Building Near Faults: Soil-Fault-Structure Interaction...

Building Near Faults: Soil-Fault-Structure Interaction

Nicolas K. Oettle, Ph.D., P.E.

Jonathan D. Bray, Ph.D., P.E. University of California, Berkeley

Funding: National Science Foundation

Grant No. 926473

Acknowledgements

Overview

Four Topics: • Effects of Past Earthquakes • 2011 Tohoku Earthquake • Dynamic Modeling • Mitigation Strategies

• Conclusions

:: 1906 San Francisco Earthquake (Lawson, 1908)

• Faults rupture up to several meters at the ground surface • Which disturbs structures and the built environment • This study addresses engineering in fault zones

:: Chi-Chi (Taiwan) Earthquake, 3 to 4.5 m of reverse fault slip (GEER)

Fault-Soil-Structure Interaction

Structural damage from earthquake fault rupture

Deformed Ground Surface

:: (Anastasopoulos et al., 2008) Fault

Fault-Soil-Structure Problem

Bedrock fault ruptures through soil with overlying structure

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)

Light Load: q = 37 kPa

Heavy Load: q = 91 kPa

• Fault-soil-structure interaction in geotechnical centrifuge • Showed importance of structure

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)

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30 35 40 45 50 55 60

Ver

tical

Dis

plac

emen

t at S

urfa

ce (m

)

Original Horizontal Position (m)

RupturedSoil

UnrupturedSoil

Deformed GroundSurface

Pulse Time History

Structure

Pulse Time History(Rigid Boundary)

orApplied Stress from1-D Site Response

(Deformable)

Soil

Bedrock

FixedBoundary

Not to Scale

2011 Tohoku Earthquake Effects of Past Earthquakes

Mitigation Strategies Dynamic Modeling

Collaborators: Jonathan Bray Paper: Oettle, N.K. and Bray, J.D. 2013, JGGE, 139:10, 1637–1647.

Collaborators: Jonathan Bray, Keith Kelson, Kazuo Konagai Paper: Oettle, N.K. et al. 2013. 2013 Geo-Congress.

Collaborators: Jonathan Bray Paper: Oettle, N.K. and Bray, J.D. 2013. JGGE, 139:11, 1864–1874.

Collaborators: Jonathan Bray, Douglas Dreger Paper: Oettle, N.K., Bray, J.D., and Dreger, D.S. Submitted: Soil Dynamics and Earthquake Engineering.

Evolution of Surface Expression

• Observation: response localizes with increasing displacement • Idea: Faults with prior seismicity could begin with localized

displacement

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35 40 45 50 55In

crem

enta

l Dis

plac

emen

t (m

)Original Horizontal Position (m)

0-0.3 m 0.6-0.9 m 2.1-2.4 m

Soil

Fault

Numerical Model

Shear Strain

Mesh

A numerical model was developed to study this effect

Elastic-perfectly plastic

Friction interface

Structural Model

7-10 kPa/floor

Model structures included three- and six-story steel moment frames attached to a reinforced-concrete mat foundation

10 m wide bays

Constitutive Model

Yield surface:

Flow rule:

:: (Beaty, 2009)

Hardening law (hyperbolic):

Validation

0.00.20.40.60.81.01.21.41.61.82.0

20 30 40 50 60 70

Ver

tical

Dis

plac

emen

t at S

urfa

ce (m

)


Dots - Centrifuge TestSolid Lines - Numerical Model

1.87 m vert.base offset



Numerical model was validated with centrifuge data

Boundary Deformation Induced Localization

Zone of High Stress Ratio

Principal Stresses

Fault Movement

Surface Deformation

• Shear band formation propagates upward with increasing displacement

• Initial K0 stress state altered to failure stress state

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

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Ver

tical

Dis

plac

emen

t at S

urfa

ce (m

)


RupturedSoil

UnrupturedSoil

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

Compression

• Based on several numerical models • Depends on soil height, failure strain, fault type

0

20

40

60

80

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120

140

0% 5% 10% 15%

Soi

l Thi

ckne

ss /

Req

uire

d V

ertic

alB

ase

Offs

et

PS Compression (Loading) Failure Strain

Solid - Reverse FaultDashed - Normal Fault

Normal and Reverse Fault Stress Fields


Shear BandPrincipal Stresses

Fault Movement

Graben

Plane StrainCompression

Unloading


Surface Deformation

Shear Band


ShearBand

Principal Stresses

Fault Movement

Surface Deformation

Plane StrainExtension Loading

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-50

0

50

100

0 50 100 150 200

t = (σ

1-σ 3

)/2(k

Pa)

s = (σ1+σ3)/2 (kPa)

Reverse FaultStress Path

Normal FaultStress Path

InitialStress

PeakStress Ratio

CriticalState

Stress Ratio

Stress PathDirections

Fundamentally different stress fields for normal and reverse faults Stress Paths: Reverse:

Normal:

Required Boundary Deformation Controlled by field stress path failure strain

0

20

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60

80

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120

140

0% 5% 10% 15%

Soi

l Thi

ckne

ss /

Req

uire

d V

ertic

alB

ase

Offs

et

PS Compression (Loading) Failure Strain


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0% 5% 10% 15% 20%

Soi

l Thi

ckne

ss /

Req

uire

d V

ertic

alB

ase

Offs

et

Stress Path-Dependent Failure Strain


• Developed the potential importance of prior fault ruptures • Elucidated the correct mechanics of fault rupture

mechanisms in soil

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Ver

tical

Dis

plac

emen

t at S

urfa

ce (m

)


