LIQUEFACTION INDUCED SETTLEMENTS OF BUILDINGS WITH SHALLOW FOUNDATIONS

Transcription

1 LIQUEFACTION INDUCED SETTLEMENTS OF BUILDINGS WITH SHALLOW FOUNDATIONS 1. Key parameters found in the field 2. Key parameters found from centrifuge experiments 3. key challenges and paths forward Kohji Tokimatsu, Tokyo Institute of Technology

2 1. Key parameters observed in the field

3 Effects of W B /H L on Normalized building settlement (S B /H L ) S B /H L (%) W B S B H L W B /H L Yoshimi, Y., and Tokimatsu, K. (1977): Settlement of buildings on saturated sand during earthquakes, Soils and Foundations, 17(1),

4 Effects of Thickness of Surface Crust Thickness of Liquefied Layer, H L (m) Ground Surface H NL H L Liquefied Layer Thickness of Crust, H NL(m) Ishihara, K. (1985): Stability of natural deposits during earthquakes, Proc., 11 th ICSMFE, Vol. 1,

5 Effects of contact pressure or stories Average Building Settlement, S B (cm) Number of Stories Tokimatsu, K., Kojima, J., Kuwayama, S., Abe, A., and Midorikawa, S. (1994): Liquefaction induced damage to buildings during 199 Luzon Earthquake, J. Geotech. Engrg., 12(2),

6 Effects of H L on Building Settlement Building Settlement, S B Estimated Thickness of Liquefied Layer, H L (m) Extensive liquefaction with building settlement more than 5 cm Extensive liquefaction with building settlement less than 5 cm Marginal zone without building settlement No liquefaction without building settlement Tokimatsu, K., Kojima, J., Kuwayama, S., Abe, A., and Midorikawa, S. (1994): Liquefactioninduced damage to buildings 199 Luzon Earthquake, J. Geotech. Engrg., 12(2),

7 Effects of aspect ratio on building settlements Normalized Building Settlement, S B /W B Building Aspect Ratio, H B /W B W B Sancio, R., Bray, J. D., Durgunoglu, T., and Onalp, A. (24): Performance of buildings over liquefiable ground in Adapazari, Turkey, Proc., 13th World Conf. on Earthquake Engineering, St. Louis, Mo., Canadian Association for Earthquake Engineering, Vancouver, Canada, Paper No S B H B

8 Ground Settlement vs Tilt Angle of Houses Damage Classification in Tilt Angle (%) 1/2~ Average Ground Settlement, S G (cm) Tokimatsu, K., Tamura, S., Suzuki, H., and Katsumata, K. (212): Building damage associated with geotechnical problems in the 211 Tohoku Pacific Earthquake, Soils and Foundations, 52(5),

9 Effects of Foundation Rigidity on Failure Mode 1964 Niigata Earthquake 211 Tohoku Earthquake Unreinforced concrete foundations Structural Damage Reinforced concrete foundations Building settlement & Tilt

10 Key Parameters Observed in the Field Building Response Number of Stories, N Aspect Ratio, W B /H B Contact Pressure, q B Mass Eccentricity Foundation Rigidity Structure to Structure Interaction Non liquefied Crust Groundwater Table Liquefiable Layer W B q B Building Settlement, S Building Tilt Structural Damage H B Ground Settlement Location of Ejecta H NL or H 1 Shear Strength Dr L H L or H 2 3D Drainage Input motion (Amax, Duration, Sequence)

11 Summary from Field Observation (1) Liquefaction-induced settlements of buildings with shallow foundations were affected by various factors, relative effects of which have not been clearly identified. (2) A simplified procedure should be pursued for better understanding of the mechanism that could explain building damage in most of the complied case histories.

