geotechnical characterization for seismic design: standard

Geotechnical Characterization for Seismic Design: Standard Penetration Testing and

Shear Wave Velocity ProfilesShear Wave Velocity Profiles

Brady R. Cox, Ph.D., P.E.Brady R. Cox, Ph.D., P.E.The University of Arkansas

Department of Civil Engineering

Geotechnical Earthquake Engineering for Seismic Design Workshop,

Department of Civil Engineering

Port-au-Prince, Haiti, November 18-19, 2010

Geotechnical Investigation: Standard Objectives • Determine the depth and thickness of soil layers (including depth

to bedrock if possible)

• Determine the location of the ground water table

• Obtain soil samples for testing

• Most common method used around the world is the Standard Penetration TestPenetration Test (SPT)

McCarthy

Standard Penetration Test (SPT): EquipmentDrill Rig

Coduto (2001)

S lit

Coduto (2001)

Split-spoon Sampler

5 OD

Coduto (2001)

5 cm OD3.5 cm ID

SPT: Procedure• Drill to the desired depth

• Drop a 63.5 kg mass on top of the drill rod from a height of 0.75 m

• Count the number of hammer blows to drive the split-spoon sampler 3 separate 15 cm intervals

• Sum of blows over the last 2 increments (i.e. the last 30 cm) is the “blow count” or N-value

C d t (2001)Coduto (2001)

• Stop if > 50 blows are needed for any 15 cm increment (refusal)

• Remove the split spoon and retrieve soil sample for characterization• Remove the split-spoon and retrieve soil sample for characterization

• Repeat the test at desired depth interval (typically every 1 – 1.5 m)

SPT: Example Boring Log

N = 11

N = 17

N = 16

N = 11

SPT: Example N-values

In-Situ Shear Wave Velocity (Vs) Measurements

• Earthquake damage is considered to be caused primarily by vertically propagating shear waves

• The velocity at which these shear waves travel through a given material (i e rock vs soil)through a given material (i.e. rock vs. soil) strongly influences the response of the material because V is directly related to shear modulusbecause Vs is directly related to shear modulus

• Therefore, a very important part of Geotechnical Earthquake Engineering is dynamic site characterization to obtain in-situ measurements of Vs

Seismic Investigation: Additional Objectives• Obtain a shear wave velocity

(Vs) profile to a depth of at least 30 m

0 06005004003002001000

Shear Wave Velocity (m/s)

30 m

• Vs reflects the shear modulus (G) of the soil according to: 50

10

(G) of the soil according to: G = *Vs2

• Vs used to obtain simplified 100

epth

(ft)

30

20

Depth (p

Seismic Site Classification via the average shear wave velocity over the top 30m (Vs30 or Vs) 150

De

40

(m)

Vs = Vs30 = 325 m/s

over the top 30m (Vs30 or Vs)

• Vs profile also needed for more advanced ground motion 200 60

50

advanced ground motion prediction via site response analysis

2002000160012008004000

Shear Wave Velocity (ft/sec)

In-Situ Shear Wave Velocity (Vs) Measurements

• Intrusive (Borehole Methods)C h l– Crosshole

– DownholeS i L i– Suspension Logging

• Non intr si e (S rface Wa e Methods)• Non-intrusive (Surface Wave Methods)– Spectral Analysis of Surface Waves (SASW)

Multi channel Analysis of Surface Waves (MASW)– Multi-channel Analysis of Surface Waves (MASW)– Refraction Microtremor (ReMi)

Crosshole: Setup and Equipment

Horizontal (H1)Geophone

H i t l (H2)


H i t l (H2)Horizontal (H2)Geophone

Vertical (V)Geophone


Vertical (V)Geophone

ReceiverCase

ReceiverCase

3D Receiver

Crosshole Hammer

Crosshole: Shear Wave Records2

Downward Impact Upward Impact

T i0

agni

tude

Trigger

Vertical Receiverin One Borehole

-2

orm

aliz

ed M

a in One Borehole

Vertical Receiverin Second Borehole

-4No

Denotes Arrival Time-6

0.0100.0080.0060.0040.0020.000-0.002

Time, sec

Denotes Arrival Time

t Vs = d / t = m/s

Crosshole: Vs Profile00

50

Thin Limestone

100

sure

men

t Dep

th, f

t Layer (?)

