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Eurocode 8 - Design of Structures for Earthquake Resistance – Part 2

Eurocode 8 - Design of Structures for Earthquake Resistance – Part 2. Pedro S. Sêco e Pinto Laboratório Nacional de Engenharia Civil and Faculty of Engineering, University of Coimbra, Portugal President of ISSMGE(2005-2009). TOPICS. Introduction Earth Retaining Structures

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Eurocode 8 - Design of Structures for Earthquake Resistance – Part 2

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  1. Eurocode 8 - Design of Structures for Earthquake Resistance – Part 2 Pedro S. Sêco e Pinto Laboratório Nacional de Engenharia Civil and Faculty of Engineering, University of Coimbra, Portugal President of ISSMGE(2005-2009)

  2. TOPICS Introduction Earth Retaining Structures Potentially Liquefiable Soils Foundation System 5. Final Remarks

  3. POTENTIALLY LIQUEFIABLE SOILS Soil investigations should include SPT or CPT tests and grain size distribution.   The seismic shear stress e can be estimated from the simplified expression: e = 0,65 grf S vo where gr is the design ground acceleration ratio, S is the soil parameter and vo is the total overburden pressure.

  4. Table 5 Magnitude scaling factors Magnitude M Seed & Idriss (1982) Idriss NCEER (1997) Ambraseys (1988) 5.5 1.43 2.20 2.86 6.0 1.32 1.76 2.20 6.5 1.19 1.44 1.69 7.0 1.08 1.19 1.30 7.5 1.00 1.00 1.00 8.0 0.94 0.84 0.67 8.5 0.89 0.72 0.44

  5. PROBABILISTIC APPROACH FOR LIQUEFACTION A new proposal presented by Cetin et al. (2001) considered advanced in relation with the previous ones, as integrates: (i) data of recent earthquakes; (ii) corrections due the existence of fines; (iii) experience related a better interpretation of SPT test; (iv) local effects; (v) cases histories related more than 200 earthquakes; (v) Baysiana theory.

  6. POTENTIALLY LIQUEFIABLE SOILS

  7. RESIDUAL STRENGTH Fig. 14 - Normalized residual strength plotted versus CPT values (after Ishihara et al., 1990)

  8. Fig. 15 - Correlation between volumetric strain and SPT (after Tokimatsu and Seed, 1987) Fig. 16 - Post cyclic liquefaction volumetric strain curves using CPT and SPT results (after Ishihara, 1993) VOLUMETRIC STRAIN

  9. POTENTIALLY LIQUEFIABLE SOILS Some topics in EC 8 that deserve consideration:  Residual strength of soil  Post – liquefaction settlements  Number of cycles versus pore pressures  Liquefaction remediation

  10. REMEDIAL MEASURES • Two categories of remedial measures against liquefaction were proposed: • (i) Solutions aiming at withstanding liquefaction - Confinement wall: stiff walls anchored in a non liquefied layer (or a bedrock) to avoid lateral spreading in case of liquefaction; Soil reinforcement - transfer of loads to a non-liquefiable layer. • (ii) Solutions to avoid liquefaction: - Soil densification: compaction grouting to minimise the liquefaction potential; - Dewatering: to lower the water table in order to minimise the risk of liquefaction; - Drainage : to facilitate the dissipation of pore pressure; - Fine grouting: to increase the soil cohesion.

  11. LIQUEFACTION REDUCE METHODS • In NEMISREF Project the following criteria for selection was used: (i) Potential efficiency; (ii) Technical feasibility; (iii) Impact on structure and environmental; (iv) Cost-effectiveness; (v) Innovation. • Two methods were selected: (i) Soil grouting using calcifying bacteria; (ii) confinement wall. • With confinement wall even if partial liquefaction could occur the final deformations will be controlled.

  12. LIQUEFACTION REDUCED METHODS Related with calcifying bacteria the objective of soil consolidation is to create a cementation between the grains of soil skeleton increasing the cohesion.

