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 Key role: pore pressure generation Liquefaction-induced damages: Key role: pore pressure generation

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

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: 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

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

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

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

Test Setup Drain pipe Drain Test No Drain Test

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

Test Results: Pore Pressure Generation Curve Threshold shear strain

Time Histories No Drain Test Drain Test

Dissipation Behavior Ru-time histories at different radial distances

Dissipation Rate

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

National Science Foundation Thank You Research Supported by National Science Foundation