Transcript Slide 1

Development of an In-Situ Test for
Direct Evaluation of the
Liquefaction Resistance of Soils
K. H. Stokoe, II, E. M. Rathje and B.R. Cox
University of Texas at Austin
W.-J. Chang
National Chi-Nan University
U.S.-Taiwan Workshop on Soil Liquefaction
November 3-5, 2003
Goal:
• develop an in-situ testing
procedure that can be used to
evaluate directly the cyclic
liquefaction resistance of soil
in terms of u vs. g for different
numbers of cycles
Key Characteristics:
• involves a limited volume of material
• shakes soil like an earthquake
Elements of the In-Situ
Liquefaction Test
•
•
•
Dynamic loading source (similar to
EQ shaking)
Embedded instrumentation array
(to monitor ground motion and
measure pore pressure generation
and dissipation)
Analysis procedure(s) to permit shear
strain time histories to be evaluated
Similar to Cyclic Strain Approach Used
in Laboratory Testing (Dobry et al. 1982)
Pore pressure
generation curve
gt
Schematic Layout of Field Setup
in First-Generation Testing
Waterproof
liner
Vibroseis
Footing
1
3.3 m
Backfill soil
2
0.3 m
1.2 m
5
0.3 m
Liquefaction sensor
Accelerometer
Settlement plate
4
3
0.3 m
0.3 m
1.2 m
Dynamic Field Source
Well-Controlled Dynamic Loading:
•
•
•
uniform cyclic amplitude (g)
specified frequency (f)
specified number of cycles (N)
Dynamic Sources:
•
•
First Generation-Vertical Vibroseis
Second Generation – T-Rex
and Liquidator
First-Generation Source:
Vibroseis - Involves Rayleigh Waves
Z Direction
Second-Generation Source:
T-Rex - Involves Shear Wave
Loading in X, Y or Z Directions
Theoretical Performance of T-Rex:
Vertical and Horizontal Modes
300
(60 kips)
250
12 Hz
Vertical Mode
200
Force,
kN
150
(30 kips)
5 Hz
100
Horizontal Modes
50
0
0
20
40
60
80
Frequency, Hz
100
120
Second-Generation Source:
Liquidator – Involves Shear Waves
Loading in X or Z Directions
Comparison of the Vertical Force
Outputs of T-Rex and Liquidator
150
1.3 Hz
Force,
kN
Liquidator
100
(20 kips)
50
T-Rex-Vertical
0
0
1
2
3
Frequency, Hz
4
5
Embedded Instrumentation Array
“Liquefaction Sensor”
•
•
•
measure soil particle motion
(2-D and 3-D geophones)
measure pore pressure generation
all measurements at same location
Settlement Plates
First-Generation
Liquefaction Sensor
8.9 cm
Filter
Shoe
3.8 cm
2.5 cm
Instrumentation Van during
Preliminary Field Trials
•
•
Sercel 408XL System, up to 2000 channels
VXI Technology System, 48-channel analyzer
Field Testing Procedure
Seismic testing
Settlement-plate elevations
Staged
testing
Apply dynamic loading for a
specific number of cycles
Data analysis
Rest for 30 minutes to 2 hours
Final settlements
Final S-wave velocities
Retrieve sensors
Saturation evaluation
In-situ density measurement
Interactive
testing
Data Analysis
• Shear strain
calculations
1. Processing of
Geophone data
2. Strain calculation
methods
• Pore pressure
processing
1. PPT data processing
2. Hydrodynamic and
Residual pore
pressure
Pore Pressure Generation Curves
Schematic Layout of Field Setup
in First-Generation Testing
Waterproof
liner
Vibroseis
Footing
1
3.3 m
Backfill soil
2
0.3 m
1.2 m
5
0.3 m
Liquefaction sensor
Accelerometer
Settlement plate
4
3
0.3 m
0.3 m
1.2 m
QUARRY TEST SITE IN AUSTIN, TEXAS
First-Generation
Vibroseis
First-Generation
Instrumentation Van
Test Series T1 – Small-Strain Level
Test T1-3 at center of the array
10
2
Shear
0
Strain
(x10-3 %) -2
20
0
0.0
0.2
0.4
0.6
0.8
Time (sec)
1.0
1.2
4
(Band-pass
filtered)
10
2
Pore
Pressure 0
Ratio, ru
-2
(%)
-4
0
0.0
1.4
20
0.2
0.4
0.6
0.8
1.0
Time (sec)
1.2
1.4
1.6
sv = 6.4 kPa
Test Series T1 – Large-Strain Level
Test T1-6 at center of the array
30
Shear
Strain
(x10-3 )
AW average
10
0
-10
SDM method
-30
0.0
0.5
Recorded ru
80
Pore
Pressure 40
Ratio, ru
0
(%)
0.0
0.5
1.0
1.5
2.0
Residual ru
1.0
1.5
Time (sec)
2.0
Pore Pressure Generation Curves for
Different Numbers of Loading Cycles
100
Note: gxz calculated by the SDM method
80
Dr = 35%
Pore
Pressure
Ratio, ru
(%)
60
n=20 cycles
40
20
0
0.0001
gt
n=10 cycles
n=5 cycles
n=2 cycles
0.001
0.01
0.1
Mean shear strain amplitude (%)
1
Installation of Embedded Sensors
Hydraulic Ram
Wire Rope
and
Electrical
Cable
Hollow Push Rod
Liquefaction Sensor
Liquefiable Layer
Loading with Rayleigh (R) Waves
Shallow
Instrumented
Zone
Rayleigh Waves
(Vertical Particle
Motion)
Loading with Shear (SH) Waves
Shallow
Instrumented
Zone
Horizontally Polarized
Shear (SH) Waves
Loading with Shear (SV) Waves
Shallow
Instrumented
Zone
Vertically Polarized
Shear (SV) Waves
Loading with Shear Waves (SV) at Depth
Instrumented
Zone at Depth
Vertically Polarized
Shear (SV) Waves
Conclusions
•
•
•
•
Development of the basic elements of a
field liquefaction test have been initiated
with first-generation equipment.
Successful measurements of ground
motion and pore pressure generation have
been conducted.
Second-generation dynamic sources,
liquefaction sensors, and data acquisition
equipment are nearly developed.
The test will continue to evolve over the
next few years, but there are already
numerous applications.
Thank you
•
•
•
•
National Science Foundation
United States Geological Survey
George E. Brown, Jr. Network for
Earthquake Engineering Simulation
(NEES)
Many graduate students at the
University of Texas