U.S.-Taiwan Workshop on Soil Liquefaction
Direct Evaluation of Effectiveness
of Prefabricated Vertical Drains in
Liquefiable Sand
Wen-Jong Chang, National Chi Nan University
Ellen M. Rathje, University of Texas at Austin
Kenneth H. Stokoe, II , University of Texas at Austin
Brady R. Cox, University of Texas at Austin
11/03/2003~11/04/2003 @ NCTU
Outline
 Introduction
 Drainage
Techniques
 Experiment Methodology
 Test Results
 Conclusion
Introduction

Liquefaction-induced damages:
Key role: pore pressure generation
Mitigation Methods
1.
Reducing the excess pore pressure
generation


2.
Densification: dynamic compaction etc.
Reinforcement: compaction grouting etc.
Quickly remove the accumulated pore
water pressure

Drainage: gravel drains, stone columns,
prefabricated vertical drains
Combination of both effects
Research Significances
 Problems
of conventional gravel drains
 mixing, clogging, installation disturbance
 Advantages of prefabricated drains
 minimum mixing, better discharge and
storage capacity, developed sites
applicable
 Goals: Quantitatively evaluate the
effectiveness of drainage alone
Drainage Techniques : Analytical
Background
 Seed
and Booker: develop chart-based
approach
 Onoue et al. : consider drain resistance,
chart-based approach
 Pestana et al. : includes drain
resistance and reservoir capacity, FEM
code (FEQDrain)
Drainage Techniques :
Experimental Works
 Onoue
et al. : large-scale in situ
experiments
 Iai et al. : shaking table test
 Yang and Ko : centrifuge test on a
trench shape drain
 Brennan and Madabhushi : centrifuge
test on a “cell”
Field Performance of Gravel
Drains
 Japan’s
experiences: sand drains
performed well in 1993 Kushiro-Oki and
1995 Hyogoken-Nambu EQ.
 Sand drains reduced ground settlements
more than 50%
 Performance cannot be solely attributed to
drainage
Prefabricated Drains

Components:
11.8 cm O.D.
Open
slotPoints
Slots
at Quarter
Slots Measure .0.13 cm

Features:
better discharge capacity & storage
capacities

0.64Filter
cm
Installation:
statically/dynamically

fabric
Plastic pipe
Rollins et al. blasting test:
1.1 cm
reducing 40~80% settlements
10.5 cm
Experiment Methodology
 Two
full-scale reconstituted
specimens
 In situ dynamic liquefaction test
 Data reduction
 Test setup
In Situ Dynamic Liquefaction
Test
 Components:
Dynamic source : Vibroseis truck
 Embedded instrumentation:
Liquefaction test sensor & DAQ
 Test layout

Vibroseis Truck
Hydraulic Ram
Liquefaction Test Sensor
Cable
Vertical
Geophone
8.9 cm
Pore
Pressure
Transducer
Filter
Horizontal
Geophone
Shoe
3.8 cm
2.5 cm
Test Layout
Vibroseis
truck
Waterproof
liner
Footing
1
3.3 m
PVC pipe
Backfill soil
2
0.3 m
1.2 m
5
Liquefaction sensor
0.3 m
Accelerometer
Settlement platform
4
3
0.3 m
1.2 m
0.3 m
Data Analysis

Pore pressure data: separate static, hydrodynamic,
and residual excess pore pressure via digital filter
 Shear strain calculation:

Displacement-Based (DB) method

Apparent Wave (AW) method
  Bu
 xz

PVv

Vah
Pore pressure generation curve & time histories
Test Setup
Drain
Drain pipe
Pipe
Vibroseis
truck
0.9 m
Waterproof
liner
0.27 m
1
Foundation
3.3 m
Vibroseis
truck
Backfill sand
2
Foundation
1.17 m
5
0.3 m
Liquefaction Test
Sensor
Settlement Plate
Backfill sand
0.29 m
Legend
0.7 m
Waterproof
liner
3.3 m
0.69 m
5
4
3
2
Legend
4
3
0.31 m
0.3 m
1.2 m
No Drain Test
0.3 m
Liquefaction Test
Sensor
0.46 m 0.3 m
0.23 m 0.15 m
Settlement Plate
1.2 m
Drain Test
1
1.15 m
Specimen Preparation
 Both
specimens using water pluviation to
construct loose, saturated specimens
 Prefabricated drain were installed prior
water pluviation  no densification
 Sensors were installed during water
pluviation process
Testing Procedure
 Loading
frequency=20 Hz for 3 seconds
 Interactive stage loading:




From small loading to largest loading level
Fully dissipation of excess pore pressure
between loading
Determine threshold shear strain
Generate pore pressure generation curve
Excess pore pressure ratio, uR (%)
Test Results:
Pore Pressure Generation Curve
100
80
60
40
Threshold shear strain
No Drain Test
Drain Test
Dr = 36%
n = 60 cycles
20
0
0.0001
0.001
0.01
Mean shear strain amplitude (%)
0.1
Time Histories
20
No Drain Test
Sensor 5
Depth=0.56 m
Shear Strain (x10 %)
10
0
-10
-20
10
0
-10
-20
0
1
2
3
4
0
Tim e (sec)
120
100
100
80
80
60
No Drain Test
Sensor 5
Depth=0.56 m
40
20
1
2
Time (sec)
3
4
Drain Test
Sensor 4
Depth=0.69 m
60
Ru (%)
Ru (%)
Drain Test
Sensor 4
Depth=0.69 m
-3
Shear Strain (x10
-3
%)
20
40
20
0
0
-20
0
1
2
Time (sec)
No Drain Test
3
4
0
1
2
Time (sec)
Drain Test
3
4
Dissipation Behavior
40
Within drain
R=0.10 m
R=0.25 m
R=0.48 m
R=0.78 m
Ru, (%)
30
20
10
0
0
2
4
6
8
Time (sec)
Ru-time histories at different radial distances
10
Dissipation Rate
100
No Drain Test
Drain Test
Ru, (%)
80
60
40
20
0
0
2
4
6
Time (sec)
8
10
Conclusions
 Drainage



alone can considerably
reduce pore pressure generation
minimize settlement
accelerate after shaking dissipation
 With
single prefabricated drain, max.
pore pressure ratio only 35% instead of
100% in No Drain Test
Conclusions (cont.)
 Drainage
alone can reduce volumetric
strain up to 75%
 Prefabricated drain can be an effective
alternative for liquefaction mitigation
 Same testing procedure can be
implemented to evaluate other
remediation techniques and current
treated sites
Thank You
Research Supported by
National Science Foundation