SEISMIC SLOPE STABILITY

Tim McCrinkSeismic Hazards Mapping Program

California Geological Survey

100th Anniversary Earthquake Conference Commemorating the 1906 San Francisco Earthquake

April 18 – 22, 2006

Topics Covered

• CGS Earthquake-Induced Landslide Zones

• Site-Specific Seismic Slope Stability Analyses

CALIFORNIA GEOLOGICAL SURVEY

EARTHQUAKE-INDUCED LANDSLIDE ZONES OF REQUIRED

INVESTIGATION

LANDSLIDE ZONES - MAPPING APPROACH SUMMARY

• INPUT DATA:– GEOTECHNICAL SHEAR TEST DATA– GEOLOGIC MAP DATA– DIGITAL TERRAIN DATA– DESIGN SEISMIC DATA

• METHODOLOGY:– INFINITE-SLOPE FAILURE MODEL– NEWMARK DISPLACEMENT ANALYSIS

ZONE MAPPING CRITERIA

• ANALYTICALLY DETERMINED – HAZARD POTENTIAL CRITERIA - NEWMARK

DISPLACEMENTS:• VERY LOW - < 5 cm• LOW - > 5 cm to < 15 cm• MODERATE - > 15 cm to < 30 cm• HIGH - > 30 cm

• EXISTING LANDSLIDE AREAS, INCLUDING EARTHQUAKE-TRIGGERED SLOPE FAILURES

Landslide Inventory

Shear Strength Groups

Geologic Stratigraphy

Geologic Structure

Digital Terrain

PSHA Data

Dip Gradient

Dip Azimuth

Slope Gradient

Slope Azimuth

Strong Motion Record

Hazard Potential Matrix

Dip-Slope Map

Slope Gradient

Geologic Material Strength Map

Landslide Hazard

Potential Map,

Zone MapZone Map

GENERALIZED WORK FLOWShear Strength Data

Shear Strength Groups

Ay vs. Disp.

Stability Analyses

GEOTECHNICAL SHEAR TEST DATA

• SHEAR TEST SOURCES

• CRITERIA FOR COLLECTION

• DATA PROCESSING

Material Strength by Geologic Unit

0 10 20 30 40 50 60Angle of Internal Friction (phi)

0

5

10

15C

ount

Coarse-Grained

Fine-Grained

Chatsworth Formation (Kc)

Strength Groups

Slope Stability Analysis

• Static Conditions:– FS = R/D = Wcos α tan φ / Wsin α = tan φ / tan α

• Dynamic Conditions:– ay = (FS - 1)g sin α (Newmark’s Equation)

W

kh(t)W

W

Rs Rd

Ns = W cos α Nd

α α

STATIC CONDITIONS DYNAMIC CONDITIONS

INFINITE SLOPE MODEL

1967-1981 USGS DEM SHADED RELIEF

1998 RADAR DEM SHADED RELIEF

DIGITAL TERRAIN DATA

Slope Gradient Map

GEOLOGIC MAP INFORMATION

• STRATIGRAPHIC INFORMATION

• GEOLOGIC STRUCTURE INFORMATION

• LANDSLIDE INVENTORY INFORMATION

GEOLOGIC MATERIAL STRENGTH MAP=

+

+

Stratigraphic InformationGEOLOGIC MAP:

Geologic Structure Information

• STRIKE AND DIP MEASUREMENTS

• FOLD AXES

GEOLOGIC MAP:

Adverse Bedding Map

Geologic Structure Information

Landslide Inventories

• EXISTING LANDSLIDES

• KNOWN EARTHQUAKE-INDUCED SLOPE FAILURES

GEOLOGIC MAP:

GEOLOGIC MATERIAL STRENGTH MAP

DESIGN SEISMIC DATA

• REPRESENTATIVE STRONG-MOTION RECORD(S)

• STRONG-MOTION RECORD SELECTION

PERMANENT SLOPE DISPLACEMENTS FROM EARTHQUAKE GROUND

MOTION

DISPLACEMENT vs. YIELD ACCELERATION

0.1

1.0

10.0

100.0

1000.0

DIS

PLA

CE

ME

NT

(cm

)

0.01 0.1 1 YIELD ACCELERATION (g)

NEWMARK DISPLACEMENTvs. YIELD ACCELERATION

30 cm

15 cm

5 cm

0.076

0.129

0.232

USC STATION #14 - Channel 3

H M L VL

HAZARD POTENTIAL MATRIX

Very Low Low Moderate High1 (36) 0 to 46% 47 to 52% 53 to 57% > 57%

2 (33) 0 to 39% 40 to 46% 47 to 49% > 49%

3 (30) 0 to 31% 32 to 37% 38 to 42% > 42%

4 (25) 0 to 22% 23 to 27% 28 to 31% > 31%

5 (16) - 0 to 2% 3 to 8% > 8%

MINDEGO HILL QUADRANGLE HAZARD POTENTIAL MATRIX

Geologic Material

Strength Group (Average Phi)

