Reinforcement of the Seismic Interaction… Mohanad Talal Alfach
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moment M
p
which is the maximum allowable bending
moment of the pile. Cakir and Mohammed 2013
studied the seismic retrofitting of historical masonry
bridges using micropiles and the seismic performance of
this technique. Likewise Momenzadeh et al. 2013
confirmed the micropiles’ seismic retrofit efficiency in
poor soil conditions with high structural load demands
of the San Francisco Bay area bridge. Furthermore
Doshi et al. 2017 have analyzed the seismic behavior
of soil-piles-micropiles-bridge and confirmed the
beneficial effect of the added micropiles in reducing
settlement bending moment and shear force. Ousta and
Shahrour 2001 studied the seismic behavior of
micropiles in saturated soils. Sadek and Shahrour 2003
showed that the inclination of micropiles results in an
improvement of lateral stiffness bending moment and
axial force. Alsaleh and shahrour 2006 confirmed that
nonlinearities of the soil and micropile-soil interface
have a significant effect on the seismic response of the
micropiles’ group as well as that of the structure. The
research conducted in this study provides a thorough
analysis of the soil-pile-bridge under seismic loading.
Particular attention is paid to the influence of
nonlinearity of concrete piles and the behavior of soil-
pile-bridge reinforced by adding a group of micropiles.
The study is carried out using a three-dimensional model
by means of the calculation code Flac 3D.
Soil-Pile Structure System and Numerical Model
The model consists of a group of piles implanted in
soil. The modeling of the behavior of such system under
seismic loading requires specific methods to take into
consideration the interaction between those different
components namely the soil-piles pile-pile piles –cap
interaction and all piles-cap-soil with the structure. The
boundaries of the model should be put sufficiently away
from the structure to minimize the effect of wave
reflection which leads to a dense mesh. To overcome
this difficulty we use specific borders which prevent the
waves from reflecting on the model. FLAC 3D is used
in this study. This code uses the Lagrangian
representation of movement. It is based on the explicit
finite difference method to solve the equations of
dynamic equilibrium.
Reference Example Elastic
The reference example consist of a group of 23
floating piles with length L
p
10.5 m. The group is
implanted into a layer of homogeneous soil with a depth
of 15 m and embedded in a cap of 1 m thickness
Figure 1. The characteristics of soil piles and
superstructure are given in Tables 1 and 2. The
mechanical and geometrical characteristics of the
reference example are plotted in Figure 1.a. The pile
heads D
p
80 cm are embedded in a cap of thickness
e
c
1m with rigid contact spacing between piles is
S 3.75D
p
3 m. To avoid the complexity of soil-cap
interaction the cap was placed 0.5 m above the soil. In
this reference example the behavior of soil-pile-
structure is assumed to be elastic with Rayleigh damping
for the soil the factor of damping used is 5 for the
soil and 2 for the structure. The fundamental
frequency of soil is 0.67 Hz. The superstructure is
modeled by a column which supports a lumped mass in
its head M350 tons. The rigidity of the superstructure
and its frequency assumed fixed at the base are equal
to K
st
86840 kN/m and F
st
2.5 Hz. They were
determined by the following expressions:
The frequency of the superstructure taking into
consideration the soil-structure interaction is f
st flex
1.1
Hz including SSI.

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Jordan Journal of Civil Engineering Volume 13 No. 2 2019
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Table 1. The elastic property of the soil and piles’ materials
Table 2. The elastic property of the superstructure
E and are the density Young modulus and
coefficient of Poisson. ζ is the factor of damping. D
p
is
the pile diameter. EA and EI are the axial and bending
stiffness. The used mesh shown in Figure 1.b includes
3856 zones of 8 nodes and 138 three-dimensional
beams of 2 nodes. The mesh was refined around the piles
and near the superstructure where inertial forces induce
high stresses.
a System geometry
Seismic loading
S375 Dp3 m
S
4 m
10 m
1 m
1 m
Mst350 T
f
st
flexible
1 Hz Superstructure
05 m
Pile Cap
Dp80 cm
x
y
z

