Study of Passive Isolation of Deep Foundations in Sandy ... - Ejge.com

Study of Passive Isolation of Deep
Foundations in Sandy Soil by
Rectangular Trenches
- 1297 -
Mehrab Jesmani
Associate Professor, Department of Civil Engineering, Imam Khomeini
International University, Qazvin, Iran
Jesmani@ikiu.ac.ir
Arash Moghadam Fallahi
Department of Civil Engineering, Imam Khomeini
International University, Qazvin, Iran
Hamed Faghihi Kashani
Department of Civil Engineering, Imam Khomeini
International University, Qazvin, Iran
Hamed.Faghihey@Gmail.com
ABSTRACT
Wave barriers are intended to mitigate the transmission of vibrations in the soil actively or passively including
open and in-filled trenches, sheet piles, etc. In most previous studies, the researchers haven’t reached to an
agreement in effective parameters such as the height of the trenches and also the effect of these parameters on
screening induced by shallow foundations.
In this study, the passive screening has been evaluated in sandy soils with the help of open trenches against
deep foundation vibration by ANSYS software as two-dimensional to carry out an extensive parametric study
on passive isolation. Because of the assessed strain less than 10 -3 the linear soil behavior has been utilized.
KEYWORDS: Passive Isolation, Vibration Reduction, RayLeigh Wave, Pile foundations, Sand soil,
Rectangular trenches, Body Waves, ANSYS Program.
INTRODUCTION
Isolating the sensitive structures against vibrations which are undesirable and created by industrial
machineries foundation, vehicle traffic, explosions, earthquakes, and may lead to fatigue and failure of these
structures have became an important subject in engineering science. Generally, installing wave barriers near the
sensitive structures to mitigate adverse effects of vibrating is known as passive isolation.
Regarding the literature on ground-borne vibrations, Barkan (1962), as the first scientist, used screening
against vibration waves with open trenches and reported that open trench dimensions are large enough relative to
the wavelength of the surface motions. Neumeuer (1963) performed field tests to evaluate the effects of
vibrations on a wood factory in Berlin which were created by subway by using Bentonite in-filled trenches
concluding that the wave amplitude could decrease approximately 50 percent.

Vol. 16 [2011], Bund. Q 1298
Woods(1968-1969) conducted a series of field tests to study the screening performance of different
governing parameters of trenches in active and passive isolation system and also defined amplitude reduction
Ratio (Arr) Concluding reduction of displacement amplitude could be achievable when trench height is deep
enough and also the thickness of the open trenches doesn’t have an obvious effect on reduction of displacement
amplitudes. Woods et al (1974) simulated vibration in half-space employing the principal of holography to
investigate the screening efficiency of hollow cylindrical piles as barriers in passive system.
Wass (1972), Haupt (1977) and Segol et al. (1978) simulated the efficiency of distance and shape of the
open trenches on amplitude reduction by using finite element method (FEM) Aboudi (1973)carried out a
research to evaluate the ground surface response of wave barriers under a time-dependent surface load in elastic
half-space through finite difference method (FDM) Fuyuki M, Matsumoto Y(1980)studied the efficiency of open
trench barrier on reaching Rayleigh waves by using a two-dimensional model through finite difference method
(FDM) May, T.W., and Bolt, B.A. (1982) conducted a research to evaluate the efficiency of open trench on
compression and shear waves under the assumption of a plane strain condition.
Beskos et al. (1985-1991) studied the efficiency of open, in-filled trenches isolation in continuously
homogeneous and non-homogenous soils under assumption of a plain strain condition by using boundary
element method (BEM) Ahmad and Al-Hussaini (1991-1996) concentrated on simplified design methodologies
for vibration screening of machine foundations by trenches using a three-dimensional boundary element
algorithm.
Yeh.C.S et al. (1997) simulated open in-filled trenches on train induced ground motions by using a FEM
analysis. Kattis et al. (1999) examined the isolation screening efficiency of pile barriers and open, in-filled
trenches. They found out that trenches are more efficient than pile barriers, except for the vibration with large
wavelength, where deep trenches are impractical. Hollow piles were observed to be more efficient than concrete
piles. Shrivastava (2002) conducted a research to evaluate the effectiveness of open and filled trenches for
screening Rayleigh waves because of impulse loads in a 3D FE model.
Shen-Haw Jo, Hung-Ta Lin (2004) worked on analysis of train-induced vibrations and vibration reduction
schemes above and below critical Rayleigh wave speeds by using finite element method (FEM). The results
show that the foundations of adjacent buildings also have effects on vibration screening. Adam M., Estorff O.
(2005) evaluated the efficiency of open and filled trenches in reducing the six-storey building vibrations due to
passing trains using a two-dimensional FEM Analysis. The results show an 80% reduction in the building
vibrations and internal forces. El Naggar (2005) inspected the effectiveness of open and filled trenches in
reducing the pulse-induced waves for shallow foundations resting on an elastic half-space by using ANSYS
software. Because of the two-dimensional model, the behavior of wave motions wasn’t obvious enough behind
the trenches.
Celebi E. et al. (2006) presented two mathematical models and numerical techniques for solving problems
associated with the wave propagation in a track and an underlying soil owing to passing trains in the frequency
domain. The results show that an open trench with appropriate geometric properties can noticeably reduce the
wave frequencies. G.Y. Gao et al. (2006) explored the efficiency of pile barriers on reduction of ground
vibrations by using three-dimensional model. The similarity of thin piles and open trenches in vibration
screening isolation was reported and also the results show that the net distance between piles is an important
factor in reducing vibration.
Karlstrom and Bostrom (2007) examined the efficiency of active screening isolation in one and two sides of
a train railway on reduction of vibration amplitudes. The results showed that using open trenches could
noticeably reduce the vibration amplitudes especially at frequencies in the range of 2-8 Hertz, Tsai et al. (2007)

