlements; (3)
reduced earth pressures; (4) to avoid excavation
and reﬁll employed.
Figure1.Typical geosynthetic reinforced piled
embankment.
The design of reinforced piled embankments is
a complex soil-structure interaction problem
involving embankment ﬁll, geosynthetic
reinforcement, a pile group, and the soft underlying
soil see Love and Milligan (2003) [9].
In both design and analysis of geosynthetic
reinforced piled embankments analytical and
numerical techniques are utilized. A number of
design methodologies have been proposed for
Guido (1987), Carlsson (1987) [3], Hewlett và
Randolph (1988) [6], BS8006 (1995) [1], Zaeske
and Kempfert (1997) [8].
There are numerous types of numerical methods
of simulating a geosynthetic reinforced piled
embankments; two of the most common modes of
analysis are Finite Element Method (FEM) and the
Finite Difference Method (FDM). In FEM method,
the two-dimensional finite element model
(FEM2D) and three-dimensional finite element
model (FEM3D) widely used in recent years.
concluded that the simu-lation of a reinforced
embankment by 2D numerical analysis was in
good agreement with 3D analyses of the problem.
In this study, through analyzing case study in Hung
Loi Metro, Can Tho City, Viet Nam. The analysis
Valuate the effect of embankment height and pile spacing to the
behavior of the Geosynthetic - Reinforced Piled mmbankment
using FEM
Phan Tran Thanh Truc
Mien Trung University of Civil Engineering, Viet Nam. E-mail: phantranthanhtruc@muce.edu.vn
Le Lam Gia
Can Tho University, Viet Nam. Email: lglam@ctu.edu.com
Takenori Hino
Institute of Lowland and Marine Research, Saga University, Japan. Email: hino@ilt.saga-u.ac.jp
influences of various parameters like pile spacing,
embankment height on the performance of GRPE
like like vertical stress on soil, differential
settlements, stress concentration ratio has
investigated. A comparative analysis of Plaxis 2D
and Flaxis 3D is presented in this paper.
2. OVERVIEW OF DESIGN METHODS
2.1. The Guido Method
This is a very simplified approach. The
reinforcement has to bear only small pyramids. The
original paper (Guido 1987) has nothing to do with
that approach. Pyramids geometry is always the
same independent of the fill. Because the pyramids
are very flat, surcharge on embankment is rarely
taken into account.
Figure 2. The Guido Method
2.3. The Swedish Method
Accoding to Carlsson (1987) who used a simplified
approach which is always pyramids of 75° wall
inclination, independent of type and strength of
embankment fill. Better than the “Guido Method”:
pyramids can reach the embankment surface and
thus include surcharge. Dimensioning of
reinforcement based on the “membrane theory”.
Figure 3. The Swedish Method
2.2. The British Standard 8006 Method
First approaches explained in John (1987), further
develop ments shown e.g. in Jones et al (1990),
finally fixed as Standard in 1995. 3D arching
assumption in the embankment fill: always a semi-
sphere, independent of type and strength of fill. It
cuts the embankment surface rarely, thus traffic
load hardly ever taken into account. “Membrane
theory” for the tensile force in reinforcement.
Reinforcement concentrated in one layer on top of
piles. No support of reinforcement upward
counterpressure from the soft soil between the
piles: “free hanging system”.
Figure 4. The BS 8006 Method
2.4. The New German Method
The development started in 1995. Focal points
were to im- prove the stress redistribution model
for the embankment body and to find a way for a
reasonable consideration of a possible upward soft
soil counterpressure between the piles (Kempfert et
al (1999), Zaeske (2001)). The draft for a new
chapter in EBGEO (1997) is ready. It includes a
new “multi- shell arching” theory and a strain-
related counterpressure. Only one or maximum two
strong reinforcement layers directly on top of piles
are strongly recommended.
Figure 5. The New German Method
3. HUNG LOI METRO PROJECTS
3.1. Location of The Study Area
Location in this study is at location Parking
Area of Hung Loi Metro, Can Tho City. Where
using solutions GRPE is aplied (Fig. 6). The
model uses circular pile diameter D = 300mm
(B20), spacing pile s = 4000mm, the parameters of
pile cap 1500x1500x300mm, above two layers of
geotextile and compacted sand 500 mm, base
course (0x40mm) 270 mm, subbase course
250mm, reinforced concrete pavers (B25) 180mm.
Figure 6. The Cross section of the model designed of IGW at Location Parking Area of Hung Loi Metro, Can
Tho City
Figure 7. Geological cross section of the study area.