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Reinforcement of the Seismic Interaction… Mohanad Talal Alfach
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b 3D numerical mesh with absorbing boundaries 138 beam elements and 6978 nodes
Figure 1: Problem under consideration
Seismic Loading Real
The seismic loading chosen in this research is the one
recorded in Kocaeli Turkey on 17/08/1999 Station
AMBARLI KOERI source. The seismic loading is
applied as a speed at the base of the soil as shown in
Figure 2. The maximum amplitude of this loading is
40 cm/s maximum acceleration 0.247 g.
The spectrum of Fourier corresponds to the used
seismic loading illustrated in Figure 2. We note that
the frequencies involved are less than 3 Hz with a
maximum peak for F 0.9 Hz which is between the
fundamental frequency of the soil F1 0.67 Hz and
the frequency of the structure Fss 1.1 Hz. Also note
that a first peak is observed for frequency F 0.6 Hz
which is very close to the fundamental frequency of the
soil.
Figure 2: Kocaeli earthquake record 1999
a displacement b velocity c acceleration d Fourier spectra of velocity component
15
60 m
40 m

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Jordan Journal of Civil Engineering Volume 13 No. 2 2019
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Table 3 shows the efforts induced in the piles under
Kocaeli earthquake loading. In order to compare the
obtained results the induced efforts are normalized to
inertial forces of the superstructure as follows:
Table 3. Reference example: response of a group of 23 piles under Turkey loading 1999
Seismic
Loading
ɑst
m/s²
ɑCap
m/s²
Internal Forces Normalized Forces
Central Piles Corner Piles Corner Piles
Tmax
kN
Mmax
kN.m
Tmax
kN
Mmax
kN.m
Tmax Mmax
Turkey 11.28 8.385 675.8 954.4 1016.1 1099 0.196 0.05
∗
∗
where:
m
st
: the bending moment at the base of the
superstructure.
T
cap
and
st
denote the shear force induced at the cap and
the acceleration of the superstructure mass.
H
st
: superstructure height.
Influence of Nonlinearity of Concrete Piles
Results
The numerical simulations were carried out for the
case of frictional soil C2 kPa φ30° 20° under
seismic loading recorded in Turkey 1999 with
maximum amplitude V
max
0.4 m/s. The results
obtained in the case of linear behavior of concrete piles
were compared with the results obtained for elasto-
plastic behavior of the piles and plastic bending moment
M
p
500 kN.m which is the maximum moment the
piles can support. The results show a significant
decrease in the internal forces induced in the piles with
the nonlinear behavior of the piles. This result is
confirmed in Figure 3 and Table 4. The acceleration of
the superstructure for elastic pile behavior is 23.5
higher than that obtained for elastoplastic behavior. The
comparison of elastic and elastoplastic responses Table
4 reveals a reverse trend for the acceleration at the cap.
The profile of bending moment shows that plasticity has
attained over a large part of the pile and is not located
only in the pile head plastic hinge. This result shows
that the collapse of the structure is very probable with
the elastoplastic behavior of the piles. On the other hand
we note that only in case of nonlinear behavior of the
piles we obtain a reduction of maximum normal forces
by about 24.6 and 34 for maximum shear forces
in external piles. Also there is an attenuation in the
spectral amplitude of the structure velocity as shown in
Figure 4. This is due to the damping induced by the
plasticity of the piles. It is important to note that the
presence of plasticity in the piles influences the
distribution of the maximum shear forces between the
central and external piles.
Table 4. Influence of nonlinear behavior of concrete pile on dynamic forces in piles
frictional soil earthquake of Turkey Vg 40 cm/s
Model
Concrete
ɑst
m/s
2
ɑCap
m/s
2
Dynamic Forces Normalized Forces
Central Piles Corner Piles Corner Piles
Nmax
kN
Tmax
kN
Mmax
kN.m
Nmax
kN
Tmax
kN
Mmax
kN.m
Tmax Mmax
Elastic 9.567 6.592 38.2 511.7 897.9 1840.2 917.3 1140 0.211 0.062
Plastic Mp500 kN.m 7.744 8.327 38.8 450.8 500 1386.2 604.1 500 0.154 0.033