conducted a numerical research using three-dimensional BEM to evaluate the active screening isolation of pile
barriers in shallow foundations against vertical loading. They also examined the effectiveness of pile
dimensions, wave frequencies, screening location and pile materials on active screening isolation. They reported
that steel pipe piles are the most effective screening and concrete hollow pile barriers can be ineffective due to
its stiffness, they added that the effectiveness of pile is more important than the distance between piles in pile
barriers isolation.
Jesmani et al. (2008) explored the efficiency of geometrical properties of an open trench in the ground
vibration active isolation of deep foundations resting on a homogenous half-space clay soil by using a threedimensional
finite element method (FEM).
Depending on the obtained results, there is an optimal ratio of trench depth to pile length and installing a
deeper trench is uneconomically practical. The efficiency of trench in very deep pile foundations majorly is
independent on trench depth and location. From the above review, researchers mainly focused on active
isolations to reduce the vibration of shallow foundations by using open and in-filled trenches in which the
Rayleigh waves play an important role in transmission of ground vibration. They mostly investigated the
vibration reduction in cohesive soils. In this study, however, the ground passive vibration isolation of deep
foundations, generating Rayleigh waves, has been investigated in sandy soils.
PROPAGATION AND ATTENUATION CHARACTERISTICS
OF DEEP FOUNDATIONS
The waves which are produced from deep pile foundations in the ground are elastic waves and they are in
the form of shear waves, compression waves and surface waves (Figure1) (Attewell and Farmer, 1973)
Vertically polarized shear waves are generated by soil-shaft contact which propagates radially from the shaft
on a cylindrical surface; meanwhile, compression and shear waves propagate radially in all directions from the
toe on a spherical wave front especially at the pile toe, and Rayleigh waves propagate radially on a cylindrical
wave front along the surface. In elastic half space, both Rayleigh and body waves decrease in amplitude by
increasing the distance from the pile foundation because of the geometrical damping.
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Vol. 16 [2011], Bund. Q 1300
Figure 1: Wave propagation induced by deep foundation (Attewell and Farmer, 1973)
Wolf (1994)theoretically presents an equation between attenuation of ground vibration in the far field and
the distance from vibration source; � �� which r is the distance and n is the geometrical attenuation coefficient.
The latter is equal to 0.5 for surface waves propagating on a cylindrical wave front and equal to 1 for body
waves propagating on a spherical wave front in the interior of the half space and equal to 2 for body waves
propagating along the surface.
PROBLEM DEFINITION AND ASSUMPTIONS
In two-dimensional model the thickness of foundation is assumed 1.5m in order to be satisfied with the
foundation rigidity under compressive loads and evaluated under assumption of isotropic and homogenous with
linear soil behavior for low deformations.
A rigid concrete (table1) of a width of 4m (Bf),with 3 piles of diameter of 70cm and various length (D)
resting on a soil layer with dynamically and statically different characteristics (table2,3) of a limited thickness
underlain by a hard stratum at a depth of Hm and Length Lm is subjected to a harmonic (f = 50 Hz) compressive
concentrated load P0sin(ωt) (Figure 2) an open trench of depth H and width w is located at a distance of L from
the edge of the foundation Table 4)
Table 1: Model concrete properties
Young’s
Modulus
Poisson’s
Ratio
Density Material
Damping
� � � � � �

