Passive Isolation of Deep Foundations

8/13/2019 Passive Isolation of Deep Foundations

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Study of Passive Isolation of Deep

Foundations in Sandy Soil by

Rectangular Trenches

Mehrab Jesmani

Associate Professor, Department of Civil Engineering, Imam Khomeini

International University, Qazvin, Iran

[email protected]

Arash Moghadam Fallahi

Department of Civil Engineering, Imam KhomeiniInternational University, Qazvin, Iran

Hamed Faghihi Kashani

Department of Civil Engineering, Imam KhomeiniInternational University, Qazvin, Iran

[email protected]

ABSTRACTWave 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 havent 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 studyon passive isolation. Because of the assessed strain less than 10-3the 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 thesensitive 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.
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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 doesnt have an obvious effect on reduction of displacement

amplitudes. Woods et al (1974) simulated vibration in half-space employing the principal of holography toinvestigate 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 boundaryelement 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 concretepiles. 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 reductionschemes 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 wasnt 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 thewave 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 vibrationscreening 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)


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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 three-

dimensional 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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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 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 andDensity. 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 dont 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).

H

Lm

B


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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.


8/22

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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 trenchfor assessing trench effectiveness.

Arr=

To assess the screening effectiveness of trench, parameter Aarr which is the average of amplitude reduction

ratio is employed and its calculated along all radial lines near the trench and in the length of one Rayleigh wave

length.

Aarr=

Arr i

where,

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 its 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 Bfand by going far from this value (L=0.08 Bf) the trench will be useless.


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Vol. 16 [2011], Bund. Q 1306

Figure 7:Effect of trench depth (D=0, Loading frequency=50Hz,Soil 1)


Pile's length (D) =0 m

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.700.80

0.90

1.00

0 5 10 15 20 25 30 35

Depth of trench (m)

Averageamplitudereductionratio

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)


0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

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)


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Figure 10:Effect of trench depth (D=0, Loading frequency=50Hz, Soil 2)


0.00

0.10

0.20

0.30

0.40

0.500.60

0.70

0.80

0.90

1.00

0 5 10 15 20 25 30 35

Depth of trench (m)

Averageamplitudere

ductionratio

D=15 ,L=20 D=15 ,L=35 D=15 ,L=50 D=15 ,L=70



0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

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



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Figure 11:Effect of trench depth (D=10, Loading frequency=50Hz, Soil2)

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 byincreasing 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 relatedto the pile length, Aarr decreases by decreasing the distance between foundation and trench (L) Thus, for


0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

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



0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 5 10 15 20 25 30 35

Depth of trench (m)

Averageamp

litudereductionratio

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)


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passive screening in the case of HD the trench location 4


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Figure 15:Effect of trench Location (H=25, Loading frequency=50Hz, Soil 1)


Depth of trench= 25 (m)

0.56

0.57

0.58

0.59

0.60

0.61

0.62

0.63

0.64

0.00 0.50 1.00 1.50 2.00 2.50 3.00

Normalized tre nch location (L/H)

Averageamplitudereduction

ratio

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)


0.00

0.20

0.40

0.60

0.80

1.00

1.20

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00

Normalized trench location (L/H)

Average

amplitudereduction

ratio

D=0 ,H=10 D=5 ,H=10 D=10 ,H=10 D=15 ,H=10



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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 ; A: For the trenches near vibration source (deep foundations), increasing in thepile 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.

Depth of tre nch= 20 (m)

0.68

0.70

0.72

0.74

0.76

0.78

0.80

0.82

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00


Averageamplitud

ereduction

ratio

D=0 ,H=20 D=5 ,H=20 D=10 ,H=20 D=15 ,H=20



0.57

0.58

0.59

0.60

0.61

0.62

0.63

0.64

0.65

0.66

0.00 0.50 1.00 1.50 2.00 2.50 3.00


Averageamplitudereduction

ratio

D=0 ,H=25 D=5 ,H=25 D=10 ,H=25 D=15 ,H=25



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Vol. 16 [2011], Bund. Q 1312

Figure 19:Effect of pile length (H=10, Loading frequency=50Hz, Soil 1)


Depth of trench =10 (m)

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0 5 10 15 20

Pile Length (m )

Averageamplitud

e

reductionratio

L=20 L=35 L=50 L=70

Linear (L=20) Linear (L=35) Linear (L=50) Linear (L=70)


0.76

0.78

0.80

0.82

0.84

0.86

0.88

0 5 10 15 20

Pile Length (m)

Averageamplitude

reductionratio

L=20 L=35 L=50 L=70



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0.68

0.70

0.72

0.74

0.76

0.78

0.80

0 5 10 15 20

Pile Length (m)

Averageamplitude

reduct

ionratio

L=20 L=35 L=50 L=70



0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 5 10 15 20

Pile Length (m)

Averageamplitude

reductionratio

L=20 L=35 L=50 L=70



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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 dont have any effects on Aarr, as can be seen in figure 26.

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:

Soil Chart

D10 L50 H30

0.41

0.45

0.48

0.40

0.41

0.42

0.43

0.44

0.450.46

0.47

0.48

0 0.5 1 1.5 2 2.5 3 3.5

Soil Number

Averageamplitu

de

reductionfactor

E=30 Mpa ,=30 E=40 Mpa ,=35 E=50 Mpa ,=40

D=10,L=50,H=30,Frequency=5Hz

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0 1 2 3 4 5 6

Time( Second )

Averageam

plitudereduction

Ratio

D=10,L=50,H=30,Frequency=5Hz


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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 Bfand by going far from this value (L=0.08 Bf) the trench will be useless.

For passive screening in the case of HD the trench location 4


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Passive Isolation of Deep Foundations