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Reinforcement of the Seismic Interaction… Mohanad Talal Alfach
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Figure 3: Influence of nonlinear behavior of concrete pile on dynamic forces in the corner piles
frictional soil earthquake of Turkey Vg 40 cm/s
Figure 4: Influence of nonlinear behavior of concrete pile on the superstructure head spectral velocity
Fourier spectra diagram
frictional soil earthquake of Turkey Vg 40 cm/s
0
0.01
0.02
0.03
0.04
0.05
0.06
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Fréquence HZ
Amplitude
béton élastique
béton p Mp500 KN.m
Elastic Behavior of Concrete
Plastic Behavior of Concrete pile
Mp500 KN.m
Frequency Hz
0
2
4
6
8
10
12
0 200 400 600 800 1000 1200
M KN.m
H m
béton élastique
béton p Mp500 KN.m
Elastic behavior of concrete
Plastic behavior of concrete
Mp 500 KN.m
0
2
4
6
8
10
12
0 200 400 600 800 1000
T KN
H m
béton élastique
béton p Mp500 KN.m
Elastic Behavior of Concrete
Plastic Behavior of Concrete
Mp 500 KN.m
a Bending Moment b Shear Force

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Jordan Journal of Civil Engineering Volume 13 No. 2 2019
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Piles’ Interaction Reinforcement
The site observations of Lizzi and Carnevale 1981
Pearlman et al. 1993 Mason 1993 Herbst 1994 as
well as recent research have demonstrated that the
micropiles’ system constitutes a reliable tool as
reinforcement technique for existing structures. The
facility of installation especially in difficult access
locations represents its main asset. The use of such
system in seismic sites provides great benefits because
this system of foundations is characterized by good
flexibility and ductility which are very appreciated
properties for structures exposed to seismic risks. In the
field of numerical modeling the main research on the
seismic behavior of micropiles focused on their use as
foundation of new structures Sadek 2003 Al Saleh
2006. In this part we examine the response of soil-pile-
structure system reinforced by a group of micropiles.
The model used is an identical system to that previously
studied with a foundation of 6 piles that will be
reinforced by a group of 4 micropiles 22. The
micropiles’ diameter is D
m
0.25 m Figure 5. The
implementation of the reinforcement solution used takes
place by enlarging the existing cap in which the
micropiles will be built in order to rigidify the
foundation system. The interface between the piles or
micropiles and the soil is supposed elastic. The
calculations were carried out with assuming the value of
piles’ plastic moment M
p
500 kN.m. The applied
loading is the record of Turkey 1999 but with a
maximum amplitude V
max
0.6 m/sec.
This amplitude was chosen in order to induce the
development of plasticity over a large part of the piles
which justifies the reinforcement. The soil
characteristics are identical to those used in the previous
sections frictional soil: C2 kPa φ30° 20°. In
order to limit the number of reinforcement elements 4
elements and perform a qualitative analysis the
behavior of the micropiles will be assumed to be linear
elastic even if the induced forces exceed their bearing
capacity. For a real study the number of micropiles must
be optimized according to applied seismic loading.
Figure 5: Micropile reinforcement scheme
S375 D3 m
Superstructure
Dp80 cm
4 m
10 m
05 m
05 m
x
Seismic Loading
Piles
1m
S375D3 m
Mst350
T
Cap
y
3m Sm
Dm025 m
15 m
Micropiles

