1. 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
NCTU

2. Outline Introduction Drainage Techniques Experiment Methodology
Test Results
Conclusion

3. Introduction Key role: pore pressure generation
Liquefaction-induced damages:
Key role: pore pressure generation

4. Mitigation Methods Reducing the excess pore pressure generation
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

5. 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

6. 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)

7. 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”

8. 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

9. Prefabricated Drains Components: Features: Installation:
better discharge capacity & storage capacities
Installation:
statically/dynamically
Rollins et al. blasting test: reducing 40~80% settlements
Open slot
Filter fabric
Plastic pipe

10. Experiment Methodology
Two full-scale reconstituted specimens
In situ dynamic liquefaction test
Data reduction
Test setup

11. In Situ Dynamic Liquefaction Test
Components:
Dynamic source : Vibroseis truck
Embedded instrumentation: Liquefaction test sensor & DAQ
Test layout

12. Vibroseis Truck
Hydraulic Ram

13. Liquefaction Test Sensor

14. Test Layout Vibroseis truck Waterproof liner PVC pipe Backfill soil
Footing 2 1
0.3 m
3.3 m 5
1.2 m
Liquefaction sensor
0.3 m
Accelerometer 3 4
Settlement platform
0.3 m
0.3 m
1.2 m

15. 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
Pore pressure generation curve & time histories

16. Test Setup
Drain pipe
Drain Test
No Drain Test

17. 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

18. 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

19. Test Results: Pore Pressure Generation Curve
Threshold shear strain

20. Time Histories
No Drain Test
Drain Test

21. Dissipation Behavior
Ru-time histories at different radial distances

22. Dissipation Rate

23. 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

24. 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

25. National Science Foundation
Thank You
Research Supported by
National Science Foundation