Young’s
Modulus
Table 2: Static properties of the sandy soils
Poisson’s
Ratio
Specific
Weight
- 1301 -
Density Material
Damping
Soil’s Mechanical
Parameters
�� � �� � �� � �
Soil
( ��
��) -
( ��
��) (��
��) -
( ��
��) Degree
(1) 30 000 0.35 17.5 1783.89 5% 10 30
(2) 50 000 0.35 19 1936.80 5% 10 40
Dynamical
Properties
Soil
Concret
(
e
�
��) -
( ��
��) -
(1) 2E+10 0.2 2500 2%
Table 3: Dynamic properties of the sandy soils
�� Shear
modulus
( ��
� �)
� �
Shear wave
velocity
�
�
K
� � = �.� �
-
� �
Rayleigh wave
velocity
(1) 16000 91 0.936 85
(2) 27000 118 0.936 110
Table 4:Geometric Properties of the trench and Deep foundation
Explanation Value
Trench Depth 10,15,20,25,30
Pile Length(m) 0,5,10,15,20
Trench Location(m) 20,35,50,70
Trench Width(m) 1
�
�

Vol. 16 [2011], Bund. Q 1302
Hm
Bf
Lm
Figure 2: Problem definition of passive isolation by open trench
GEOMETRIC MODEL
In order to reduce the computation time and in accordance with the axisymmetric ½ of the actual model is
built in two-dimensional model. To prevent any wave reflection from the base of model, the depth of model is
not less than 30m (Jesmani 2008).
FINITE ELEMENT MODEL STRATEGIES
The properties which affect the wave propagation in low strain are Stiffness, Damping, Poisson, Ratio and
Density. Stiffness and damping are more important and effective than other properties because the predicted
strains are lower than 10 -3 ; therefore, linear elastic is applied to simulate the soil behavior and the above
parameters are involved (Figure 3) ( Ishihara. K, 1996).
In two-dimensional model the computation is done under the assumption of plain strain condition. To
simulate the behavior of soil in places which we have stress concentration and we need exact strains, two
dimensional PLANE82, In places which the strains and stresses gradient don’t play an important role in results,
two-dimensional PLANE42, and for simulating the behavior of concrete foundation, two-dimensional plane82
have been employed.
To evaluate the behavior of soil and the foundation such as sliding or any probable separation at the soil
structure interface, two-dimensional surface-to-surface contact elements (TARGE169, CONTA171) have been
employed. Because of the rigidity of the foundation compared with the underlying soil, the soil surface and pile
are taken as contact surface and target surface. Normal contact stiffness and maximum contact friction
coefficient are presumed to be equal to 1 and 0.6 respectively. PLANE42 is defined by four nodes having two
degrees of freedom at each node and PLANE82 is the developed form of PLANE42 with eight
nodes(figure4)and all of them have plasticity, creep, swelling, stress stiffening, large deflection and large strain
capabilities (ANSYS Manual).

Figure 3 : Soil behavior models in accordance with magnitude of strain
Figure 4: provided modeling elements (L:PLANE42 R:PLANE82)
MESHING AND BOUNDARY CONDITION
The mesh dimension of 0.25 times of the shortest Rayleigh waves length have been taken with loading
frequency equal to 50 Hz near the foundation and 1.5 times of the longest Rayleigh waves length have been
taken with loading frequency equal to 2Hz ( 1� 35
Rayleigh wave length) for other elements.
For distant elements from the trench outer edge, the element size increase gradually.
Boundary conditions are defined to be restrained in the X and Y direction. The hard stratum underlying the
soil layer has been defined to be a rigid boundary. Meshing method is shown in Figure 5.
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Vol. 166
[2011], Bund. B Q
Figure F 5: Thhe
geometry of the finitee
element moodel
used
To investtigate
the val lidity of the oobtained
resuults
from the current studyy
we comparre
our results with the
mmodel
which Beskos et al.
(1986) has ddefined
underr
the assumpttion
of plain sstrain
in two-ddimensional
mmodel
by
uusing
BEM aand
also the researches r whhich
Al-Hassaaini
(1991) annd
M. Heshaam
El Naggarr
(2005) havee
done by
uusing
ANSYSS
software (v version 5.7). Figure (6) exxhibits
an agreeement
betweeen
the currennt
FEM modeel
and the
aabove
researcches.
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0 0.5 1.0
1.5 2.0 2.5
MODEL
VVERIFICCATION
3.0 3.5 4.0
4.5 5.0 5.5 66.0
6.5 7.0 7. 5 8.0 8.5 9.0
Figure 6: comparative
ddiagram
Presennt
stady
Naggaar
2004
Ahmadd
1991
Beskos
1986
9.5 10.0
1304

