07 bathurst recent research
TRANSCRIPT
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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
cyclic shear strain (%)
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
cyclic shear strain (%)
0.00001 0.0001 0.001 0.01 0.1 1 10 100
dampingratio(%)
0
10
20
30
40
50
60
70
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
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:
Parametric numerical studies:
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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Parametric numerical studies:
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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Parametric numerical studies:
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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Parametric numerical studies:
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
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
maximum wall force with geofoam t = 0.1H
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
maximum wall force with geofoam t = 0.1H
maximum wall force with geofoam t = 0.2H
maximum wall force-control case
H = 3 mEPS22f = 0.3f11
3 m-high wall with EPS22 excited at 0.3 f11
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Parametric numerical studies:
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