07 Bathurst Recent Research

7/30/2019 07 Bathurst Recent Research

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Recent Research on EPS Geofoam

Seismic Buffers

Richard J. Bathurst and Saman Zarnani

GeoEngineering Centre at Queens-RMC

Canada


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What is a wall (SEISMIC) buffer? A compressible inclusion placed between a rigid wall and the retained soil

Purpose: To reduce lateral earth pressure by allowing controlled yielding ofbackfill (soil straining)

Can be used for both static and dynamic loading conditions

For static case, reduction of pressure to near active case (quasi-active)

For dynamic earth pressure case, the concept of earth pressure reductionis the same except that the loads are higher

The product of choice is expanded polystyrene geofoam (EPS)

buffer

rigid basement wall

retained soil

buffer

Geofoam blocks


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First example of EPS seismic buffer Inglis et al. 1996

Deep basement in Vancouver BC Canada

Numerical analysis (FLAC) showed that the EPS seismic buffer

(1 m thick) could reduce seismic forces on the rigid basement

walls by up to 50%


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PROOF OF CONCEPT


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One control wall without buffer and 6 walls with

different buffer densities were tested

(Bathurst, R.J., Zarnani, S. and Gaskin, A. 2007. Shaking table testing of geofoam seismic buffers.

Soil Dynamics and Earthquake Engineering, Vol. 27, No. 4, pp. 324-332.)

Experimental study:

General arrangement of shaking table tests


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View of geofoam buffer during construction

1.4 m


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Wall #EPS bulk density(kg/m3) EPS initialtangent Youngs

modulus (MPa)

EPSThickness

(m)

EPS type(ASTM C

578)

1 Control structure (rigid wall with no seismic buffer)

2 16 4.7 0.15 I

3 12 3.1 0.15 XI

4 14 0.6 0.15 Elasticized

5

6

(50% removed by

cutting strips)

1.6 0.15 XI

6

6

(57% removed by

coring)

1.3 0.15 XI

7

1.32

(89% removed by

coring)

0.34 0.15 XI

Experimental study:

Properties of EPS geofoam buffer material

Note: Density of unmodified EPS geofoam = 12 kg/m3


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Experimental study:

Properties of backfill soil

artificial sintered synthetic olivine material(JetMag 30-60)

silica-free

Property Value

Density 1550 kg/m3

Peak angle of friction 51

Residual friction angle 46

Cohesion 0 kPaRelative density 86%

Dilation angle 15


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Experimental study:

Table excitation

Time (s)

0 10 20 30 40 50 60 70 80 90 100

Acceleration(g)

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.60.8

1.0

stepped-amplitude

sinusoidal base input

excitation frequency = 5Hz

3-secondwindow

Time (s)

39 40 41 42

Acceleration(g)

-1.0-0.8-0.6-0.4-0.20.0

0.20.40.60.81.0


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Experimental study:

Buffer forces


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Experimental study:

Total force versus (peak) acceleration

acceleration (g)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

horizontalwallforce(kN)

0

2

4

6

8

10

12

14

16

18

20

22

24

Wall 1

(no buffer)

Wall 2buffer density =16 kg/m

3

Wall 7

buffer density =1.32 kg/m3

Ftotal

(Zarnani, S. and Bathurst, R.J. 2007. Experimental Investigation of EPS geofoam seismic buffers

using shaking table tests, Geosynthetics International, Vol. 14, No. 3, pp. 165-177.)


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Experimental study:

Buffer compressive strains and stresses


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Experimental study:

Dynamic geofoam modulus


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Experimental study:

Dynamic geofoam modulus

range of modulus valuesbased on correlationsreported by Bathurst et al. (2006)

geofoam bulk density (kg/m3)

0 2 4 6 8 10 12 14 16 18

initialelasticYoung'smodulus,Ei(MPa)

0.1

1

10

range of values reported

in the literature

(Bathurst et al. 2006a)average

maximum

minimum

modifiedEPS


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NUMERICAL MODEL VERIFICATION


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Numerical studies:

Model in FLACA slip and separation interface

with friction angle of 15


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Numerical study:

actual shaking


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Constitutive models Soil modeled as a purely frictional, elastic-plastic

material with Mohr-Coulomb failure criterion

e

Elastic

Perfectly plastic

Soil M-C model

Geofoam buffer material modeled as a linear elastic,

purely cohesive material

Geofoam

Elastic

1%


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Numerical studies:

Numerical results - Forces

(Zarnani, S. and Bathurst, R.J. 2008. Numerical modeling of EPS seismic buffer shaking table tests,Geotextiles and Geomembranes. Vol. 26, No. 5, pp. 371-383.)

time (s)

0 10 20 30 40 50 60 70 80 90 100

totalwallforce(N

/m)

0

2000

4000

6000

8000

10000

12000

14000

experimental

numerical

Wall 2, EPS = 16 kg/m3

time (s)

0 10 20 30 40 50 60 70 80 90 100 110

totalwallforce(N

/m)

0

2000

4000

6000

8000

10000

12000

experimental

numerical

Wall 7, EPS = 1.32 kg/m3

Wall 2, EPS =16 kg/m3 Wall 7, EPS =1.3 kg/m3

Ftotal


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Influence of constitutive model on numericalresults?