12 2. Key parameters found in the lab

13 Description of Centrifuge Shaking Table Tests 7 22 Experimental Variables Contact pressure Building height and width Mass eccentricity ratio Groundwater depth Thickness and Density of Liquefied soil 8 silica sand (Dr=5% or 65%) (model scale) (unit mm) 7 silica sand (Dr=5% or 65%) Input PGA Centrifuge Acceleration 5g with small container 25g with large container 7 silica sand (Dr=5%) silicon oil (5cst) 2 (1m) 13

14 Absolute Settlement (cm) Relative Settlement (cm) Effects of Groundwater Table 2 1st M2L 1 M3L M4L Groundwater Table (m) Tilt Angle (rad) Groundwater Table (m) 14

15 Relative Settlement (cm) Absolute Settlement (cm) Effects of Soil Density & Location of Liquefied Soil Case 1 Case 2 Case Case 1 2 Case 2 1 Case Groundwater Table (m) Tilt Angle (rad) Dr=65% Case 1 Case 3 Case Groundwater Table (m) Dr=65% Dr=8 ~9% 建物模型 7MSを用いた全実験モデルにおける Dr=5% Dr=5% Dr=9% 1 回目加振時の実験結果 Dr=9% Dr=5% Case 1 Case 2 Case 15 3

16 Relative Settlement (cm) Absolute Settlement (cm) Acc max =4m/s 2 Acc max =2m/s 2 Effects of Input Motions 4 3 Acc max =4m/s Acc max =2m/s Groundwater Table (m) Tilt Angle (rad) Acc max =4m/s 2 Acc max =2m/s Groundwater Table (m) 16

17 F SW = R W / L W Resistance R W = Factor of Safety against Vertical Load Self Weight of Building L w = (m 1 +m 2 +m e )g m 1 K : Earth Pressure Coefficient σ v : Effective Stress Z : Thickness ofnon liquefied Crust θ : Internal Friction Angle B, L : Dimension of Foundation m 1, m 2, m e : Mass of Building g : Gravitational Acceleration B m e m 2 σv Z E.P.W.P.R 1 17

18 Factor of Safety against Overturning Moment F SMe = R Me / L me Resisting Moment R Me = Driving Moment L Me = (m 1 +m 2 )gb/2 +m e g(b/2+e) +m 1 a 1 h 1 +(m 2 +m e )a 2 h 2 K : Earth Pressure Coefficient σ v : Effective Stress Z : Thickness ofnon liquefied Crust θ : Internal Friction Angle B, L : Dimension of Foundation m 1, m 2, m e : Mass of Building g : Gravitational Acceleration a 1, a 2 : Building Acceleration h 1, h 2 : Height e :Eccentric Distance h 1 h 2 m 1 e m e m a 2 2 σv B a 2 a 1 Z E.P.W.P.R 1 18

19 Relative Settlement (cm) Absolute Settlement (cm) Factors of Safety vs Building Settlement and tilting Tilt Angle (rad) Factor of Safety against Vertical Force, F SW Ground Settlement ~5cm 5~1cm 1~15cm 15cm~ Factor of Safety against Overturning Moment F SMe 19

20 Summary from Centrifuge Experiment (1) The relative settlement and tilt angle of a building decreased as the crust thickness (H NL ) and the density of liquefied soil (D rl ) increased or the thickness of the liquefied soil (H L ) and building contact pressure (q B ) decreased. (2) The effects of soil liquefaction on building damage were well accounted for by the safety factors against vertical force and static and dynamic overturning moments of the building together with the liquefaction severity of the underlying liquefiable deposit, represented by the integration of liquefaction-induced volumetric strain with depth.

21 3. KEY CHALLENGES AND PATHS FORWARD (1) Compilation/revisit of well documented case histories of liquefaction induced ground and building settlements during resent earthquakes. (2) Centrifuge experiments to identify relative effects of key parameters on building settlements. (3) Development of a simplified procedure that can explain liquefaction-induced building damage in most of the complied case histories and centrifuge experiments.

22 3. KEY CHALLENGES AND PATHS FORWARD (CONT.) (4) Refinement of numerical and design procedures to estimate ground and building settlements, which should be substantiated by well documented case histories and centrifuge experiments. (5) Development of cost effective mitigation techniques taking into account the key parameters controlling settlement and tilting of buildings.

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