Recent Research on EPS Geofoam Seismic Buffers Richard J. Bathurst and Saman Zarnani GeoEngineering Centre at Queen s-rmc Canada
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 of backfill (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 reduction is the same except that the loads are higher The product of choice is expanded polystyrene geofoam (EPS) rigid basement wall retained soil Geofoam blocks buffer
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%
PROOF OF CONCEPT
Experimental study: General arrangement of shaking table tests 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.)
View of geofoam buffer during construction 1.4 m
Experimental study: Properties of EPS geofoam buffer material Wall # EPS bulk density (kg/m 3 ) EPS initial tangent Young s modulus (MPa) EPS Thickness (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 7 6 (50% removed by cutting strips) 6 (57% removed by coring) 1.32 (89% removed by coring) 1.6 0.15 XI 1.3 0.15 XI 0.34 0.15 XI Note: Density of unmodified EPS geofoam = 12 kg/m 3
Experimental study: Properties of backfill soil artificial sintered synthetic olivine material (JetMag 30-60) silica-free Property Value Density 1550 kg/m 3 Peak angle of friction 51 Residual friction angle 46 Cohesion 0 kpa Relative density 86% Dilation angle 15
Experimental study: Table excitation 1.0 0.8 0.6 Acceleration (g) 0.4 0.2 0.0-0.2-0.4-0.6-0.8-1.0 0 10 20 30 40 50 60 70 80 90 100 stepped-amplitude sinusoidal base input excitation frequency = 5Hz Time (s) Acceleration (g) 1.0 0.8 0.6 0.4 0.2 0.0-0.2-0.4-0.6-0.8-1.0 39 40 41 42 Time (s) 3-second window
Experimental study: Buffer forces
Experimental study: Total force versus (peak) acceleration F total horizontal wall force (kn) 24 22 20 18 16 14 12 10 8 6 4 2 0 Wall 1 (no buffer) Wall 2 buffer density =16 kg/m 3 Wall 7 buffer density =1.32 kg/m 3 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 acceleration (g) (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.)
Experimental study: Buffer compressive strains and stresses
Experimental study: Dynamic geofoam modulus
Experimental study: Dynamic geofoam modulus 10 initial elastic Young's modulus, E i (MPa) 1 maximum average minimum modified EPS range of values reported range of in modulus the literature values based on (Bathurst correlations et al. 2006a) reported by Bathurst et al. (2006) 0.1 0 2 4 6 8 10 12 14 16 18 geofoam bulk density (kg/m 3 )
NUMERICAL MODEL VERIFICATION
Numerical studies: Model in FLAC A slip and separation interface with friction angle of 15
Numerical study: actual shaking
Constitutive models Soil modeled as a purely frictional, elastic-plastic material with Mohr-Coulomb failure criterion Perfectly plastic Elastic Soil M-C model e Geofoam buffer material modeled as a linear elastic, purely cohesive material Elastic 1% Geofoam
Numerical studies: Numerical results - Forces F total 14000 12000 Wall 2, EPS = 16 kg/m 3 experimental 12000 10000 Wall 7, EPS = 1.32 kg/m 3 experimental total wall force (N / m) 10000 8000 6000 4000 2000 numerical total wall force (N / m) 8000 6000 4000 2000 numerical 0 0 10 20 30 40 50 60 70 80 90 100 0 0 10 20 30 40 50 60 70 80 90 100 110 time (s) time (s) Wall 2, EPS =16 kg/m 3 Wall 7, EPS =1.3 kg/m 3 (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.)
Influence of constitutive model on numerical results?
Simple M-C model
Equivalent Linear Method (ELM) unload-reload cycles with hysteresis behavior modulus degradation and damping ratio variation
Influence of material constitutive model, ELM Shear modulus variation Damping ratio variation
Resonant column testing of geofoam specimens
Cyclic load testing of geofoam specimens using PIV
EPS material properties for ELM hysteresis model G / G max 1.0 0.8 0.6 0.4 0.2 0.0 a) EPS type 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 Athanasopoulos et al. (2007) Athanasopoulos et al. (1999) 0.00001 0.0001 0.001 0.01 0.1 1 10 100 cyclic shear strain (%) 30 b) 25 damping ratio (%) 20 15 10 Athanasopoulos et al. (1999) 5 0 0.00001 0.0001 0.001 0.01 0.1 1 10 100 cyclic shear strain (%)
Influence of material constitutive model, ELM 1.0 0.8 a) G / G max 0.6 0.4 fit with FLAC default function Sand modulus degradation & damping curves 0.2 0.0 0.00001 0.0001 0.001 0.01 0.1 1 10 100 70 60 range of shear modulus values for sand (Seed and Idriss 1970) b) cyclic shear strain (%) fit with FLAC default function damping ratio (%) 50 40 30 20 range of damping ratio values for sand (Seed and Idriss 1970) 10 0 0.00001 0.0001 0.001 0.01 0.1 1 10 100 cyclic shear strain (%)
Numerical studies: Influence of material constitutive model Comparison of numerical results (RIGID wall) wall force (kn/m) 20 18 16 14 12 10 8 6 experimental, Test 1, Rigid control wall numerical (ELM, with hysteresis damping) numerical (linear elastic-plastic, with constant Rayleigh damping) rigid wall geofoam F a) 4 2 0 0 20 40 60 80 100 time (s) (Zarnani, S. and Bathurst, R.J. 2009. Influence of constitutive model on numerical simulation of EPS seismic buffer shaking table tests. Geotextiles and Geomembranes, Vol. 27, No. 4, pp. 308-312.)
