2015. 4. 1(Wednesday) Seoul National University Geotechnical & Geo-environmental Engineering Lab

1. 2015 Wednesday seminar Evaluation of the Strength and Deformation Characteristics in Cohesive Soils caused by Back Pressure 2015. 4. 1(Wednesday) Seoul National University Geotechnical & Geo-environmental Engineering Lab Park, Jaechan

2. 2015 Wednesday seminar Contents 1. Introduction 2. Test plans 2.1 Field condition & Test condition 2.2 Back pressure failure test 2.3 Back pressure cyclic test 3. Test Results 4. Conclusion 3.1 Back pressure failure test results 3.2 Back pressure cyclic test results

3. 2015 Wednesday seminar 1. Introduction • Background • Caused by urbanization and climate change, the damages of heavy rain and drought were increasing • Excessive reliance on surface water has problems. January 2009, Yeongam Haksan reservoir July 2011, Seoul Gangnam area To solve these problems, an underground reservoir which can save water in the aquifer during rainy season, and use the water at dry season is under development

4. 2015 Wednesday seminar 1. Introduction • Background Filling pipe River Pretreatment Lift riser Injection well Pumping well Post-cleaning water Clay layer Sand layer • Cracks might occur right above a confined aquifer due to the change of pore pressure • Evaluation of the strength and deformation characteristics of lower side of the clay caused by pore water pressure change is needed

5. 2015 Wednesday seminar 1. Introduction • Literature Review • The strength and deformation characteristics of the samples had been evaluated by compression or extension tests • According to Cho & Finno(2009), results of tests which have different direction of stress path have different values in terms of strength or volumetric strain • The effect of the change of pore water pressure to sample behavior is not fully investigated in Korea Cho&Finno(2009) • The samples obtained from the reservoir site were tested for the stability of clay layer and deformation characteristics • (1)Back pressure failure testand (2)back pressure cyclic test were conducted

6. 2015 Wednesday seminar 2. Test plan/ 2.1 Field conditions & Test conditions • Basic properties • Samples obtained from Busan • Field conditions • Ground water level: 2m • Test conditions • H=140mm, D=70mm, cylindrical shape • Saturated by 150kPaBack pressure (B value > 0.95) • K0=0.55 for NC(Normally Consolidated) clay

7. 2015 Wednesday seminar 2. Test plans/ 2.2 Back pressure failure test • Objective • To decide the size of back pressure that occurs failure, by simulating the phenomenon that the increment of pore water pressure arises from the bottom of clay layer • Procedure • Apply back pressure to bottom of sample (1.0kPa/min) • Maintain constant externaltotal stress during applying back pressure Loading Frame 1.0 kPa/min

8. 2015 Wednesday seminar 2. Test plans/ 2.3 Back pressure cyclic test • Objective • To evaluate the effects of (1)the magnitude of back pressure applying and reducing and (2)the number of cycles to the stability and deformation characteristics of samples • Procedure • Apply and Reduce the back pressure by rate of 1kPa/min • 60%(35kPa)of failure back pressure(60kPa) and 80%(50kPa) of failure back pressure were performed each • Maintain constant externaltotal stress during applying and reducing back pressure • No time lag between applying and reducing back pressure, not to make excess pore pressure dissipate Loading Frame 1.0 kPa/min

9. 2015 Wednesday seminar 3. Test results/ 3.1 Back pressure failure test results • Test results(σ1=180kPa, σ3=99kPa consolidation) • (+): sample compression, (-): sample extension • Extension at initial → compression • Reaching at back pressure increment 60kPa, the sample begins to compress and deviator stress is not maintained constantly Volumetric strain-Back pressure increment Axial Displacement-Back pressure increment Deviator stress-Back pressure increment Failure occurs at 60kPa of back pressure increment

10. 2015 Wednesday seminar 3. Test results/ 3.1 Back pressure failure test results • Test results(σ1=180kPa, σ3=99kPa consolidation) • Mohr-Coulomb failure envelope from 3 times of CK0U TXC (NC clay, c’=0, Φ’=29.1º) • Applying back pressure with constant external stress makes Mohr circle to move horizontally without size change • Similar results with the results of back pressure failure test(60kPa) Mohr circle failure state Mohr circle after consolidation 57kPa

11. 2015 Wednesday seminar 3. Test results/ 3.2 Back pressure cyclic test results • Test results(σ1=180kPa, σ3=99kPa consolidation) • 60% of failure pressure(35kPa) cycle test, 10 cycles • 80% of failure pressure(50kPa) cycle test, 21 cycles 체적변형률-시간 축방향 변위-시간 Volumetric strain-Back pressure increment Deviator stress-Back pressure increment Axial Displacement-Back pressure increment 축차응력-시간 축방향 변위-시간 체적변형률-시간 축차응력-시간 Deviator stress-Back pressure increment Axial Displacement-Back pressure increment Volumetric strain-Back pressure increment

12. 2015 Wednesday seminar 3. Test results/ 3.2 Back pressure cyclic test results • Test results(σ1=180kPa, σ3=99kPa consolidation) • 60% of failure pressure(35kPa) cycle test, 10 cycles • 80% of failure pressure(50kPa) cycle test, 21 cycles • As the number of iterations increases, displacement and volumetric strain are generated accumulatively • Compression occurs • Volume increases at initial phase, soon it decreases • Due to the difference between rate of excess pore pressure dissipation • Deviator stress is kept constant • Case of 60% : displacement0.236mm, 10 cycles(0.0236mm/cycle) • Case of 80% : displacement0.623mm, 21 cycles(0.0297mm/cycle) 체적변형률-시간 축방향 변위-시간 Volumetric strain-Back pressure increment Deviatoric stress-Back pressure increment Axial Displacement-Back pressure increment 축차응력-시간 축방향 변위-시간 체적변형률-시간 축차응력-시간 Deviatoric stress-Back pressure increment Axial Displacement-Back pressure increment Volumetric strain-Back pressure increment

13. 2015 Wednesday seminar 4. Conclusions • Conclusions • Evaluation of the strength and deformation characteristics of clay layer through the back pressure failure tests & back pressure cyclic tests and verification the results of back pressure failure test with triaxial tests • Back pressure failure tests • Evaluate Strength and deformation characteristics occurring when back pressure applied • Failure occurred without any change of external stress • Define ‘Back pressure failure state’ at the point of maximum extension occurs, deviator stress starts to decrease • Verify the results with triaixal tests • Back pressure cyclic tests • Simulate the phenomenon occurs during injection and pumping at reservoir • As iteration increasesdisplacement is generated accumulatively → expected to be due to the difference in the dissipation rate of excess pore pressure • Applying:extension, reducing: compression → compression is dominant • Although compression occurred, did not reach the failure

14. 2015 Wednesday seminar Thank you.