1. 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 U.S.-Taiwan Workshop on Soil Liquefaction 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 1. Reducing the excess pore pressure generation  Densification: dynamic compaction etc.  Reinforcement: compaction grouting etc. 2. 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 mixing, clogging, installation disturbance  Advantages of prefabricated drains minimum mixing, better discharge and storage capacity, developed sites applicable 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 experiments  Iai et al. : shaking table test  Yang and Ko : centrifuge test on a trench shape drain 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 Prefabricated Drains  Components:  Features: better discharge capacity & storage capacities better discharge capacity & storage capacities  Installation: statically/dynamically statically/dynamically  Rollins et al. blasting test: reducing 40~80% settlements Open slot Filter fabric Plastic pipe

10. Experiment Methodology 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 Dynamic source : Vibroseis truck Embedded instrumentation: Liquefaction test sensor & DAQ Embedded instrumentation: Liquefaction test sensor & DAQ  Test layout

12. Hydraulic Ram Vibroseis Truck

13. Liquefaction Test Sensor

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

15. Data Analysis  Pore pressure data: separate static, hydrodynamic, and residual excess pore pressure via digital filter  Shear strain calculation: Displacement-Based (DB) method Displacement-Based (DB) method Apparent Wave (AW) method Apparent Wave (AW) method  Pore pressure generation curve & time histories

16. Test Setup No Drain Test Drain Test Drain pipe

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 From small loading to largest loading level Fully dissipation of excess pore pressure between loading Fully dissipation of excess pore pressure between loading Determine threshold shear strain Determine threshold shear strain Generate pore pressure generation curve 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 R u -time histories at different radial distances

22. Dissipation Rate

23. Conclusions  Drainage alone can considerably reduce pore pressure generation reduce pore pressure generation minimize settlement minimize settlement accelerate after shaking dissipation 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