3.2. Geometric Modelling of The Study
Fig. 8. shows the geometry of the geometry of a
geosynthetic-reinforced piled embankment. This
model is presented and conducted by the study of
Zaeke (1997) [10]. Accordingly, Zaeke
recommended set of models and input parameters
for studying as follows:
The diameter of the pile cap to spacing pile =
a/s = 0.75 ( ≥ 0.15)
Spacing pile - the diameter of the pile cap =
s - a = 4 - 1.5 = 2.5 (≤ 2.5)
Spacing pile - the diameter of the pile cap =
s - a = 4 - 1.5 = 2.5 ≥ 1.4H = 1.4x1.2 = 1.68
(s - a ≤ 1.4H)
Pile Stiffness to Soft Soil Stifnes = Epile/ESoft
Soil = 256 ( ≥ 0.15)
Fig. 8 shows the geometry of the piled
embankments on soft soils reinforced with
geosynthetic. The numerical model simulates the
pile with plate elements, the reinforced layers with
geogrid elements and the soil-pile contact area with
interface element. Furthermore, the Mohr-
Coulomb plastic model is used for soil. The GRPE
construction is modelled with staged construction
and consolidation ( after 7 months).
(a) (b)
Figure 8. (a) The geometry of GRPE in 2D analysis, (b) The geometry of GRPE in 3D analysis.
3.3. Parameter
Table 1. Soils and Foundation Data Parameters
Soils
γw
(KN/m3)
kx
(m/day)
ky
(m/day)
φ(o)
Eoed
(KN/m2)
ν
Rinter
Sand
19.00
0.10
0.05
30
7,500
0.25
0,8
Soft Clay CH1
16.57
1.85E-01
1.04E-01
5.9
657.5
0.278
0.8
Plastic Clay
CH2
15.04
0.713E-4
0.475E-4
2.2
5038
0.270
0.8
Plastic Clay
CL1
16.86
1.85E-01
1.04E-01
6.1
674.1
0.275
0.8
Liquid Clay
CH3
16.18
0.713E-4
0.475E-4
4.6
625.5
0.279
0.8
Sandy Clay
CH4
18.75
0.1
0.05
15.7
1812
0.290
0.8
Sandy Clay
CL2
19.00
0.1
0.05
21.5
2,724
0.292
0.8
Table 2. Pile Data Parameters
Identification
Young's Modulus (KN/ m2)
A (m2)
Pile
2.9E+7
0.3*0.3
0.15
Table 3. Geosynthetic Data Parameters
Identification
EA (KN/ m)
Geosynthetic
3000
0.17
4. RESULTS AND DISSCUTIONS
4.1. Results and discussion in the first case study
Influence analysis influence of the embankment
height H to vertical stress on the soil s,
differential settlement S, stress concentration
ratio n of GRPE with input parameters show in
Table [1, 2, 3].
For each of the embankment height H = 1, 1.2,
2, 3, 4m, increasing load from 0 , 5, 10, 20, 40, 60,
80, 100, 120, 150KN with the pile spacing was
kept constant equal to s = 4m to consider the
impact of the embankment height H to the
behaviour of GRPE through two different
computer programs: Plaxis 2D and Plaxis 3D.
(a) (b)
Figure. 9. Relationship between load q (KN), differential settlement S(mm), and embankment height H (m)
(Case: s = 4m) through: (a) Plaxis 2D (FEM2D), (b) Plaxis 3D (FEM3D).
(a) (b)
Figure 10. Relationship between load q (KN), stress concentration ratio n, and embankment height H (m) (Case:
s = 4) through: (a) Plaxis 2D (FEM2D), (b) Plaxis 3D (FEM3D).
(a) (b)
Figure 11. Relationship between load q (KN), vertical stress on the soil s, and embankment height H (m)
(Case: s = 4m)
through: (a) Plaxis 2D (FEM2D), (b) Plaxis 3D (FEM3D).
(a) (b) (c) (d)
Figure 12. The arching effect in soil through parameter vertical stresses in Plaxis 2D (Case: s = 4m):
(a) H=1.0m, (b) H=1.2m, (c) H=2m, (d) H=4m
.
Differential Settlements, vertical stress on
soil
Dierential settlements were deﬁned as the
dierence in settlement between the top of the pile
cap and midspan between two dierential adjacent
pies at the base of the embankment.
Fig. 9(a, b), Fig. 11(a, b) shows the differential
of settlements ΔS and vertical stress on the soil σs
increases with different trends when we fixed pile
spacing s = 4m and increasing load from 0 KN to
150 KN. Differential of settlements ΔS and
vertical stress on the soil σs increases rapidly when
embankment height H increased from 1m to 2m
and almost unchanged when embankment height
H increased from 3m to 5m. And the differential of
settlements ΔS and vertical stress on the soil σs
through Plaxis2D, and Plaxis 3D have the same
trend when increasing the embankment height.
