Direct Evaluation of Effectiveness of Prefabricated Vertical Drains in Liquefiable Sand

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 11/03/2003~11/04/2003 @ NCTU

2. Outline • Introduction • Drainage Techniques • Experiment Methodology • Test Results • Conclusion

3. Introduction • 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: 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 1.2 m 5 Liquefaction sensor 0.3 m Accelerometer 4 3 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. Threshold shear strain Test Results:Pore Pressure Generation Curve

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. Thank You Research Supported by National Science Foundation