RupturedSoil

UnrupturedSoil


Pulse Time History

Structure



(Deformable)

Soil

Bedrock

FixedBoundary

Not to Scale







2011 Tohoku Aftershock

Site

Down

Fault

Ridge

Up

Three years ago, a fault ruptured through this ridge

Site Geology

The tertiary ridge overlies cretaceous bedrock

Field Deformation Measurements

:: (Karabacak et al., 2011)

Terrestrial LiDAR fault deformation on ridge by Prof. Kazuo Konagai of the University of Tokyo

Field Deformation Measurements

LiDAR measured 3D deformation field as determined from the original pool elevation

Conceptual Site Geometry

Pool/Gym

Tertiary RidgeSedimentary Rocks

CretaceousAbukuma Bedrock

Not to Scale

South

1.2 m

A tertiary ridge overlying cretaceous bedrock with the pool and gymnasium on the ridge, above the fault

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

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Vert

ical

Dis

plac

emen

t (m

)


2% (or higher) Failure Strain Needed

5 m

50 m

20 m

Improved Building Performance

:: (GEER, 2011)

• 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

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tical

Dis

plac

emen

t at S

urfa

ce (m

)


RupturedSoil

UnrupturedSoil


Pulse Time History

Structure



(Deformable)

Soil

Bedrock

FixedBoundary

Not to Scale







Dynamic Modeling

• Very near fault ground motions • Based on numerical work by

Dreger et al. (2011) • Slip rate 0.5 to 1.0 m/s

:: Dreger et al. (2011)


Pulse Time History

Structure



(Deformable)

Soil

Bedrock

FixedBoundary

Not to Scale

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0

100

200

300

Acce

lera

tion

(cm

/s2 ) This Study

Dregeret al. (2011)

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20

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80

Velo

city

(cm

/s)

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17 18 19 20 21

Dis

plac

emen

t (cm

)

Time (s)

Change in Soil Stresses

• Very unusual site response due to fling-type motions • Could cause tension in soil if fast enough slip velocity

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1.60

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350

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Bou

ndar

y V

eloc

ity (m

/s)

Stre

ss (k

Pa)

Time (s) Reverse Fault, 0.8 m/s

σyy

σxx

Vboundary

Increase in Fault Diversion • Free-field solution not changed considerably • A dynamic analysis influences the amount of building

movement

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Mov

emen

t of S

truct

ure

as F

ract

ion

ofP

seud

o-S

tatic

Mov

emen

t

Slip Velocity (m/s)

Pseudo-StaticDisplacement

Dynamic Displacement

Free-FieldBoundary Condition

Pseudostatic: 0.8 m/s:

Structural Mass

Increasing the size of the structure decreases building movement

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Mov

emen

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truct

ure

as F

ract

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ofP

seud

o-S

tatic

Mov

emen

t

Mat Pressure (kPa)

Pseudo-Static Displacement

Dynamic Displacement

• Fault rupture dynamics can have a moderate affect on the predicted soil-structure interaction

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30 35 40 45 50 55 60

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tical

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plac

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t at S

urfa

ce (m

)


RupturedSoil

UnrupturedSoil


Pulse Time History

Structure



(Deformable)

Soil

Bedrock

FixedBoundary

Not to Scale







Design Strategies for Mitigation

1. Spread fault deformation 2. Design structure to move 3. Divert the fault

Design Strategies: Engineered Fill

Limited ShearBand Propagation

Fault Movement

Distributed Ground Deformation

Spreading ofFault MovementDuctile

EngineeredFill

500

1,000

1,500

2,000

2,500

0 0.3 0.6 0.9

Mom

ent i

n 2n

d Fl

oor (

kN·m

)

Vertical Fault Displacement (m)

Stiff Previously Ruptured Native Soil

More DuctileEngineered Fill

Less DuctileEngineered Fill

Yielding

• Engineered fill spreads fault deformation • Note: first plastic behavior in structure typically bending in

beams at beam-column joints

Design Strategies: Mat Foundations

Thick mats support structure and limit distress to superstructure

Thinner Mat Columns

Floor Beams

Thicker Mat


500

1,000

1,500

2,000

2,500

0 1 2 3

Mom

ent i

n 2n

d Fl

oor (

kN·m

)

Mat Thickness (m)

0 m 0.05 m 0.1 m0.2 m 0.3 m 0.6 m

Reverse Fault

Vertical Fault Displacement:

Yield Moment

• Increasing mat thickness decreases structural loads • Can decrease structural distress to elastic levels or

to life-safety levels


Thick mats are resilient to complex fault deformation

Thick MatFoundation

Fault 2Fault 1

Design Strategies: Fault Diversion

:: 1999 Kocaeli Earthquake (Lettis et al., 2000)

Heavy structures diverted fault rupture in Kocaeli


Ground improvement diverts well located faults

Reverse Fault

Soil

Ground�Improvement

Six-storyStructure

Normal FaultSoil

Diaphragm Wall

Three-storyStructure

Tiebacks


Structural walls can divert well located faults


Reverse Fault

Soil

Anchors

Three-storyStructure

Ground anchors can hold a building to one side of a fault


Fault

Seismic Gap

Structure

Excavation

Gaps or soft ground can accommodate fault displacement

Mitigation strategies can work and come in a variety of styles

Conclusions

• Historic seismicity affects soil-fault-structure interaction • Can explain localized fault deformation observed • Japan showed how soil can spread fault rupture • Slip rate also affects the boundary deformation solution • Safe structural design in fault zones is achievable

• Spread fault deformation • Design structure to move • Divert the fault

Building Near Faults: Soil-Fault-Structure Interaction...

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