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Reinforcement of the Seismic Interaction… Mohanad Talal Alfach
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Effect of Pile-Micropile Spacing
In order to analyze the influence of pile-micropile
spacing on the seismic response of the system
numerical simulations were carried out for several
values of pile-micropile spacing S
m
1.5 2 and 3 m.
Tables 5 and 6 and Figure 6 give the results of the
comparison between system response before and after
reinforcement for several pile-micropile spacings.
Firstly it is noted that reinforcement with micropiles can
constitute an effective reinforcement solution. In fact
there is a strong reduction in the internal forces in the
piles after reinforcement. Figure 6 compares the
bending moment envelopes in the piles with and without
reinforcement. After reinforcement we note that the
plasticized area of the pile is reduced to a point located
at the head of the pile which can be accepted by seismic
codes and does not jeopardize the structure.
Furthermore it can be seen that the maximum bending
moment decreases with the increase of pile-micropile
spacing. A similar tendency is observed for the shear
force envelope in the pile. The reinforcement incites a
reduction of the normal force by about 45 in the pile
head. The influence of pile-micropile spacing is not very
significant despite a small decrease of the internal forces
with the spacing increase. In terms of the normal forces
induced in the piles the reinforcement was very
beneficial in reducing the forces taken by the piles. In
fact a 75 attenuation of the maximum normal force in
the piles after reinforcement is obtained. However the
influence of pile-micropile spacing on the normal force
in the piles is not important but it incites a significant
attenuation of normal force in the micropiles. By
examining the spectral response of the velocity at the top
of the superstructure Figure 7 we note a decrease in
the maximum amplitude with the increase of pile-
micropile spacing. This reflects an increase in the
stiffness of the structure and explains the reduction of
the dynamic forces in the piles. This result agrees with
those obtained by Sadek 2003 concerning the increase
in the rigidity of the system and the reduction of the
lateral amplification with the spacing. Figure 8 shows
the envelope of the bending moment and shear force
induced in the micropiles under seismic loading. It can
be seen that the spacing does not have a significant effect
on the distribution of these forces. These results confirm
the measurements performed in centrifuges by Fukui et
al. 2001.
Table 5. Influence of nonlinear behavior of concrete pile on dynamic forces in piles
Model
Concrete
Mp 500 kN.m
Acc
mass
m/s
2
Acc
Cap
m/s
2
Dynamic Forces
Central Piles Corner Piles
Nmax
kN
Tmax
kN
Mmax
kN.m
Nmax
kN
Tmax
kN
Mmax
kN.m
Before
reinforcement
8.712 14.25 36.3 489.1 500 1553.2 691.5 500
After
reinforcement
S1.5 m
8.432 8.4 8.6 289.1 500 376 359.6 500
After
reinforcement
S2 m
8.358 8.759 0.5 281.9 500 422.6 339.7 500
After
reinforcement
S3 m
7.608 8.131 0.4 247.2 500 331 291 500

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Jordan Journal of Civil Engineering Volume 13 No. 2 2019
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Table 6. Influence of pile-micropile spacing on the dynamic forces in the reinforcing micropiles
Model
Dynamic Forces Micropile
Nmax
kN
Tmax
kN
Mmax
kN.m
S1.5 m 984 602.7 1090
S2 m 841.3 596.6 1080
S3 m 594 491.4 926.8
Figure 6: Influence of pile-micropile spacing on dynamic forces in the corner piles
0
2
4
6
8
10
12
0 100 200 300 400 500 600
M KN.m
H m
avant le renforcement
aprèsS1.5 m
après S2 m
après S3 m
Before reinforcement
After
After
After
0
2
4
6
8
10
12
0 100 200 300 400 500 600 700 800
T KN
H m
avant le renforcement
après S1.5 m
après S2 m
après S3 m
After
After
After
Before Reinforcement
a Bending Moment b Shear Force

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Reinforcement of the Seismic Interaction… Mohanad Talal Alfach
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Figure 7: Influence of pile-micropile spacing on the superstructure head spectral velocity
Figure 8: Influence of pile-micropile spacing on the dynamic forces in the reinforcing micropiles
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0 0.5 1 1.5 2 2.5 3
Fréquence Hz
Amplitude
avant le renforcement
après S1.5 m
après S2 m
après S3 m
Before reinforcement
After
After
After
Frequency Hz
0
2
4
6
8
10
12
0 200 400 600 800 1000 1200
M KN.m
H m
S1.5 m
S2 m
S3 m
0
2
4
6
8
10
12
0 100 200 300 400 500 600 700
T KN
H m
S1.5 m
S2 m
S3 m
a Bending Moment b Shear Force

