Ground Modification for Liquefaction Mitigation

1. Ground Modification for Liquefaction Mitigation January 11, 2013 Kansas City, MO Tanner Blackburn, Ph.D., P.E. Assistant Chief Engineer

2. Presentation Summary • Determining liquefaction susceptibility • NCEER guidelines • Mitigation methods • Densification • Reinforcement • Drainage

3. Geotechnical Seismic Hazards • Liquefaction • Bearing capacity • Excessive settlement • Lateral spreading • Slope Stability • Cyclic shear strength • Kinematic loading of slopes/earth

4. Liquefaction • Function of: • Earthquake magnitude • Distance from site • Groundwater conditions (current or ‘high water’?) • Depth to ‘liquefiable’ strata (svo, rd) • Common Input Parameters: • Peak Ground Acceleration (PGA) • Magnitude (M)

5. Liquefaction • National Center for Earthquake Engineering Research (NCEER) Summary Report (1997 Meeting, published in JGGE, 2001). • Seed and Idriss (1971): • Normalized by vertical effective stress:

6. Liquefaction • Resistance to liquefaction • Referred to as Cyclic Resistance Ratio (CRR) or CSRfield • Function of: • Geologic history (deposit type, age, OCR) • Soil structure (relative density, clay content) • Groundwater conditions • Factor of Safety = CRR/CSR

7. Liquefaction • Evaluation of CRR (NCEER, 1997): • SPT blow count (N) • Corrected blow count • Need fines content • Corrected clean sand blow count – N1(60)CS • CPT tip resistance (qc) and sleeve friction (fs) • Shear wave velocity (Vs) • Corrections for magnitude (M) • Scaling factor (MSF) – apply to F.S.

8. Liquefaction – SPT Analysis

9. Liquefaction – CPT Analysis • To address FC: • (qc1N)csinstead of qc1N • (qc1N)cs = Kc*qc1N • Kc = f(qc, fs, svo, s’vo) • This eliminates need for sampling to determine FC.

10. Liquefaction – Shear Wave

11. Liquefaction - MSF

12. Example • Loose Sand • (N1)60 at 15’ depth = 10 • Fines Content < 5% (SW/SP) • Water table during earthquake @ 5’ depth • Soil Parameters: • svo’=1176 psf • svo= 1800 psf • rd = 0.97 • PGA=0.15g • M=5.8

13. Example (cont’d) • CSR = (0.65)(0.15)(1800/1176)(0.97) • CSR = 0.15 • Using NCEER figure for (N1)60= 10: CRR=0.11 • MSF ≈2 • FS = MSF*(CRR/CSR) = 2*(0.11/0.15) = 1.47 • Note the influence of MSF!

14. Liquefaction - FS

15. Liquefaction – Cohesive Materials • Strength loss – not technically liquefaction • ‘Seismic softening’ • ‘Chinese’ Criteria (Seed et al. 1983) • Function of wc, LL, clay content • Not well accepted anymore... • Bray and Sancio (2006) • No defined criteria, but good overview. • Boulanger and Idriss (2006, 2007) • Chris Baxter at URI - Silts

16. Liquefaction – Lateral Spreading • Lateral spreading can occur in gradual slopes (<2°) • Must design for static and dynamic driving forces with residual undrained shear strengths • Even for cohesionless materials

17. Liquefaction-induced Settlement Zhang et al., 2002 Tokimatsu and Seed, 1987 Ishihara and Yoshimine, 1992

18. Liquefaction Mitigation • Increase strength ( CRR) • Ground improvement (densification or grouting) • Decrease driving stress ( CSR) • Shear reinforcement with ‘stiffer’ elements within soil mass • Decrease excess pore pressure quickly • Reduce drainage path distance with tightly spaced drains

19. Mitigation - Densification • Increase cyclic shear strength (CRR) by increasing relative density of cohesionless materials • Advantages: • Field Verifiable! • Conduct field testing before and after treatment • Employed for over 50 years, through several large magnitude earthquakes. • Several peer-reviewed documents describing the methods, efficiency, and mechanics of densification. • Approved by CA Office of Statewide Health Planning and Development (OSHPD) for hospital and school construction.

