Soil Shearing during Earthquake Surface Fault Rupture Nicolas K. - - PowerPoint PPT Presentation

Soil Shearing during Earthquake Surface Fault Rupture

Nicolas K. Oettle, P.E.

Graduate Researcher University of California, Berkeley Advisor: Jonathan D. Bray, Ph.D., P.E. Professor of Geotechnical Engineering University of California, Berkeley Funding: National Science Foundation Grant No. 926473

Surface Fault Rupture Through Soil

:: (Bransby et al., 2008)

• Important for buildings, overpasses, railroads
• Centrifuge tests (Univ. of Dundee):

Centrifuge Incremental Surface Response

0.00 0.05 0.10 0.15 0.20 0.25 0.30 35 37 39 41 43 45 47 49 51 53 55 Incremental Vertical Displacement (m) Original Horizontal Position (m) 0-0.3 m 0.6-0.9 m 2.1-2.4 m

• Response localizes with increasing displacement

Footwall Hanging Wall Reverse Fault

Why does Localization Occur?

Zone of High Stress Ratio Principal Stresses Fault Movement Surface Deformation

• Numerical analysis with FLAC
• Shear band formation and alteration of K0 stress

What if Past Earthquakes have Already Ruptured the Soil?

• Current research does not address this
• Fault rupture may already be localized
• Weakened shear zone likely remains intact
• Initial stress state partially intact
• Stress relaxation
• Correct initial stress unknown

:: (Lade et al., 2010)

Centrifuge Test w/ Prior Rupture

1/5000 1/500 1/50 1/5 30 35 40 45 50 55 60 Angular Distortion (m/m) Original Horizontal Position (m) Ruptured Soil Unruptured Soil 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 30 35 40 45 50 55 60 Vertical Displacement at Surface (m) Original Horizontal Position (m) Ruptured Soil Unruptured Soil

Angular Distortion Surface Displacement

• Assumed continuation of prior EQ in FLAC
• Ignores stress relaxation

Potential Effect of Stress Relaxation

• Significant, but shear band also important
• Reality probably in between ruptured/unruptured

0.00 0.05 0.10 0.15 0.20 0.25 0.30 10 15 20 25 30 Vertical Displacement at Surface (m) Original Horizontal Position (m) Ruptured Soil (ss-ls) Ruptured Soil (stress reset) Unruptured Soil (ls) Ruptured Soil (no strain softening)

Prior Displacement Required to Localize?

Compression

0% 2% 4% 6% 8% 10% 12% 0% 5% 10% 15% Vertical Fault Movement to Soil Thickness (m/m) PS Compression (loading) Failure Strain Dashed-Normal Faults Solid-Reverse Faults

• Based on a number of FLAC analyses
• Depends on soil height, failure strain, fault type

Why Normal/Reverse Different?

• 150
• 100
• 50

50 100 50 100 150 200 t=(σ1-σ3)/2 (kPa) s=(σ1+σ3)/2 (kPa) Reverse Fault Normal Fault Initial Stress Peak Stress Ratio Critical State Stress Ratio

Zone of High Stress Ratio Shear Band Principal Stresses Fault Movement Graben Plane Strain Compression Unloading Zone of High Stress Ratio Surface Deformation Shear Band Zone of High Stress Ratio Shear Band Principal Stresses Fault Movement Surface Deformation Plane Strain Extension Loading

Reverse: Normal: Stress Paths:

• Result of stress path

Effect of Field Stress Path

0% 2% 4% 6% 8% 10% 12% 0% 5% 10% 15% Vertical Fault Movement to Soil Thickness (m/m) PS Compression (loading) Failure Strain Dashed-Normal Faults Solid-Reverse Faults 0% 1% 2% 3% 4% 5% 6% 7% 8% 0% 5% 10% 15% 20% Vertical Fault Movement to Soil Thickness (m/m) Field Stress Path Failure Strain Solid-Reverse Faults Dashed-Normal Faults

• Controlled by field stress path failure strain

Three Conclusions

• 1. Past earthquakes could significantly affect

fault rupture through soil

• 2. Caused by shear banding and stress

development

– Correct initial stress unknown (stress relaxation)

• 3. Normal faults more easily influenced than

reverse

– Result of field stress path