The vertical and horizontal axes illustrate the Arr (Woods 1968-1969) and trench location normalized by
Rayleigh wave length respectively.
RESULTS FROM THE FINITE ELEMENT ANALYSIS
Woods (1986-1969) put forward Arr which is a ratio of amplitude with trench to amplitude without trench
for assessing trench effectiveness.
Arr=
��������� ���� ��� ������ �������
��������� ������� ��� ������ �������
To assess the screening effectiveness of trench, parameter Aarr which is the average of amplitude reduction
ratio is employed and it’s calculated along all radial lines near the trench and in the length of one Rayleigh wave
length.
where,
Aarr= �
�
�
���
∑ Arr
i= is the radial distance between the outer edge of the foundation and trench.
n= is the number of studied points along the radial distances.
The curves which illustrate the changes of Aarr against the trench location are normalized by trench depth.
EFFECT OF TRENCH DEPTH
The effect of trench depth has been illustrated in figures7 through 12.These figures illustrate:
- Increasing the depth of trench in passive screening, cause a decrease in Arr and this is a tangible behavior
that is reported in many published researches.
- In comparison to these figures it’s observed that by increasing the length of the piles, the diagrams flush
with each other and show that by increasing the length of the piles, the distance between trench and vibration
source can be avoided.
- In a constant depth of trench, in all figures by increasing the quantity of L, Arr increases, and the optimal L
is near to 50m which is 0.08 Bf and by going far from this value (L=0.08 Bf) the trench will be useless.
- 1305 -
i

Vol. 16 [2011], Bund. Q 1306
Average amplitude reduction ratio
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
Pile's length (D) =0 m
0.00
0 5 10 15 20 25 30 35
Depth of trench (m)
D=0 ,L=20 D=0 ,L=35 D=0 ,L=50 D=0 ,L=70
Poly. (D=0 ,L=20) Poly. (D=0 ,L=35) Poly. (D=0 ,L=50) Poly. (D=0 ,L=70)
Figure 7: Effect of trench depth (D=0, Loading frequency=50Hz,Soil 1)
Average amplitude reduction ratio
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
Pile's length (D) =10 m
0.00
0 5 10 15 20 25 30 35
Depth of trench (m)
D=10 ,L=20 D=10 ,L=35 D=10 ,L=50 D=10 ,L=70
Poly. (D=10 ,L=20) Poly. (D=10 ,L=35) Poly. (D=10 ,L=50) Poly. (D=10 ,L=70)
Figure 8: Effect of trench depth (D=10, Loading frequency=50Hz,Soil 1)