Numerical studies: Influence of material constitutive model Comparison of numerical results (EPS wall) 20 18 16 experimental, Test 2, EPS density = 16 kg/m 3 numerical (ELM, with hysteresis damping) numerical (linear elastic-plastic, with constant Rayleigh damping) b) wall force (kn/m) 14 12 10 8 6 4 2 0 0 20 40 60 80 100 time (s) (Zarnani, S. and Bathurst, R.J. 2009. Influence of constitutive model on numerical simulation of EPS seismic buffer shaking table tests. Geotextiles and Geomembranes, Vol. 27, No. 4, pp. 308-312.)
PARAMETRIC NUMERICAL STUDY
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 / f 11 ) 1 (m) 5 (m) 0 EPS19 0.3 3 (m) 15 (m) 0.025 EPS22 0.5 6 (m) 30 (m) 0.05 EPS29 0.85 0.7g 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 f 11 = natural frequency of the wall-backfill system
Parametric numerical studies: Model excitation Variable amplitude sinusoidal acceleration record: t u ( t) e t sin(2 ft) 0.8 0.6 0.4 f = 1.25 Hz f / f 11 = 0.5 for 6 m high wall acceleration (g) 0.2 0.0-0.2-0.4-0.6-0.8 0 2 4 6 8 10 12 14 16 18 time (s)
Parametric numerical studies: Material properties of backfill soil 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 Property Value Unit weight 18.4 kn/m 3 Friction angle 38 Cohesion 3 kpa Shear modulus 6.25 MPa Bulk modulus 8.33 MPa
Parametric numerical studies: Material properties of EPS geofoam Modeled as purely cohesive material with elastic-perfectly plastic Mohr-Coulomb failure criterion Property Type EPS19 EPS22 EPS29 Density (kg/m 3 ) 19 22 29 Yield (compressive) strength (kpa) 81.4 102 150 Shear strength (kpa) 40.7 51 75 Young s modulus (MPa) 5.69 6.9 9.75 Poisson s ratio 0.1 0.12 0.16
Parametric numerical studies: Example wall force-time response 3 m-high wall with EPS22 excited at 0.3 f 11 300 250 H = 3 m EPS22 f = 0.3 f 11 Control wall Control wall wall maximum maximum wall wall force force wall with with force geofoam with t geofoam = t = 0.05 H t = 0.05 H maximum maximum wall force wall with force geofoam with geofoam t = 0.1 H t = 0.1 H maximum wall force with geofoam t = 0.2 H wall force (kn/m) 200 150 100 maximum wall force-control case 50 0 0 2 4 6 8 10 12 14 16 18 time (s)
Parametric numerical studies: New design and performance parameters 3 E Elastic modulus of geofoam Buffer stiffness K (MN/m ) t geofoam thickness Isolation efficiency peak force (rigid wall) peak force (seismic buffer) peak force (rigid wall) 100% (Zarnani, S. and Bathurst, R.J. 2009. Numerical parametric study of EPS geofoam seismic buffers, Canadian Geotechnical Journal Vol. 46, No. 3, pp. 318-338.)
Design charts 70 60 70 0.3 f a) H = 1 m 11 1.4 f 11 b) H = 3 m EPS19 EPS19 60 EPS22 EPS22 0.3 f 11 1.4 f 11 EPS19 EPS19 EPS22 EPS22 isolation efficiency (%) 50 40 30 20 EPS29 EPS29 isolation efficiency (%) 50 40 30 20 EPS29 EPS29 10 10 0 0 50 100 150 200 0 0 50 100 150 isolation efficiency (%) 70 60 50 40 30 20 10 K = E/t (MN/m 3 ) K = E/t (MN/m 3 ) 70 c) H = 6 m 0.3 f 11 1.4 f 11 EPS19 EPS19 60 d) H = 9 m EPS22 EPS22 EPS29 EPS29 50 40 30 20 10 isolation efficiency (%) 0.3 f 11 1.4 f 11 EPS19 EPS19 EPS22 EPS22 EPS29 EPS29 0 0 20 40 60 80 100 0 0 10 20 30 40 50 K = E/t (MN/m 3 ) K = E/t (MN/m 3 )
Influence of earthquake record 0.8 0.6 acceleration (g) 0.4 0.2 0.0-0.2-0.4-0.6-0.8 17 0 10 20 30 40 50 60 time (s) Kobe earthquake (1995)
Conclusions 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 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/m 3 was observed to be the practical range for the design of these systems to attenuate earthquake loads.
Recent example of EPS application as seismic buffer Queen Elizabeth Water Reservoir - Vancouver - Sandwell Engineering Protected with EPS geofoam from Beaver Plastics
Recent Research on EPS Geofoam Seismic Buffers Tusen Takk