Building Near Faults: Soil-Fault-Structure Interaction Nicolas K. Oettle, Ph.D., P.E.
Acknowledgements Jonathan D. Bray, Ph.D., P.E. University of California, Berkeley Funding: National Science Foundation Grant No. 926473
Overview • Faults rupture up to several meters at the ground surface • Which disturbs structures and the built environment • This study addresses engineering in fault zones Four Topics: • Effects of Past Earthquakes • 2011 Tohoku Earthquake • Dynamic Modeling • Mitigation Strategies • Conclusions :: 1906 San Francisco Earthquake (Lawson, 1908)
Fault-Soil-Structure Interaction Structural damage from earthquake fault rupture :: Chi-Chi (Taiwan) Earthquake, 3 to 4.5 m of reverse fault slip (GEER)
Fault-Soil-Structure Problem Bedrock fault ruptures through soil with overlying structure Deformed Ground Surface Fault :: (Anastasopoulos et al., 2008)
Soil Ductility (Bray et al., 1994) • Showed that soil failure strain is large controlling factor in soil response • Critical in estimating damage to earthen dams
Geotechnical Centrifuge Tests (Bransby et al., 2008) • Fault-soil-structure interaction in geotechnical centrifuge • Showed importance of structure Light Load: q = 37 kPa Heavy Load: q = 91 kPa
Numerical Model of SFSI (Anastasopoulos et al., 2008) • Numerical SFSI model for evaluating fault rupture in soil • Advanced the state-of-the-art in simulation capabilities Building in fault zones is still controversial
Project Motivation • Study the fundamentals of boundary deformation problems • Analyze dynamic effects of near-field fault slip rate • Evaluate mitigation strategies for structures in fault zones :: Darfield Earthquake (Quigley et al., 2011)
Effects of Past Earthquakes 2011 Tohoku Earthquake 0.7 Vertical Displacement at Surface (m) 0.6 0.5 Ruptured Unruptured Soil Soil 0.4 0.3 0.2 0.1 0.0 30 35 40 45 50 55 60 Original Horizontal Position (m) Collaborators: Jonathan Bray, Keith Kelson, Kazuo Collaborators: Jonathan Bray Konagai Paper: Oettle, N.K. and Bray, J.D. 2013 , JGGE, Paper: Oettle, N.K. et al. 2013 . 2013 Geo-Congress. 139:10, 1637–1647. Dynamic Modeling Mitigation Strategies Structure Deformed Ground Surface Pulse Time History (Rigid Boundary) or Soil Applied Stress from Fixed 1-D Site Response Boundary (Deformable) Not to Scale Bedrock Pulse Time History Collaborators: Jonathan Bray, Douglas Dreger Collaborators: Jonathan Bray Paper: Oettle, N.K., Bray, J.D., and Dreger, D.S. Paper: Oettle, N.K. and Bray, J.D. 2013 . JGGE, 139:11, Submitted: Soil Dynamics and Earthquake Engineering . 1864–1874.
Evolution of Surface Expression • Observation: response localizes with increasing displacement • Idea: Faults with prior seismicity could begin with localized displacement 0.3 Incremental Displacement (m) 0.2 0.1 0.0 35 40 45 50 55 Original Horizontal Position (m) 0-0.3 m 0.6-0.9 m 2.1-2.4 m
Numerical Model A numerical model was developed to study this effect Shear Soil Strain Fault Mesh
Structural Model Model structures included three- and six-story steel moment frames attached to a reinforced-concrete mat foundation 10 m wide bays Elastic-perfectly plastic 7-10 kPa/floor Friction interface
Constitutive Model Yield surface: Flow rule: Hardening law (hyperbolic): :: (Beaty, 2009)
Validation Numerical model was validated with centrifuge data 2.0 Vertical Displacement at Surface (m) Dots - Centrifuge Test 1.8 Solid Lines - Numerical Model 1.87 m vert. 1.6 base offset 1.4 0.98 m vert. 1.2 base offset 1.0 0.8 0.6 0.7 m vert. 0.4 base offset 0.2 0.0 20 30 40 50 60 70 Original Horizontal Position (m)
Boundary Deformation Induced Localization • Shear band formation propagates upward with increasing displacement • Initial K 0 stress state altered to failure stress state Zone of High Stress Ratio Surface Deformation Principal Stresses Fault Movement Shear Band Boundary Deformation
The Effect from Previous Ruptures • Fault rupture may already be localized – Weakened shear zone – Existing stress state :: 1906 San Francisco Earthquake (Lawson, 1908)