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Simple M-C model


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Equivalent Linear Method (ELM)

unload-reload cycles with

hysteresis behavior

modulus degradation and

damping ratio variation


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Influence of material constitutive model, ELM

Shear modulus

variation

Damping ratio

variation


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Resonant column testing of geofoam specimens


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Cyclic load testing of geofoam specimens using PIV


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EPS material properties for ELM hysteresis model

cyclic shear strain (%)

0.00001 0.0001 0.001 0.01 0.1 1 10 100

G

/Gmax

0.0

0.2

0.4

0.6

0.8

1.0


0.00001 0.0001 0.001 0.01 0.1 1 10 100

dampingratio

(%)

0

5

10

15

20

25

30

a)

b)

Athanasopoulos et al.(2007)

Athanasopoulos et al.(1999)

Athanasopouloset al. (1999)

EPStype confinement

D24 - 0 kPa

D24 - 30 kPa

D24 - 60 kPa

D30 - 0 kPa

D30 - 30 kPa

D32 - 60 kPa

D15 - 0 kPa

D15 - 20 kPa

D29 - 0 kPa

D29 - 20 kPa

used in this study

Ossa & Romo(2008)

current study


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Influence of material constitutive model, ELM


0.00001 0.0001 0.001 0.01 0.1 1 10 100

dampingratio(%)

0

10

20

30

40

50

60

70


0.00001 0.0001 0.001 0.01 0.1 1 10 100

G

/Gmax

0.0

0.2

0.4

0.6

0.8

1.0

b)

a)

fit with FLAC default function

range of shear modulus values for sand(Seed and Idriss 1970)

fit with FLAC default function

range of damping ratio values for sand(Seed and Idriss 1970)

Sand modulusdegradation &damping curves


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Numerical studies:

Influence of material constitutive model

Comparison of numerical results (RIGID wall)

(Zarnani, S. and Bathurst, R.J. 2009. Influence of constitutive model on numerical simulation of EPSseismic buffer shaking table tests. Geotextiles and Geomembranes, Vol. 27, No. 4, pp. 308-312.)

time (s)

0 20 40 60 80 100

wallforce(kN/m)

0

2

4

6

8

10

12

14

16

18

20a)

geofoam

rigid wall

F

experimental, Test 1, Rigid control wall

numerical (ELM, with hysteresis damping)

numerical (linear elastic-plastic,with constant Rayleigh damping)


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Numerical studies:

Influence of material constitutive model

Comparison of numerical results (EPS wall)

(Zarnani, S. and Bathurst, R.J. 2009. Influence of constitutive model on numerical simulation of EPSseismic buffer shaking table tests. Geotextiles and Geomembranes, Vol. 27, No. 4, pp. 308-312.)

time (s)

0 20 40 60 80 100

wallforce(kN

/m)

0

2

4

6

8

10

12

14

16

18

20

b)experimental, Test 2, EPS density = 16 kg/m3

numerical (ELM, with hysteresis damping)

numerical (linear elastic-plastic,with constant Rayleigh damping)


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PARAMETRIC NUMERICAL STUDY


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Parametric numerical studies:

Matrix of variables

Wall height (H)

backfill width (B)

Thickness of

geofoam (t / H)*Type of EPS

geofoam#

Input excitation

Peak

acceleration(f / f11)

1 (m) 5 (m) 0 EPS19

0.7g

0.3

3 (m) 15 (m) 0.025 EPS22 0.5

6 (m) 30 (m) 0.05 EPS29 0.85

9 (m) 45 (m) 0.1 1.2

0.2 1.4

0.4

t = seismic buffer thickness = 0 to 3.6 m# based on ASTM D6817-06 f = predominant frequency of the input excitation and

f11 = natural frequency of the wall-backfill system


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Variable amplitude sinusoidal acceleration record:


Model excitation

)2sin()( fttetu t

time (s)

0 2 4 6 8 10 12 14 16 18

acce

leration(g)

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

f = 1.25 Hz

f / f11 = 0.5 for 6 m high wall


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Material properties of backfill soil

