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Ground Modification for Liquefaction Mitigation

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Presentation on theme: "Ground Modification for Liquefaction Mitigation"— Presentation transcript:

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: Common Input Parameters:
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 Function of:
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)cs instead 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 Soil Parameters: (N1)60 at 15’ depth = 10
Fines Content < 5% (SW/SP) Water table during 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

21

22 Liquefaction Mitigation-Densification
Loose sand zone Hospital site Vibro-replacement to 45 ft.

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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. tsoil tinc tsoil

27 GI for Large Earthquakes
Large magnitude earthquakes: PGA ~ g M >7 Typical CSR values ~ 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

37

38 Figure : Basic Treatment Patterns (Bruce 2003)
Grouting and Ground Treatment, Proc. the 3rd International Conference, ASCE, Edited by Lawrence F. Johnsen, Donald A. Bruce, and Michael J. Byle, New Orleans, LA, 2003. 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 – Soilcrete Grid per 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 Plant Southport, NC
Spoil Deposit Batch Plant N Point out maintenance building and storage facility, location of batch plant and intake canal 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)

51 EQ Drain Installation

52 EQ Drain Design Concept
Based on radial dissipation theory (just like vertical consolidation, but radial geometry) Change in PP per cycle depends on PP of previous cycle NL based on CSR of soil, SPT, Fines Neq, td are functions of earthquake, but there are correlations to magnitude Assume periodic wave form DeAlba et al., 1975

53 Derivations Factor of safety is inverse of Ru Settlement

54 EQ Drain Design Graphical solutions to diff equation (JGS):
Address drain size, well resistance Provides Ru, but no settlement calculations FEQDrain – Finite Element software program Provides Ru and settlement calculations Both methods need the following: Soil permeability, kh Soil compressibility, mv, Earthquake duration, td Number of earthquake cycles, Neq Drain spacing (trial values)

55 EQ Drain with Stone Column Installation

56 Stone Column Installation with EQ Drains

57 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

58 Questions


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