Effect of Historical Seismicity • Assumed continuation of prior earthquake • More localized deformation field 0.7 Vertical Displacement at Surface (m) 0.6 0.5 Ruptured Unruptured Soil Soil 0.4 0.3 0.2 0.1 0.0 30 35 40 45 50 55 60 Original Horizontal Position (m)
Effect of Prior Ruptures on SSI Without prior rupture: • Broad deformation • Fault splitting With prior rupture: • Localized displacement • Foundation separation
Boundary Displacement Required for Localization • Based on several numerical models • Depends on soil height, failure strain, fault type 140 Soil Thickness / Required Vertical 120 100 Base Offset 80 60 Solid - Reverse Fault Dashed - Normal Fault 40 20 0 Compression 0% 5% 10% 15% PS Compression (Loading) Failure Strain
Normal and Reverse Fault Stress Fields Fundamentally different stress fields for normal and reverse faults Reverse: Stress Paths: Zone of High Stress Ratio Surface Deformation 100 Normal Fault Shear Band Stress Path 50 Plane Strain Stress Path Extension Loading Directions Initial t = ( σ 1 - σ 3 )/2 (kPa) 0 Stress Principal Stresses Fault Movement Reverse Fault Stress Path Normal: -50 Surface Deformation Graben Zone of High Stress Ratio Zone of High Stress Ratio Critical -100 State Plane Strain Stress Ratio Compression Shear Band Unloading Peak Stress Ratio -150 Shear Band 0 50 100 150 200 Principal Stresses s = ( σ 1 + σ 3 )/2 (kPa) Fault Movement
Required Boundary Deformation Controlled by field stress path failure strain 140 140 Soil Thickness / Required Vertical Soil Thickness / Required Vertical 120 120 100 100 Base Offset Base Offset 80 80 60 60 Solid - Reverse Fault Solid - Reverse Fault Dashed - Normal Fault Dashed - Normal Fault 40 40 20 20 0 0 0% 5% 10% 15% 0% 5% 10% 15% 20% PS Compression (Loading) Failure Strain Stress Path-Dependent Failure Strain • Developed the potential importance of prior fault ruptures • Elucidated the correct mechanics of fault rupture mechanisms in soil
Effects of Past Earthquakes 2011 Tohoku Earthquake 0.7 Vertical Displacement at Surface (m) 0.6 0.5 Ruptured Unruptured Soil Soil 0.4 0.3 0.2 0.1 0.0 30 35 40 45 50 55 60 Original Horizontal Position (m) Collaborators: Jonathan Bray, Keith Kelson, Kazuo Collaborators: Jonathan Bray Konagai Paper: Oettle, N.K. and Bray, J.D. 2013 , JGGE, Paper: Oettle, N.K. et al. 2013 . 2013 Geo-Congress. 139:10, 1637–1647. Dynamic Modeling Mitigation Strategies Structure Deformed Ground Surface Pulse Time History (Rigid Boundary) or Soil Applied Stress from Fixed 1-D Site Response Boundary (Deformable) Not to Scale Bedrock Pulse Time History Collaborators: Jonathan Bray, Douglas Dreger Collaborators: Jonathan Bray Paper: Oettle, N.K., Bray, J.D., and Dreger, D.S. Paper: Oettle, N.K. and Bray, J.D. 2013 . JGGE, 139:11, Submitted: Soil Dynamics and Earthquake Engineering . 1864–1874.
2011 Tohoku Aftershock Three years ago, a fault ruptured through this ridge Fault Ridge Site Down Up
Site Geology The tertiary ridge overlies cretaceous bedrock
Field Deformation Measurements Terrestrial LiDAR fault deformation on ridge by Prof. Kazuo Konagai of the University of Tokyo :: (Karabacak et al., 2011)
Field Deformation Measurements LiDAR measured 3D deformation field as determined from the original pool elevation
Conceptual Site Geometry A tertiary ridge overlying cretaceous bedrock with the pool and gymnasium on the ridge, above the fault Pool/Gym Tertiary Ridge Sedimentary Rocks South Cretaceous Abukuma Bedrock 1.2 m Not to Scale
Numerical Modeling • At least 20 m of deformable media is necessary to deform the ground surface this broadly • Elasto-plastic analysis indicates 2% or higher axial failure strain without prior ruptures matches the LiDAR data • Subsequent geophysics confirm this model 0.0 20 m 5 m Vertical Displacement (m) -0.2 -0.4 50 m -0.6 -0.8 2% (or higher) Failure -1.0 Strain Needed -1.2 35 45 55 65 Original Horizontal Position (m)
Improved Building Performance • 20 m of deformable media changed surface expression of the boundary deformation problem from localized to broad • Ridge likely had no previous ruptures at this location • Other areas with soft sediments on this fault ruptured discretely and likely had prior ruptures :: (GEER, 2011)
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