Average amplitude reduction ratio
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
Pile's length (D) =15 m
0.00
0 5 10 15 20 25 30 35
Depth of trench (m)
Figure 9: Effect of trench depth (D=15, Loading frequency=50Hz,Soil 1)
Average amplitude reduction ratio
D=15 ,L=20 D=15 ,L=35 D=15 ,L=50 D=15 ,L=70
Poly. (D=15 ,L=20) Poly. (D=15 ,L=35) Poly. (D=15 ,L=50) Poly. (D=15 ,L=70)
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
Pile's length (D) =0 m
0.00
0 5 10 15 20 25 30 35
Depth of trench (m)
D=0 ,L=20 D=0 ,L=35 D=0 ,L=50 D=0 ,L=70
Poly. (D=0 ,L=20) Poly. (D=0 ,L=35) Poly. (D=0 ,L=50) Poly. (D=0 ,L=70)
Figure 10: Effect of trench depth (D=0, Loading frequency=50Hz, Soil 2)
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Vol. 16 [2011], Bund. Q 1308
Average amplitude reduction ratio
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
Pile's length (D) =10 m
0.00
0 5 10 15 20 25 30 35
Figure 11: Effect of trench depth (D=10, Loading frequency=50Hz, Soil2)
Average amplitude reduction ratio
Depth of trench (m)
D=10 ,L=20 D=10 ,L=35 D=10 ,L=50 D=10 ,L=70
Poly. (D=10 ,L=20) Poly. (D=10 ,L=35) Poly. (D=10 ,L=50) Poly. (D=10 ,L=70)
1.20
1.00
0.80
0.60
0.40
0.20
Pile's length (D) =15 m
0.00
0 5 10 15 20 25 30 35
Depth of trench (m)
D=15 ,L=20 D=15 ,L=35 D=15 ,L=50 D=15 ,L=70
Poly. (D=15 ,L=20) Poly. (D=15 ,L=35) Poly. (D=15 ,L=50) Poly. (D=15 ,L=70)
Figure 12: Effect of trench depth (D=15, Loading frequency=50Hz, Soil2)
EFFECT OF TRENCH LOCATION
As can be seen in figures13 through 18, when the depth of the trench is approximately near the pile length
(D=10), there is a minimum Aarr within the boundary of 4-6 normalized trench location ( �� �
), and by
increasing the depth of the trench into 20m and 25m the minimum Aarr takes place within the boundary of 2-3
and 0.5-1.5 normalized trench location ( �� �
) respectively. Hence, by increasing the depth of the trench related
to the pile length, Aarr decreases by decreasing the distance between foundation and trench (L) Thus, for

passive screening in the case of H≈D the trench location 4< �� �

Vol. 16 [2011], Bund. Q 1310
Average amplitude reduction
ratio
0.64
0.63
0.62
0.61
0.60
0.59
0.58
0.57
Depth of trench= 25 (m)
0.56
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Normalized trench location (L/H)
D=0 ,H=25 D=5 ,H=25 D=10 ,H=25 D=15 ,H=25
Poly. (D=0 ,H=25) Poly. (D=5 ,H=25) Poly. (D=10 ,H=25) Poly. (D=15 ,H=25)
Figure 15: Effect of trench Location (H=25, Loading frequency=50Hz, Soil 1)
Average amplitude reduction
ratio
1.20
1.00
0.80
0.60
0.40
0.20
Depth of trench= 10 (m)
0.00
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00
Normalized trench location (L/H)
D=0 ,H=10 D=5 ,H=10 D=10 ,H=10 D=15 ,H=10
Poly. (D=0 ,H=10) Poly. (D=5 ,H=10) Poly. (D=10 ,H=10) Poly. (D=15 ,H=10)
Figure 16: Effect of trench Location (H=10, Loading frequency=50Hz, Soil 2)

Average amplitude reduction
ratio
Figure 17: Effect of trench Location (H=20, Loading frequency=50Hz, Soil 2)
Average amplitude reduction
ratio
0.82
0.80
0.78
0.76
0.74
0.72
0.70
Depth of trench= 20 (m)
0.68
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00
Normalized trench location (L/H)
D=0 ,H=20 D=5 ,H=20 D=10 ,H=20 D=15 ,H=20
Poly. (D=0 ,H=20) Poly. (D=5 ,H=20) Poly. (D=10 ,H=20) Poly. (D=15 ,H=20)
0.66
0.65
0.64
0.63
0.62
0.61
0.60
0.59
0.58
Depth of trench= 25 (m)
0.57
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Normalized trench location (L/H)
D=0 ,H=25 D=5 ,H=25 D=10 ,H=25 D=15 ,H=25
Poly. (D=0 ,H=25) Poly. (D=5 ,H=25) Poly. (D=10 ,H=25) Poly. (D=15 ,H=25)
Figure 18: Effect of trench Location (H=25, Loading frequency=50Hz, Soil 2)
EFFECT OF PILE LENGTH
Figures 19 through 24 illustrate the effect of pile length on Arr and as a result it can be reported that:
For pile length D< �� 2
; there is a reduction in Aarr and it shows the significant function of open trench
screening.
For the pile length D> �� 2
; A: For the trenches near vibration source (deep foundations), increasing in the
pile length could have a significant effect on Aarr decrease.
B: For the farther trenches, increasing in pile length could have a significant effect on decreasing the
function of trench barriers (increase in Aarr) and it can be as the result of decreasing in wave amplitude far from
the vibration source.
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Vol. 16 [2011], Bund. Q 1312
Average amplitude
reduction ratio
0.96
0.94
0.92
0.90
0.88
0.86
Depth of trench =10 (m)
0.84
0 5 10 15 20
Pile Length (m)
L=20 L=35 L=50 L=70
Linear (L=20) Linear (L=35) Linear (L=50) Linear (L=70)
Figure 19: Effect of pile length (H=10, Loading frequency=50Hz, Soil 1)
Average amplitude
reduction ratio
0.88
0.86
0.84
0.82
0.80
0.78
Depth of trench =15 (m)
0.76
0 5 10 15 20
Pile Length (m)
L=20 L=35 L=50 L=70
Linear (L=20) Linear (L=35) Linear (L=50) Linear (L=70)
Figure 20: Effect of pile length (H=15, Loading frequency=50Hz, Soil 1)