Property Value

Unit weight 18.4 kN/m3

Friction angle 38

Cohesion 3 kPa

Shear modulus 6.25 MPa

Bulk modulus 8.33 MPa

loose to medium dense sand

modeled as frictional material with elastic-perfectly plastic Mohr-

Coulomb failure criterion

small cohesion to ensure numerical stability at the unconfined

soil surface when models were excited at high frequencies


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Material properties of EPS geofoam

Modeled as purely cohesive material with elastic-perfectly

plastic Mohr-Coulomb failure criterion

PropertyType

EPS19 EPS22 EPS29

Density (kg/m3) 19 22 29

Yield (compressive)

strength (kPa)81.4 102 150

Shear strength (kPa) 40.7 51 75

Youngs modulus (MPa) 5.69 6.9 9.75

Poissons ratio 0.1 0.12 0.16


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Example wall force-time response

time (s)

0 2 4 6 8 10 12 14 16 18

wallforce(kN/m)

300

250

200

150

100

50

0

Control wall

maximum wall force-control case

H = 3 mEPS22f = 0.3f11

time (s)

0 2 4 6 8 10 12 14 16 18

wallforce(kN/m)

300

250

200

150

100

50

0

Control wall

maximum wall force with geofoam t = 0.05H


H = 3 mEPS22f = 0.3f11

time (s)

0 2 4 6 8 10 12 14 16 18

wallforce(kN/m)

300

250

200

150

100

50

0

Control wall




H = 3 mEPS22f = 0.3f11

time (s)

0 2 4 6 8 10 12 14 16 18

wallforce(kN/m)

300

250

200

150

100

50

0

Control wall





H = 3 mEPS22f = 0.3f11

3 m-high wall with EPS22 excited at 0.3 f11


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New design and performance parameters

3 E Elastic modulus of geofoamBuffer stiffness K (MN/m )t geofoam thickness

100%wall)(rigidforcepeak

buffer)(seismicforcepeakwall)(rigidforcepeakefficiencyIsolation

(Zarnani, S. and Bathurst, R.J. 2009. Numerical parametric study of EPS geofoam seismicbuffers, Canadian Geotechnical Journal Vol. 46, No. 3, pp. 318-338.)


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Design charts

K = E/t (MN/m3)

0 50 100 150 200

isolationefficiency(%)

0

10

20

30

40

50

60

70

K = E/t (MN/m3)

0 50 100 150

isolationefficiency(%)

0

10

20

30

40

50

60

70

K = E/t (MN/m3)

0 20 40 60 80 100

isolationefficie

ncy(%)

0

10

20

30

40

50

60

70

K = E/t (MN/m3)

0 10 20 30 40 50

isolationefficie

ncy(%)

0

10

20

30

40

50

60

70

a) H = 1 m b) H = 3 m

c) H = 6 m d) H = 9 m

0.3f11

1.4f11

EPS19

EPS22

EPS29

EPS19

EPS22

EPS29

0.3f11

1.4f11

EPS19

EPS22

EPS29

EPS19

EPS22

EPS29

0.3f11

1.4f11

EPS19

EPS22

EPS29

EPS19

EPS22

EPS29

0.3f11

1.4f11

EPS19

EPS22

EPS29

EPS19

EPS22

EPS29


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Kobe earthquake (1995)

time (s)

0 10 20 30 40 50 60

acceleration(g)

-0.8

-0.6

-0.4

-0.20.0

0.2

0.4

0.6

0.8

17

Influence of earthquake record


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Experimental shaking table test results and numerical simulations

demonstrated proof of concept for using EPS geofoam material as a seismic

buffer to attenuate dynamic earth pressures against rigid retaining walls.

The magnitude of seismic load reduction in shaking table models was as high

as 40% for the softest geofoam.

The numerical simulations of the experiments showed similar reductions in

seismic-induced lateral earth force observed in physical tests.

A verified FLAC numerical model was used to carryout a parametric study to

investigate the influence of different parameters on buffer performance and

isolation efficiency:

Significant load attenuation occurs by introducing a thin layer of geofoam(> 0.05H) at the back of the wall and the attenuation increases as the

thickness of the buffer increases.

The least stiff EPS geofoam in this study resulted in the largest load

attenuation.

Conclusions


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The practical quantity of interest to attenuate dynamic loads

using a seismic buffer is the buffer stiffness defined as:

K = E / t

For the range of parameters investigated in this study,K < 50 MN/m3

was observed to be the practical range for the design of these

systems to attenuate earthquake loads.

Conclusions

Recent example of EPS application as seismic buffer


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Queen Elizabeth Water Reservoir - Vancouver - Sandwell Engineering

Protected with EPS geofoam from Beaver Plastics

Recent example of EPS application as seismic buffer


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Recent Research on EPS Geofoam

Seismic Buffers

Tusen Takk

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