HAZARD POTENTIAL

(% Slope)

HAZARD POTENTIAL

EARTHQUAKE-INDUCED LANDSLIDE ZONES OF REQUIRED

INVESTIGATION

LANDSLIDE HAZARD ZONE METHOD LIMITATIONS

• NOT CONSISTENTLY IDENTIFIED:– SHATTERED RIDGES– RIDGE-TOP SPREADING– FAILURE OF FLAT AREAS ABOVE HEAD SCARPS– LANDSLIDE RUNOUT

• FRACTURE DENSITY OR ORIENTATION• OUT-DATED TERRAIN INFORMATION• TOPOGRAPHIC AMPLIFICATION, DIRECTIVITY

SITE-SPECIFIC SEISMIC SLOPE STABILITY

Seismic Slope StabilityElements

• Investigation:– Office – Field– Laboratory

• Analyses:– Seismic– Site Response– Stability

Site Investigation Considerations• Geology, Topography, Aerial Photo Analysis

– Existing Slope Failures– Ridge-Top Failures– Cliffs, Rock Fall– Lateral Spread Features– Nearby Active Faults

• Adverse Bedding / Discontinuity Conditions• Shear Wave Velocity

Material Strength For Dynamic Conditions

From Blake et al., 2002

Seismic Analysis Considerations

• Pseudo-Static Analysis– Peak Horizontal Ground Acceleration (PGA)– Magnitude (M)– Distance (r)

• Deformation Analyses– PGA, M, r– Duration (D5-95)– Mean Period of Ground Motion (Tm)– Strong Motion Time Histories

Pseudo-Static Slope Stability Analysis

Basic Concept

From Franklin & Chang 1977

Origin of the Seismic Coefficient Approach

• Terzaghi (1950)– Severe – 0.1– Violent – 0.25– Catastrophic – 0.5

• Seed (1979)– Use k = 0.15, FS => 1.15

• Hynes-Griffin & Franklin (1984)– Use k = 0.5 * PGA, FS => 1.0, 80% strengths

SP 117 Implementation Committee “Screening Analysis”

• k = feq x (PGAr / g)• FS = 1.0

• Assumptions:– Significant Displacements of 5 or 15 cm– Stiff Soil or Soft Bedrock– Relatively Thin Slide Mass (H <~ 20 m)– Ts/Tm <~ 0.5

Determining feq

Determining feq

Example Pseudo-Static Analysis

Static Analysis

FS = 2.2

Ground Motion Estimation - PGA

Ground Motion Estimation - M, r

Seismic Coefficient - 5cm Criteria

k = f eq * MHAr = 0.56 * 0.8 = 0.45

Seismic Coefficient - 15cm Criteria

K = 0.46 * 0.8 = 0.37

Pseudo-Static Analysis

FS = 0.93

Deformation Slope Stability Analysis

Types of Deformation Analyses

• Sliding Block Models

• Stress-Deformation Models

Sliding Block Model

From Kavazanjian, et al., 1997

Types of Sliding-Block Analyses

• Rigid-Block Analysis

• Decoupled Analysis

• Coupled Analysis

Selecting the Appropriate Sliding Block Analysis

From: Wartman, et al., 2003

Performing A Rigid Block Analysis

• Pseudo-Static Analysis - ky

• Select Appropriate Time Histories

• Double Integrate Time Histories to Determine Cumulative Displacements

Example Rigid Block Analysis

Determine Yield Coefficient

Select Earthquake Time Histories

Perform Displacement Calculations

What Do the Displacement Numbers Mean?

• Sliding Block Displacements Provide an Index of Probable Seismic Slope Performance– SP 117

• <10cm – Damage Unlikely• >10cm, <100cm – Serious Deformations• >100cm – Slopes Unstable

– Implementation Committee• Buildings - <5cm• Non Building Areas - <15cm• Strain Softening Materials

– Peak Strengths - <5cm– Residual Strengths - <15cm

Internet Resources For Seismic Slope Stability

• Landslide-Related Information at CGS:– http://www.consrv.ca.gov/cgs/geologic_hazards/la

ndslides/index.htm• Seismic Hazard Zone Maps From CGS:

– http://gmw.consrv.ca.gov/shmp/ • CGS Special Publication 117

– http://www.conservation.ca.gov/cgs/shzp/webdocs/sp117.pdf

Internet Resources For Seismic Slope Stability

• Interactive Ground Motion Estimates:– http://www.consrv.ca.gov/CGS/rghm/pshamap/pshamain

.html– http://earthquake.usgs.gov/hazmaps

• Implementation Committee Report:– http://www.scec.org/resources/catalog/hazardmitigation.

html#land

• Newmark Analysis Software:– http://earthquake.usgs.gov/resources/software/slope_perf

.php