150

Mea

s

150

Site 2 Boreholes 41C-41A Crosshole 41C-41B Crosshole

2001000080006000400020000

SV-Wave Velocity, fps

Downhole: Setup and Equipment

BHG-3 Borehole Receiver

Borehole BH-4

Downhole: Setup and Equipment

Instrumented Sledgehammer

Shear WaveTraction Plank BHG-3

Control Box

LaptopDynamic

Si l A lSignal Analyzer

Downhole: Travel Time vs. Depth

Downhole: Vs Profile

Suspension Logging: Setup and Waveforms

Cable Head

7-Conductor cable

Diskette

OYO PS-160Logger/Recorder

Head Reducer

Upper Geophone

Winch

with Data

Lower Geophone

Filter Tube

Source

Source Driver

Weight

Overall Length ~ 25 ftOverall Length 25 ft

Depth Sequential Waveform ArrivalsCourtesy of GeoVision

Surface Wave MethodsVertically Oriented SourceVertically Oriented Source

SASW SetupReceiver 1 Receiver 2d d

(Impact, Random, or Steady-State Vibration) Receiver 1 Receiver 2d d

(Impact, Random, or Steady-State Vibration)

Vertically Oriented Velocity Transducers

Layer 1

Vertically Oriented Velocity Transducers

Layer 1

Multi-Layered SolidLayer 2

Multi-Layered SolidLayer 2

MASW Setup

SASW Equipment

Dynamic Signal Analyzer Geophones and Sledgehammer

MASW Equipment

12 – 60 Geophones

Sledge Hammer & Drop Weight Sources

Vibroseis Source

Surface Wave DispersionLow frequency

Layer 1Layer 1

VerticalParticle Motion

Vertical Particle Motion

1

Air

Layer 1Layer 1



1

Air

Layer 1Layer 1



1

Air

Low frequency surface waves have long wavelengths

Layer 2

Layer 1

Layer 2

Layer 12

1

Layer 2

Layer 1

Layer 2

Layer 12

1

Layer 2

Layer 1

Layer 2

Layer 12

1(), while high frequency waves have short

Depth Depth

Layer 3Layer 3

Depth Depth

Layer 3Layer 3

Depth Depth

Layer 3Layer 3wavelengths

W i h Depth Deptha. Material

Profilec. Longer

Wavelength, 2

b. Shorter Wavelength, 1

Depth Deptha. Material

Profilec. Longer

Wavelength, 2


Depth Deptha. Material

Profilec. Longer

Wavelength, 2


Waves with different frequencies/ qwavelengths sample different depths

Surface wave velocity (Vr) is close to shear wave velocity (Vs):Vs ~ 1.1*Vr

Example SASW Dispersion Curve

5000

Wavelength (m)1 10 100 1000

Experimental Disp. Curve

sec)

4000

/sec

)1200Receiver Spacings = 5, 10, 20, 25, 40, 50, 150, 300, 450, and 600 ft.

Velo

city

(ft/s

3000

Velo

city

(m/

800

Phas

e V

1000

2000

Phas

e V

400

1 10 100 1000 100000

1000

0

Wavelength (ft)1 10 100 1000 10000

Inversion to Obtain Vs ProfileWavelength (m)

c)

4000

50001 10 100 1000

c)

1200

Experimental Disp. CurveTheoretical Disp.Curve

Velo

city

(ft/s

ec

2000

3000

Velo

city

(m/s

ec

800 Shear Wave Velocity (ft/sec)0 2000 4000 6000 8000

Phas

e

1000

2000

Phas

e V

400

0

100

0

200

Wavelength (ft)1 10 100 1000 10000

0 0

epth

(m)

100

200

Dep

th (f

t)

400

600

D

300D

800

1000max/2

Shear Wave Velocity (m/sec)0 500 1000 1500 2000

1200

Seismic Site ClassificationRequired by Seismic Provisions in Building CodesRequired by Seismic Provisions in Building Codes

IBC (2009) ASCE 7-05

IBC & ASCE Codes – Seismic Site Classification

Vs N SuSite Class: A - F> 1 500 m/s> 1,500 m/s760 – 1,500 m/s360 – 760 m/s180 360 /180 – 360 m/s< 180 m/s

V i f d b it i di tl l t d t th

ASCE 7-05

Vs is preferred because it is directly related to the shear stiffness of the soil deposit (G = Vs

2)

Preview Importance of Seismic Site ClassificationIBC and ASCE Code – Design Response Spectra

Little Rock, Arkansas

Soft Soil (Site Class E)

AR

( )Horizontal Earthquake Force70% of the Structure Weight

Hard Rock (Site Class A)Horizontal Earthquake Force25% of the Structure Weight

0.2-sec(~ 2-story building)

Seismic Site Classification via Vs (i.e. Vs30)

ASCE 7-05

Example SitesShear Wave Velocity (m/s) Shear Wave Velocity (m/s)

0

5

02000150010005000

0

5

04003002001000

40

20

10

5

40

20

10

5

60

40

Dep

th (f

t)

20

15

Depth (m

) 60

0

Dep

th (f

t)

20

15

Depth (m

)

80 25

20

80 25

20

100

80006000400020000

30 100

160012008004000

30Vs = Vs30 = 1015 m/s Vs = Vs30 = 250 m/s

80006000400020000


160012008004000


Site Class B Site Class D

Seismic Site Classification via N

ASCE 7-05

Seismic site classification via blow count (N) is possible, but classification via Vs is preferred because Vs is a material property that stronglybecause Vs is a material property that strongly influences ground motions

Questions?Questions?

geotechnical characterization for seismic design: standard

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