  13. EARTH RETAINING STRUCTURES • For the pseudo–static analysis of rotating structures the seismic coefficients can be taken as: • Kh = grf S/ gr • KV =  0,5 FH when the ratio vg/gr is greater than 0.6 • KV =  0,33 FH otherwise • Where gr is the design ground acceleration ratio and the factor r takes the values listed in Table 6. Table 6. Factor affecting the horizontal seismic coefficient Type of retaining structure r Free gravity walls that can accept a displacement dr  300  S (mm) 2 As above with dr  200  S (mm) 1.5 Flexural r.c. walls, anchored or braced walls, r.c. walls founded on vertical piles, restrained basement walls and bridge abutments 1.0

  14. EARTH RETAINING STRUCTURES

  15. EARTH RETAINING STRUCTURES Topics that deserve additional consideration: (i) Design methods for the computation of permanent displacements that allow the couple computation of rotation and translation; (ii) For retaining walls of medium heights (greater than 6 m) the computed displacements are larger than the values listed by EC 8; (iii) The permanent displacements should be related with the height of the wall; (iv) The good behaviour of geogrid – reinforced soil retaining walls in comparison with reinforced concrete cantilever retaining walls, during the occurrence of earthquakes, should be stressed.

  16. FOUNDATION SYSTEM Piles and piers shall be designed to resist the following action effects: (i) inertia forces from the superstructure; and (ii) kinematic forces resulting from the deformation of the surrounding soil due the propagation of seismic waves. For the computation of internal forces along the pile, as well as the deflection and rotation at the pile head, both discrete or continuum models can be used. The following effects shall be included: (i) flexural stiffness of the pile; (ii) soil reactions along the pile; (iii) pile–group effects; and (iv) the connection between pile and structure.

  17. Figure 18 - Soil structure interaction problem (after Gazetas and Mylonakis, 1998)

  18. Figure 17 - Bounding surface for cohesive soils (after Pecker, 1997) Theoretical and experimental studies to provide bearing capacity solutions to include the effect of soil inertia forces led to the inequality:  (N,V,M,F) < 0 where = 0, defines the equation of the bounding surface.

  19. Figure 19 - Conceptual subdomains for dynamic soil structure analyses (after Pecker and Pender, 2000) The engineering approach considers two sub-domains: i) a far field domain where the non linearities are negligible; ii) a near field domain in the neighbouring of the foundation where the effects of the geometrical and material linearities are concentrated.

  20. FOUNDATION SYSTEM The following topics deserve more consideration: i) The influence of pile cap; ii) The moment rotation capacity of pile footing ; iii) The incorporation of the non linear behaviour of the materials in the methods of analysis; iv) The instrumentation of the piles for design purposes; v) Some guidelines about group effects, related with the number of piles, spacing, direction of loads, soil types and construction methods of piles.

  21. MITIGATION METHODS • For the evaluation of mitigation methods a preliminary analysis of the following solutions was performed: (i) Stiffening solutions - hard layer, reinforced concrete walls, soil stiffening at foundation level and inclined piles; (ii) Soft material barriers - soft layer, expanded polystyrene (EPS) walls, air-water balloons and soft caisson; (iii) oscillators. • For the criteria of selection the following factors were used: Potential efficiency, technical feasibility, impact on structure and environment, cost-effectiveness and innovation.

  22. MITIGATION METHODS • Soil stiffening (inclined micro-piles) • Reinforced concrete walls

  23. MITIGATION METHODS • Deformable soft barriers (soft caisson)

  24. Figure 8 - New Tagus crossing site

  25. Figure 9 - Simplified geological profile

  26. Liquefaction Assessment ● SPT Tests ● CPT Tests ● Seismic Tests

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  31. Models ● Total Stress Model Shake Program ● Effective Stress Model Dynaflow Program

  32. Sieve Characteristics of the Materials

  33. Cable Stayed BridgeEvaluation of Liquefaction Potential Material a1

  34. Summary TableLiquefaction Susceptible Zones

  35. Vertical pile load tests Figure 10 - Load settlements curves

  36. Vertical pile load tests Table 13 - Failure Loads

  37. Horizontal pile load tests Figure 11 - Measured load displacement curve

  38. Horizontal pile load tests Figure 12 - Computed values for piles displacements, bending moments and shear forces

  39. Dynamic pile load tests Shaker Velocity transducers

  40. Dynamic pile load tests Shaker Accelerometers

  41. Dynamic pile load tests Finite element mesh First two vibration modes

  42. Dynamic pile load tests Variation of maximum displacement Displacement transfer function

  43. RECEPTION TESTS FOR PILES TV camera Core sampling Sonic testsSonic diagraphy tests

  44. HYPPOCRATES • The Art is long • The Life is short • Experience is fallacious • And Decision is difficult

  45. RAPHAEL

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