Average amplitude
reduction ratio
0.80
0.78
0.76
0.74
0.72
0.70
Depth of trench =20 (m)
0.68
0 5 10 15 20
Pile Length (m)
Figure 21: Effect of pile length (H=20, Loading frequency=50Hz, Soil 1)
Average amplitude
reduction ratio
1.20
1.00
0.80
0.60
0.40
0.20
L=20 L=35 L=50 L=70
Linear (L=20) Linear (L=35) Linear (L=50) Linear (L=70)
Depth of trench =10 (m)
0.00
0 5 10 15 20
Pile Length (m)
L=20 L=35 L=50 L=70
Linear (L=20) Linear (L=35) Linear (L=50) Linear (L=70)
Figure 22: Effect of pile length (H=10, Loading frequency=50Hz, Soil 2)
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Vol. 16 [2011], Bund. Q 1314
Average amplitude
reduction ratio
Figure 23: Effect of pile length (H=15, Loading frequency=50Hz, Soil 2)
Average amplitude
reduction ratio
Depth of trench =15 (m)
0.92
0.90
0.88
0.86
0.84
0.82
0.80
0.78
0.76
0 5 10 15 20
Pile Length (m)
L=20 L=35 L=50 L=70
Linear (L=20) Linear (L=35) Linear (L=50) Linear (L=70)
0.82
0.80
0.78
0.76
0.74
0.72
0.70
Depth of trench =20 (m)
0.68
0 5 10 15 20
Pile Length (m)
L=20 L=35 L=50 L=70
Linear (L=20) Linear (L=35) Linear (L=50) Linear (L=70)
Figure 24: Effect of pile length (H=20, Loading frequency=50Hz, Soil 2)
EFFECT OF SOIL PROPERTIES
Figure 25 shows that increasing the Young's modulus lead to an increase in Aarr and the steep of this
increase in relation with Aarr increase is about 5 percent. This increase in stiff soils can be focused on as the
result of damping decrease and also as the result of wave velocity increase which leads to an increase in Raleigh
wave length.

Average amplitude
reduction factor
0.48
0.47
0.46
0.45
0.44
0.43
0.42
0.41
0.40
Figure 25: Effect of soil properties
EFFECT OF LOADING TIME
Generally, in this model increasing the loading time until 1.5s leads to an increase in Aarr and upper
amounts of loading time don’t have any effects on Aarr, as can be seen in figure 26.
Average amplitude reduction
Ratio
0.41
Soil Chart
D10 L50 H30
0 0.5 1 1.5 2 2.5 3 3.5
0.60
0.50
0.40
0.30
0.20
0.10
Soil Number
Figure 26: Effect of loading time
CONCLUSIONS
In this research, a two-dimensional finite element analysis has been conducted to evaluate the effects of
passive open-trench screening system on decreasing the amplitude of Rayleigh waves by employing ANSYS
computer program and the following conclusions could be distilled:
- 1315 -
0.45
E=30 Mpa ,φ=30˚ E=40 Mpa ,φ=35˚ E=50 Mpa ,φ=40˚
D=10,L=50,H=30,Frequency=5Hz
0.00
0 1 2 3 4 5 6
Time( Second )
D=10,L=50,H=30,Frequency=5Hz
0.48

Vol. 16 [2011], Bund. Q 1316
By determining an optimal depth for piles, the distance between trench and vibration source can be avoided.
The optimal distance for L is 0.08 Bf and by going far from this value (L=0.08 Bf) the trench will be useless.
For passive screening in the case of H≈D the trench location 4< �� �
�� 2
pile length variations could have an important effect on the open trench function.
Open trench screening with constant depth is more effective in collapsible soils than stiff soils and this can
be as the result of Rayleigh wavelength reduction in collapsible soils.
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