Liquefaction and Foundations

Transcription

1 Liquefaction and Foundations Amit Prashant Indian Institute of Technology Gandhinagar Short Course on Seismic Design of Reinforced Concrete Buildings November, 2012 What is Liquefaction? Liquefaction occurs in saturated soils All pores are completely filled with water; Soil is no more thirsty!! The water in the pores exerts the pressure on soil particles called Pore Pressure that influences how tightly the soil particles are pressed together 2 1

2 Liquefaction: Tilting of Buildings Niigata Earthquake, Japan, Bhuj Earthquake, India, 2001: Damage to rail-road and highway; sand boiling due to liquefaction 4 2

3 Sand boils in a room in Lunya-Li in Yuanlin Town, Taiwan, 1999 Sand boils in a house at Shetou, Taiwan, Liquefaction-induced settlement at Taichung Port, Taiwan,

4 Liquefaction-induced settlement at Adapazari, Kocaeli earthquake, Liquefaction sand boils during Christchurch earthquake,

5 Where does Liquefaction commonly occur? Liquefaction occurs in saturated soil only, its effects are most commonly observed in low-lying areas near bodies of water such as rivers, lakes, bays, and oceans, where poorly graded sandy or silty soil is present at shallow depths. Port and wharf facilities are often located in areas susceptible to liquefaction, and many have been damaged by liquefaction in past earthquakes. 9 How does Liquefaction happen? Under EQ ground motion Shear stresses generated at bed-rocks Transmitted upwards Loose cohesionless soils tend to densify and settle Pore water is forced out from the ids Shaking duration too short for pore water to drain Pore water pressure increases Effective stress reduces In critical state, soil looses stiffness & shear strength 10 5

6 Aftermath of Large Ground Oscillations Large Ground Oscillations Tilting Lateral Spreading Uplift Subsidence 11 Liquefaction Potential Evaluation using Cyclic Stress Approach (Youd et al., Oct 2001, ASCE) Earthquake loading is characterized by Equivalent stress induced by Earthquake Liquefaction resistance is characterized by Stress required for Liquefaction For Liquefaction evaluation: compare loading and resistance. Youd et al. (2001): Liquefaction resistance of soils: summary report from the 1996 nceer and 1998 nceer/nsf workshops on evaluation of liquefaction resistance of soils. ASCE Journal of Geotechnical and Geoenvironmental Engineering 12 6

7 Evaluation of Liquefaction potential av a CSR ' g max ' r d CRR CRR7. 5 km k k F s CRR ; If Fs 1.0; Liquefacti on CSR CSR = Cyclic Stress Ratio Seismic demand on the Soil due to Earthquake CRR = Cyclic Resistance Ratio Capacity of Soil to resist Liquefaction 13 Characterizing Earthquake Load Irregular time history of shear stress is converted to an equivalent series of uniform stress cycles at an amplitude of 65% of peak shear stress (Seed et al. 1975). av a 0.65 g max 14 7

8 Stress Reduction Coefficient, r d av a 0.65 g max r d r d = z for z 9.15m (2a) r d = z for 9.15m < z 23m (2b) 15 Cyclic Stress Ratio (CSR) Seismic demand on the soil Seed & Idriss (1971) av a CSR ' g max ' r d ' Total vertical overburden stresses Effective vertical overburden stresses 16 8

9 CRR Cyclic Resistance Ratio (CRR) Capacity of soil to resist liquefaction Liquefaction case histories used to characterize CRR in terms of measured in-situ test parameters (Whitman 1971) Parameter generally used SPT CPT Shear Wave Velocity (SWV) CRR CRR7. 5 km k k Liquefaction Observed Boundary No Liquefaction Observed Field Test Parameter 17 Cyclic Resistance Ratio (CRR) SPT Correlations CRR 7.5 for SPT data CRR for M7.5 eqk for Corrected Blow Count (N 1 ) 60 normalized for 100 kpa overburden pressure and hammer efficiency of 60% Correction factors for non-standard SPT values (Youd et al. 2001) 18 9

10 Cyclic Resistance Ratio (CRR) CPT Correlations CRR 7.5 for CPT data CRR for M7.5 eqk for Normalized cone tip resistance Correction factors for grain characteristics, and thin layer effect q c1n cs (Youd et al. 2001) 19 Cyclic Resistance Ratio (CRR) SWV Correlations CRR 7.5 for Shear wave velocity data CRR for M7.5 Earthquake for normalized shear wave velocity Correction factors (Youd et al. 2001) 20 10

