# 4 COX Seismic Site Characterization(11-18-10)

Geotechnical Characterization for Seismic
Design: Standard Penetration Testing and
Shear Wave Velocity Profiles
The University of Arkansas
Department of Civil Engineering
Geotechnical Earthquake Engineering for Seismic Design Workshop,
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 Test
(SPT)
McCarthy
Standard Penetration Test (SPT): Equipment
Drill Rig
Coduto (2001)
Split-spoon
S
lit
Sampler
Coduto (2001)
5 cm OD
3.5 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
• Stop if &gt; 50 blows are needed for any 15 cm increment (refusal)
• Remove the split
split-spoon
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.
(i e rock vs.
vs soil)
strongly influences the response of the material
because 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
• Obtain a shear wave velocity
(Vs) profile to a depth of at least
30 m
Shear Wave Velocity (m/s)
0
200
300
400
500
600
0
10
• Vs reflects the shear modulus
(G) of the soil according to:
G = *Vs2
50
Deepth (ft)
20
30
100
Vs = Vs30 = 325 m/s
40
150
50
• Vs profile also needed for more
prediction via site response
analysis
200
0
400
800
1200
1600
Shear Wave Velocity (ft/sec)
60
2000
Depth ((m)
• Vs used to obtain simplified
p
Seismic Site Classification via
the average shear wave velocity
over the top 30m (Vs30 or Vs)
100
0
In-Situ Shear Wave Velocity (Vs) Measurements
• Intrusive (Borehole Methods)
–C
Crosshole
h l
– Downhole
– Suspension
S
i Logging
L
i
• Non-intrusive
Non intr si e (Surface
(S rface Wave
Wa e Methods)
– Spectral Analysis of Surface Waves (SASW)
– Multi-channel
Multi channel Analysis of Surface Waves (MASW)
– Refraction Microtremor (ReMi)
Crosshole: Setup and Equipment
Horizontal (H1)
Geophone
H i t l (H2)
Horizontal
Geophone
Vertical (V)
Geophone
Case
Crosshole Hammer
Crosshole: Shear Wave Records
2
Downward Impact
Upward Impact
Noormalized Maagnitude
Ti
Trigger
0
in One Borehole
-2
in Second Borehole
-4
Denotes Arrival Time
-6
-0.002
0.000
0.002
0.004
0.006
0.008
Time, sec
t
Vs = d / t = m/s
0.010
Crosshole: Vs Profile
0
Meassurement Depth, ft
50
Thin
Limestone
Layer (?)
100
150
Site 2 Boreholes
41C-41A Crosshole
41C-41B Crosshole
200
0
2000
4000
6000
SV-Wave Velocity, fps
8000
10000
Downhole: Setup and Equipment
Borehole BH-4
BHG-3
Downhole: Setup and Equipment
Shear Wave
Traction Plank
Instrumented
Sledgehammer
BHG-3
Control Box
Laptop
Dynamic
Signall Analyzer
Si
A l
Downhole: Travel Time vs. Depth
Downhole: Vs Profile
Suspension Logging: Setup and Waveforms
7-Conductor cable
OYO PS-160
Logger/Recorder
Diskette
with Data
Winch
Upper Geophone
Lower Geophone
Filter Tube
Source
Source Driver
Weight
Overall Length ~ 25 ft
Courtesy of GeoVision
Depth Sequential Waveform Arrivals
Surface Wave Methods
Vertically Oriented Source
d
d
SASW Setup
Vertically Oriented Velocity Transducers
Layer 1
Layer 2
Multi-Layered Solid
MASW Setup
SASW Equipment
Dynamic Signal Analyzer
Geophones and Sledgehammer
MASW Equipment
12 – 60 Geophones
Sledge Hammer &amp; Drop Weight Sources
Vibroseis Source
Surface Wave Dispersion
Low frequency
surface waves have
long wavelengths
(), while high
frequency waves
have short
wavelengths
Air
Layer
Layer11
Vertical
Particle Motion
1
Vertical
Particle Motion
2
Layer22
Layer
Layer 3
Layer
Waves with
W
ih
different
frequencies/
q
wavelengths sample
different depths
a. Material
Profile
Depth
Depth
c. Longer
b. Shorter
Wavelength,  2
Wavelength,  1
Surface wave velocity (Vr) is close to shear wave velocity (Vs):
Vs ~ 1.1*Vr
Example SASW Dispersion Curve
Wavelength (m)
1
5000
10
100
1000
Experimental Disp. Curve
Receiver Spacings = 5, 10, 20, 25,
40, 50, 150, 300, 450, and 600 ft.
1200
3000
800
2000
400
1000
0
1
10
100
Wavelength (ft)
1000
0
10000
Phase V
Velocity (m//sec)
Phase V
Velocity (ft/s
sec)
4000
Inversion to Obtain Vs Profile
Wavelength (m)
1
5000
10
100
1000
Experimental Disp. Curve
Theoretical Disp.Curve
1200
3000
800
2000
400
Phase V
Velocity (m/sec
c)
Shear Wave Velocity (ft/sec)
0
1000
0
2000
4000
6000
8000
0
200
100
1
10
100
Wavelength (ft)
1000
0
10000
400
600
200
800
max/2
1000
1200
0
500
1000
1500
2000
Shear Wave Velocity (m/sec)
300
Depth (m)
0
Depth (ft)
D
Phase Velocity (ft/sec
c)
4000
Seismic Site Classification
Required by Seismic Provisions in Building Codes
IBC (2009)
ASCE 7-05
IBC &amp; ASCE Codes – Seismic Site Classification
Site Class: A - F
Vs
N
Su
&gt; 1,500
1 500 m/s
760 – 1,500 m/s
360 – 760 m/s
180 – 360 m/s
/
&lt; 180 m/s
ASCE 7-05
Vs is
i preferred
f
d because
b
it is
i directly
di tl related
l t d to
t the
th
shear stiffness of the soil deposit (G = Vs2)
Preview Importance of Seismic Site Classification
IBC and ASCE Code – Design Response Spectra
Little Rock, Arkansas
Soft Soil ((Site Class E))
Horizontal Earthquake Force
70% of the Structure Weight
AR
Hard Rock (Site Class A)
Horizontal Earthquake Force
25% of the Structure Weight
0.2-sec
(~ 2-story building)
Seismic Site Classification via Vs (i.e. Vs30)
ASCE 7-05
Example Sites
Shear Wave Velocity (m/s)
Shear Wave Velocity (m/s)
0
500
1000
1500
0
2000
0
100
200
300
400
0
0
0
5
5
20
20
10
10
400
Depth (ft)
60
15
60
20
20
80
25
100
Vs = Vs30 = 1015 m/s
0
2000
4000
6000
Shear Wave Velocity (ft/sec)
Site Class B
30
8000
80
25
100
Vs = Vs30 = 250 m/s
0
400
800
1200
Shear Wave Velocity (ft/sec)
Site Class D
30
1600
Depth (m)
15
Depth (m)
Depth (ft)
40
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 strongly
influences ground motions
Questions?