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Jordan Journal of Civil Engineering Volume 13 No. 2 2019
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Effect of Micropile Connection
In this section we propose to analyze the effect of
the micropile-cap connection on the seismic response of
the system. Two types of connection are studied: fixed
and articulated. The analysis is carried out for pile-
micropile spacing S2 m. Figures 9 and 10 and
Tables 7 and 8 present the results obtained for the two
studied cases. By checking the induced forces in the
piles we note that reinforcement by fixed micropiles
reveals better efficiency in comparison with articulated
micropiles. For articulated micropiles the plastic
moment has attained the upper quarter of the pile.
Furthermore the maximum shear force of the piles
obtained in the case of articulated micropiles is higher
by 25-35 than the one obtained in the case of
reinforcement by fixed micropiles. This result is
accompanied by the increase of cap acceleration which
reflects lower rigidity for the system in the case of
reinforcement with articulated micropiles. The normal
effort shows comparable maximum values in both cases
fixed articulated. Concerning the internal forces
developed in the micropiles the presence of articulation
permits to relieve the induced internal forces in the pile
head. In that case the maximum moment obtained at a
depth of 2 m of the micropile is considerably less than
that developed at the top of the fixed micropiles
M1080 kN.m. This result is consistent with the
results found by Sadek 2003. The profile of the shear
force also shows a decrease in the case of articulated
micropiles which is not in favor of the internal forces
induced in the existing piles.
Table 7. Influence of micropile/shear connection on the dynamic forces in the piles
frictional soil φ30° C2 kPa ψ20° earthquake of Turkey Vg 60 cm/s
Model
Concrete
Mp 500kN.m
Acc
mass
m/s
2
Acc
Cap
m/s
2
Dynamic Forces
Central Piles Corner Piles
Nmax
kN
Tmax
kN
Mmax
kN.m
Nmax
kN
Tmax
kN
Mmax
kN.m
Fixed
Micropiles
8.358 8.759 0.5 281.9 500 422.6 339.7 500
Articulated
Micropiles
8.179 14.28 18.7 381.2 500 406.1 499.4 500
Table 8. Influence of micropile/shear connection on the dynamic forces in the micropiles
frictional soil φ30° C2 kPa ψ20° earthquake of Turkey Vg 60 cm/s
Model
Dynamic Forces
Nmax
kN
Tmax
kN
Mmax
kN.m
Fixed 841.3 596.6 1080
Articulated 655.8 359 529.7

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Reinforcement of the Seismic Interaction… Mohanad Talal Alfach
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Figure 9: Influence of micropile/shear connection on the dynamic forces in the corner piles
frictional soil φ30° C2 kPa ψ20° earthquake of Turkey Vg 60 cm/s
Figure 10: Influence of micropile/shear connection on the dynamic forces in the micropiles
frictional soil φ30° C2 kPa ψ20° earthquake of Turkey Vg 60 cm/s
0
2
4
6
8
10
12
0 100 200 300 400 500 600
M KN.m
H m
avant le renforcement
micropieux encastrés
micropieux articulés
Before reinforcement
Fixed micropiles
Articulated micropiles
Before reinforcement
Fixed micropiles
Articulated
0
2
4
6
8
10
12
0 100 200 300 400 500 600 700 800
T KN
H m
avant le renforcement
micropieux encastrés
micropieux articulés
Before reinforcement
Fixed micropiles
Articulated micropiles
a Bending Moment b Shear Force
0
2
4
6
8
10
12
0 100 200 300 400 500 600 700
T KN
H m
micropieux encastrés
micropieux articulés
Fixed micropiles
Articulated micropiles
0
2
4
6
8
10
12
0 200 400 600 800 1000 1200
M KN.m
H m
micropieux encastrés
micropieux articulés
Fixed micropiles
Articulated micropiles
a Bending Moment b Shear Force