Besides that, Plaxis 3D gives the largest value in
all cases in this study. This is related to the soil
arching effect. (Fig. 12).
The soil arching effect can be determined
visually quite accurately based on the model shown
in Plaxis 2D, Plaxis3D through parameter vertical
stresses yy. The value differential of settlements
and vertical stress on the soil will increase rapidly
when the soil arching effect have formed and
increases slowly when have not formed (Fig. 12) .
Stress concentration ratio
The stress concentration ratio n was deﬁned as the
ratio of the stress on the pile caps to the stress
acting on the soft soil layer underlying the
reinforcement.
Fig.10 (a, b) illustrates that the stress
concentration ratio increased for an increase in
embankment height. Plaxis 3D replicated the
characteristic response of the stress concentrations
with embankment height given by Flaxis 2D.
However, Plaxis 3D recorded absolute values of
stress concentrations ratio higher than Plaxis 2D.
It is obvious that increasing the embankment
height will increase the load transferred to the
piles. As a consequence, stress concentration ratio
increases with the increase of embankment height.
This results of this study are similar to many
studies of many authors as Han and Gabr (2002)
[5] through their studies using 2-D axisymmetric
finite difference analyses and 2-D axisymmetric
finite element analyses of a piled embankment (a
cell) respectively
4.2. Results and discussion in the second case
study
Influence analysis influence of the pile spacing s
to settlement of geosynthetic Sgeosynthetic, settlement
of pile Spile, with input parameters show in Table
[1, 2, 3].For each of the pile spacing s = 2, 3, 4,
5m, increasing load from 0, 5, 10, 20, 40, 60, 80,
100, 120, 150 KN with the embankment height was
kept constant equal to H = 1.2m to consider the
impact of the pile spacing s to the behaviour of
GRPE through two different computer programs:
Plaxis 2D and Plaxis 3D.
(a) (b)
Figure 13. Relationship between load q (KN), settlement of geosynthetic Sgeosynthetic (mm), and the pile spacing s
(m) (Case: H = 1.2m) through: (a) Plaxis 2D (FEM2D), (b) Plaxis 3D (FEM3D).
Figure14. Relationship between load q (KN), settlement of pile Spile (mm), and the pile spacing s (m) (Case: H =
1.2m) through: (a) Plaxis 2D (FEM2D), (b) Plaxis 3D (FEM3D
).
Figure 15. Relationship between load q (KN),
settlement of pile Spile (mm), and settlement of
geosynthetic Sgeosynthetic (Case: H = 1.2m, s = 4m)
through: Plaxis 2D (FEM2D), Plaxis 3D (FEM3D)
Fig. 13 (a,b), Fig. 14 (a,b) illustrates that the
settlement of pile and settlement of geosynthetic
increased for an increase in the pile spacing. Plaxis
3D replicated the characteristic with the pile
spacing given by Flaxis 2D. Moreover, the value
of settlement of pile and settlement of
geosynthetic tend to converge when the external
load gives very little value and having a large
differences when having an increasing upward
Fig. 15 shows the case ( q = 5 KN, S = 4m, and
H = 1.2m) the settlement of pile Spile 2D = 27.0 cm,
Spile 3D = 20.96 cm, settlement of geosynthetic
Sgeosynthetic 2D = 25.12 cm, Sgeosynthetic 3D = 18.63 cm ,
Differential settlements ΔS2D = 27.0 cm- 25.12cm
= 1.88 cm < ΔSSettlment monitoring after 7 months = 5.9 cm ,
ΔS3D = 20.96cm 18.63 cm = 2.33 cm < ΔSSettlment
monitoring after 7 months = 5.9 cm.This results showed
there is very little difference between the
differential settlements values by FEM2D, FEM3D
and settlment monitoring values after 7 months.
There are many causes of the difference in results
may be mentioned as: the conversion from 3D
model to 2D model and input parameters in
Plaxis may not accurately reflect the true stress
strain behavior of a structured soil in this study.
This results of this study are similar to many
studies of many authors as Cortlever and Gutter
(2006) [4] based on their 2-D axisymmetric
numerical analyses stated that increasing pile
spacing obviously decreases the embankment load
transferred to the piles. This leads to the decrease
of efficacy, geosynthetic tension and the increase
of maximum and differential settlements and soil
arching ratio of a piled embankment. And, these
conditions the complete arching does not exist if H
≥ 1.4 (s - a).