20. Mitigation - Densification • Methods: • Dynamic compaction • Vibro-compaction • Vibro-replacement • Blast densification • Compaction grouting

22. Liquefaction Mitigation-Densification • Loose sand zone • Hospital site • Vibro-replacementto 45 ft.

24. Liquefaction Mitigation-Densification • Sandy site • Compaction grouting for liquefaction mitigation • Urban site, no vibrations

25. Liquefaction Mitigation • Increase strength ( CRR) • Ground improvement (densification or grouting) • Decrease driving stress ( CSR) • Shear reinforcement with ‘stiffer’ elements within soil mass • Decrease excess pore pressure quickly • Reduce drainage path distance with tightly spaced drains

26. Mitigation - Reinforcement • Reduce cyclic shear stress applied to liquefiable soil by installing ‘stiffer’ elements within soil matrix that attract stress. • Can be used in non-densifiable soils (silts, silty sands). • Large magnitude EQs • Not verifiable • Post-installation CPT or SPT results will not differ from pre-installation. • Vertical load testing of elements is not applicable. tinc tsoil tsoil

27. GI for Large Earthquakes • Large magnitude earthquakes: • PGA ~0.3-1.0g • M >7 • Typical CSR values ~ 0.3-0.6 • High liquefaction potential for all soils N<30 • Densification has limited application

28. Reinforcement • Original Design Methodology • Shear stress reduction factor (KG) (Baez and Martin, 1993): GINC=Inclusion shear modulus GSoil=Soil shear modulus ARR=Ainclusion/Atotal • Strain compatibility and force equilibrium • Assumes linear elastic soil and INC behavior • CSRapplied to soil = KG * CSRearthquake

29. Mitigation - Reinforcement • 10% Area Replacement • GINC/GSOIL=5 • KG=0.7

30. Reinforcement • Methods: • Deep soil mixing • Stone Columns (aggregate piers) • New research indicates this reinforcement effect is limited • Jet Grouting

31. Mitigation - Reinforcement • Requires engineering judgment regarding input parameters • Is there a limit to the ‘inclusion’ stiffness? • What is the deformation mechanism (bending or shear)? • Is there a maximum spacing that should be used? • If the soil liquefies around a stone column, what is the strength of the stone column? • Few peer-reviewed publications or references regarding use and efficiency • Vendor/contractor ‘white-papers’ do not qualify as design standards or peer-reviewed methods • State-of-the-practice is developing

32. Liquefaction Mitigation-Reinforcement • Example of required judgment: • Say we need KG=0.8, what ARR do we need? • Stone columns? • Typical GSC/Gsoil ~ 5 (Baez/Martin, Mitchell, FHWA) • ARR = 6% (11’ grid spacing-36” columns)

33. Liquefaction Mitigation-Reinforcement • Example of required judgment: • Say we need KG=0.8, what ARR do we need? • Piles? • Typical GSteel/Gsoil ~ 2500 • W14x120 – A=0.23 ft2 • ARR = 0.01% • 50’ Spacing!!

34. Current research by Boulanger,Elgamal, et al.

35. Spatial distribution Rrd

36. Reinforcement – Panels and Grids

38. Figure : Basic Treatment Patterns (Bruce 2003)

39. Linear Elastic FE DSM Model Boulanger, Elgamal, et al. Linear Elastic Soil Profile DSM Half Unit Cell

40. Shear reduction - panels Ratio of shear stress reduction coefficients; (a) Gr= 13.5, (b) Gr= 50

41. Conclusion – SoilcreteGridper Boulanger, Elgamal et. al • DSM grids affect both: • seismic site response (e.g., amax) • seismic shear stress distributions (e.g. spatially averaged Rrd) • DSM grids on seismic site response can be significant and may require site-specific FEM analyses • The reduction in seismic shear stresses by reinforcement can be significantly over-estimated by current design methods that assume shear strain compatibility. • A modified equation is proposed for estimating seismic shear stress reduction effects. The modified equations account for non-compatible shear strains and flexure in some wall panels. • The top 2m-3m of DSM wall could potentially be the critical wall section in term of tension development.

42. Thanks to Masaki Kitazume, Tokyo Institute of Technology Provided images to HBI.

43. Thanks to Masaki Kitazume, Tokyo Institute of Technology Provided images to HBI.

44. Thanks to Masaki Kitazume, Tokyo Institute of Technology Provided images to HBI.

45. Brunswick Nuclear PlantSouthport, NC Spoil Deposit Batch Plant N Intake Canal

46. Ventura Cancer Center, CA

47. Liquefaction Mitigation • Increase strength ( CRR) • Ground improvement (densification or grouting) • Decrease driving stress ( CSR) • Shear reinforcement with ‘stiffer’ elements within soil mass • Decrease excess pore pressure quickly • Reduce drainage path distance with tightly spaced drains

48. Mitigation - Drainage • Limit excess pore pressure increase and duration of increased pore pressure during cyclic shearing by providing short drainage paths in cohesionless materials. • Not verifiable with in situ testing • Limited peer-reviewed publications or design standards. • Methods: • EQ Drains – perforated pipe installed on tight grid • Stone columns – additional feature, but not relied on for design • Permeability of stone column material • Contamination with outside material.

49. EQ Drain Theory • Reduce the excess pore pressure accumulation during earthquake

50. EQ Drain Details • Typically 75-150 mm diameter • Slotted PVC pipe with filter fabric • Typical spacing 1-2 m triangular • Installed with large steel probe with wings (densification also intended)