11 Earthquake Magnitude Correction to CRR 7.5 for Earthquake Magnitude other than 7.5 (k m ) CRR CRR 7. 5 km k k 21 Overburden Pressure Correction to CRR 7.5 for high overburden k stresses, Correction factor for CRR 7.5 to extrapolate for soil layers with overburden pressures greater than 100 kpa Simplified procedure is valid for depths less than 15 m. CRR CRR 7. 5 km k k 22 11

12 Zone of Liquefaction Initial Static Shear Correction to CRR 7.5 for initial static shear, Correction factor for CRR 7.5 to account for initial static shear stress conditions such as due to presence of embankment, heavy structures, etc. k CRR CRR7. 5 km k k 23 Factor of safety against liquefaction 0 Cyclic Shear Stress 0 1 Fs CRR F s CSR CSR CRR Depth Depth 24 12

13 Depth (m) Example: Liquefaction potential Evaluation Problem: SPT data and sieve analysis Water table at 6 m below GL Expected earthquake M7.5 & Seismic Zone V N 60 Soil Classification Percentage fine Poorly Graded Sand and Silty Sand (SP-SM) Poorly Graded Sand and Silty Sand (SP-SM) Poorly Graded Sand and Silty Sand (SP-SM) Poorly Graded Sand and Silty Sand (SP-SM) Poorly Graded Sand and Silty Sand (SP-SM) Poorly Graded Sand and Silty Sand (SP-SM) Poorly Graded Sand and Silty Sand (SP-SM) 6 25 Example: Liquefaction potential Evaluation Liquefaction potential at depth m a max 0.24 g, M 7. 5, w 3 sat 18.5 kn / m, w.8 kn / 9 m a max 0.24 g, M 7. 5, w v kpa u ( ) kpa 0 u ' v v 0 = kpa 3 sat 18.5 kn / m, w av a CSR ' g max 9.8 kn / m ' r d

14 Example: Liquefaction potential Evaluation Liquefaction potential at depth m Compute stress reduction factor r d Compute CSR z ' a / g / CSR 0.65 maz r d v v CSR / Example: Liquefaction potential Evaluation Liquefaction potential at depth m Compute CRR CRR CRR7. 5 km k k CRR Compute F s F s CRR CSR

15 Example: Liquefaction potential Evaluation Repeat for all depths v ' v Depth %Fine (kpa) (kpa) N60 C N N 60 rd CSR CRR7. 5 CRR F s Liquefaction Potential for different soils using SPT 30 15

16 Liquefaction Resistant Structures A structure that possesses ductility, has the ability to accommodate large deformations, adjustable supports for correction of differential settlements, and having foundation design that can span soft spots; which can decrease the amount of damage a structure may suffer due to liquefaction. Deformations Below Shallow Foundations during Liquefaction Volumetric Deformations Deviatoric Deformation Ground loss due to sand boil Bearing capacity failure Ratcheting 32 16

17 Liquefied Layer Thickness Settlement Settlement induced by Liquefiable layer Foundation width Liquefied Layer Thickness 33 Shallow Foundations The elements of a shallow foundation system should be tied together to make the foundation move or settle uniformly, thus decreasing the amount of shear forces induced in the structural elements resting upon the foundation. (For example: The well-reinforced perimeter and interior wall footings should be tied together to enable them to bridge over areas of local settlement and provide better resistance against soil movements.) 17

18 Shallow Foundations A stiff foundation mat is a good type of shallow foundation, which can transfer loads from locally liquefied zones to adjacent stronger ground. Buried utilities, such as sewage and water pipes, should have ductile connections to the structure to accommodate the large movements and settlements that can occur due to liquefaction. It is better to use One foundation type throughout to support a building, e.g., Raft or Piled-raft. 35 Deep Foundations Piles driven through a potentially liquefiable soil layer to a stronger layer not only have to carry vertical loads from the superstructure, but also resist the horizontal loads and bending moments induced lateral movements if that layer liquefies. Sufficient resistance should be achieved by piles of larger dimensions and/or more reinforcement. 18

19 Deep Foundations Piles should be connected to the cap in a ductile manner which could allow some rotation to occur without a failure of the connection. If the pile connections fail, the cap cannot resist overturning moments from the superstructure by developed vertical loads in piles. Deep Foundations Piles need to be checked for bearing as well as buckling with due consideration to complete pore pressure profile along depth during liquefaction. Generally, use of fewer larger diameter piles is more appropriate to aid buckling. Lateral spreading of sloping ground can cause additional lateral passive force on the piles over a significant depth, and hence the instability of pile and Bending moment requirement should be checked

20 Measures to Overcome Liquefaction 1) Vertical Stress Increase (Surcharge) 2) Stone Columns (De-watering) 3) Compaction (Densification of soil) 4) Removal of Liquefiable soil 5) Anchored Piles 6) Liquefaction Resistant Structures Thank You 20