4. CONCLUSION
The analysis and design of the geosynthetic-
reinforced piled embankment requires an in-depth
understanding of the failure mechanism in order to
choose the right analysis method. In this study,
the behaviour of of the geosynthetic-reinforced
piled embankment is approached using FEM2D,
and FEM3D in order to increase understanding
applicability of the two applied programs;
Plaxis2D, and Plaxis3D. The results suggest the
following conclusion:
The settlement of pile and settlement of
geosynthetic increased for an increase in the
pile spacing and Plaxis2D gives the largest
value in all cases in this study
The value of settlement of pile Spile and
settlement of geosynthetic Sgeosynthetic tend
to converge when the external load gives very
little value and having a large differences when
The stress concentration ratio increased for an
increase in embankment height and Plaxis 3D
recorded absolute values of stress
concentrations ratio higher than Plaxis 2D.
Having the difference in results when designing
geosynthetic-reinforced piled embankment by
3D model, and 2D model with FEM.
5. REFERENCES
BS8006 (1995), Code of Practice for
Strengthened/Reinforced Soils and Other Fills,
British Standard Institution.
D. T. Bergado and C. Teerawattanasuk., “2D and
3D numerical simulations of reinforced
embankments on soft ground,” Geotextiles and
Geomembranes, vol. 26, no. 1, pp. 3955, 2008.
Carlsson, B. (1987)., Reinforced soil, principles
Cortlever, N. G. and Gutter, H. H. (2006)., Design
of double track railway Bidor-Rawang on
AuGeo piling system according to BS8006 and
PLAXIS numerical analysis, Cofra B.V.,
Amsterdam, The Netherlands.
Han, J. and Gabr, M.A. (2002). A numerical study
of load transfer mechanisms in geosynthetic
reinforced and pile supported embankments
over soft soil. Journal of Geotechnical and
Geoenvironmental Engineering, ASCE, 128(1),
pp. 44-53.
Hewlett, W. J. and Randolph, M. F. (1988).,
Analysis of piled embankments, Ground
Engineering, 21(3), pp.12-18.
H. Slaats, Load transfer platform, bending
moments in slender piles, M.S. thesis, Technical
University Delft, 2008.
Kempfert, H. - G., Stadel, M. and Zaeske, D.
(1997)., Design of geosynthetic-reinforced
bearing layers over piles. Bautechnik, Vol. 74,
No. 12, pp. 818-825.
Love, J., and Milligan, G. (2003)., Design methods
for basally reinforced pile-supported
embankments over soft ground. Ground
Engineering, Vol. 36, No. 3, March, pp. 39-43.
McNulty, J. W. (1965)., An Experimental study of
arching in sand.” Technical Report No. I-674,
U.S. Army Engineer Waterways Experiment
Station”, Corps of Engineers, Vicksburg,
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Geotechnical engineers face several challenges when designing structures over soft soils, These include potential bearing failure, intolerable settlement, large lateral pressures and movement, and global or local instability. Geosynthetic-reinforced and pile-supported earth platforms provide an economic and effective solution for embankments, retaining walls, and storage tanks, etc. constructed on soft soils; especially when rapid construction and/or strict deformation of the structure are required. The inclusion of geosynthetic(s) in the fill enhances the efficiency of load transfer, minimizes yielding of the soil above the pile head, and potentially reduces total and differential settlements. A numerical study has been conducted to investigate pile-soil-geosynthetic(s) interactions by considering three major influence factors: the height of the fill, the tensile stiffness of geosynthetic, and the elastic modulus of pile material. While current methods have not fully addressed important effects of the geosynthetic stiffness and pile modulus on the soil arching ratio, numerical results suggested that the stress concentration ratio and the maximum tension in geosynthetic increase with the height of the embankment fill, the tensile stiffness of geosynthetic, and the elastic modulus of the pile material. The distribution of tension force in the geosynthetic reinforcement indicated that the maximum tension occurs near the edge of the pile.
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Utilizing the same constitutive models and properties of foundation soils as published by previous researchers, two full-scale test embankments, namely steel grid embankment having longer plan dimensions with length-to-width ratio of 3.0 (long embankment) and hexagonal wire mesh reinforced embankment having shorter plan dimensions with length-to-width ratio of 1.0 (short embankment), were investigated using numerical simulation in two-dimensional (2D) and three-dimensional (3D) explicit finite-difference programs, FLAC2D and FLAC3D, respectively. The 2D numerical analysis simulated the overall behavior of the steel grid reinforced “long” embankment with very reasonable agreement between the field measurements and the calculated values. On the other hand, the 3D numerical analysis simulated the overall behavior of the hexagonal wire mesh reinforced “short” embankment. Furthermore, the simulation results from the FLAC3D used in the 2D analysis agreed with the measured settlement data in the “long” embankment as well as the 2D predictions from FLAC2D. The 2D and 3D numerical analyses should be considered important factors that may affect the numerical simulation results which are consistent with the current settlement predictions with Skempton–Bjerrum corrections.
Code of Practice for
BS8006 (1995), Code of Practice for
Reinforced soil, principles for calculation
• B Carlsson
• A B Terratema