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Jordan Journal of Civil Engineering Volume 13 No. 2 2019
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Effect of inclination of the micropiles
In this section we are interested in the analysis of the
effect of the inclination of the micropiles which can be
beneficial as already reported by Sadek 2003 and Al
Saleh 2006 for micropiles used as new foundations.
We study the influence of inclination on the response of
the existing structure superstructure + piles as well as
on the response of the micropiles themselves. The soil-
pile and soil-micropile connections are assumed to be
perfectly rigid. Figures 11 and 12 and Tables 9 and
10 present the results of numerical simulations carried
out for two inclinations of the micropiles α 0° and α
15° in the case of frictional soil C2 kPa φ30°
ψ20°. In the case of inclined micropiles we note a
considerable decrease in the shear force accompanied by
an attenuation in the lateral acceleration at the
superstructure and the cap. This decrease in the shear
force is in the order of 30 compared with the vertical
reinforcement. Concerning the bending moment in the
piles the plastic moment has attained at the pile heads
in both cases In the case of inclined micropiles a
significant attenuation is observed in the upper half of
the piles. The inclination is also very beneficial for the
normal force induced in the piles where a considerable
reduction of the normal force is obtained. This beneficial
effect of the inclination confirms the results of the tests
carried out in centrifuges by Fukui et al. 2001. By
examining the forces induced in the reinforcing
micropiles it can be seen that the inclination of the
micropiles results in a significant reduction in the
maximum shear force and bending moment. This
reduction attains 53 for the shear force and 42 for
the bending moment which significantly improves the
strength of the reinforcement elements without
jeopardizing the existing piles. It should be noted that
the inclination of the micropiles leads to a moderate
increase 15 of the normal force in the micropiles.
Table 9. Influence of inclination of the micropiles on the dynamic forces in the piles
frictional soil φ30° C2 kPa ψ20° earthquake of Turkey Vg 60 cm/s
α °
Acc
mass
m/s
2
Acc
Cap
m/s
2
Dynamic Forces
Central Piles Corner Piles
Nmax
kN
Tmax
kN
Mmax
kN.m
Nmax
kN
Tmax
kN
Mmax
kN.m
0 8.358 8.759 0.5 281.9 500 422.6 329.7 500
15 6.473 6.897 0.3 206.5 500 126 231.9 500
Table 10. Influence of inclination of the micropiles on the dynamic forces in the micropiles
frictional soil φ30° C2 kPa ψ20° earthquake of Turkey Vg 60 cm/s
α °
Dynamic Forces
Nmax
kN
Tmax
kN
Mmax
kN.m
0 841.3 596.6 1080
15 978.5 276 624

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Reinforcement of the Seismic Interaction… Mohanad Talal Alfach
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Figure 11: Influence of inclination of the micropiles on the dynamic forces in the corner piles
frictional soil φ30° C2 kPa ψ20° earthquake of Turkey Vg 60 cm/s
Figure 12: Influence of inclination of the micropiles on the dynamic forces in the micropiles
frictional soil φ30° C2 kPa ψ20° earthquake of Turkey Vg 60 cm/s
0
2
4
6
8
10
12
0 100 200 300 400 500 600
M KN.m
H m
α 0°
α 15°
0
2
4
6
8
10
12
0 50 100 150 200 250 300 350
T KN
H m
α 0°
α 15°
0
2
4
6
8
10
12
0 100 200 300 400 500 600 700
T KN
H m
α 0°
α 15°
0
2
4
6
8
10
12
0 200 400 600 800 1000 1200
M KN.m
H m
α 0°
α 15°

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Jordan Journal of Civil Engineering Volume 13 No. 2 2019
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CONCLUSIONS
This study was devoted to global numerical
modeling of soil-pile-bridge interaction problem under
seismic loading. Attention was particularly given to the
influence of soil nonlinearity and development of plastic
hinges in piles. The research within the domain of this
study was conducted using a three-dimensional finite
differences’ modeling program FLAC 3D. The
consideration of non-linear behavior of concrete is
important especially if the bearing capacity of the
concrete is likely to be exceeded. It permits better
prediction of system failure under any load. The
development of plastic hinges leads to an attenuation of
the overall response of the system. It is found that the
plasticizing of the piles changes the report of
distribution of shear and normal forces between the
central and external piles. This type of behavior permits
to predict the failure of the system in the case of
plasticity extension in the concrete piles which is not
the case with a linear behavior. On the other hand it
permits an analysis of the behavior of an existing
structure requiring reinforcement. The results confirm
the efficiency of the reinforcement system with
micropiles for existing piles. The implantation of
reinforcement elements plays a decisive role in the
response of the system. Parametric study of the
reinforcement system pile-micropile spacing micropile
connection and micropile inclination was carried out.
The results reveal a slight effect of pile-micropile
spacing on the response of the system. The fixing of
micropiles in the cap allows a better attenuation of the
forces in the piles and the structure. Similarly the use of
inclined micropiles as reinforcing elements is very
beneficial to existing piles and micropiles themselves.
These conclusions confirm the results of recent
centrifuge tests on groups of piles reinforced by
micropiles
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Finn W.D.L. 2005. “A study of piles during earthquakes:
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