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

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

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

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

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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:

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

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Liquefaction Evaluation of CRR (NCEER, 1997): SPT blow count (N) Corrected blow count Need fines content Corrected clean sand blow count – N 1(60)CS CPT tip resistance (q c ) and sleeve friction (f s ) Shear wave velocity (Vs) Corrections for magnitude (M) Scaling factor (MSF) – apply to F.S.

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Liquefaction – SPT Analysis

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Liquefaction – CPT Analysis To address FC: (q c1N ) cs instead of q c1N (q c1N ) cs = K c *q c1N K c = f(q c, f s, vo, ’ vo ) This eliminates need for sampling to determine FC.

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Liquefaction – Shear Wave

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Liquefaction - MSF

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Example Loose Sand (N 1 ) 60 at 15’ depth = 10 Fines Content < 5% (SW/SP) Water table during 5’ depth Soil Parameters: vo ’=1176 psf vo = 1800 psf r d = 0.97 PGA=0.15g M=5.8

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Example (cont’d) CSR = (0.65)(0.15)(1800/1176)(0.97) CSR = 0.15 Using NCEER figure for (N 1 ) 60 = 10: CRR=0.11 MSF ≈2 FS = MSF*(CRR/CSR) = 2*(0.11/0.15) = 1.47 Note the influence of MSF!

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Liquefaction - FS

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Liquefaction – Cohesive Materials Strength loss – not technically liquefaction ‘Seismic softening’ ‘Chinese’ Criteria (Seed et al. 1983) Function of w c, 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

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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

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

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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

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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.

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

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

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Sandy site Compaction grouting for liquefaction mitigation Urban site, no vibrations

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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

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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. soil inc

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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

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Reinforcement Original Design Methodology Shear stress reduction factor (K G ) (Baez and Martin, 1993): G INC =Inclusion shear modulus G Soil =Soil shear modulus ARR=A inclusion /A total Strain compatibility and force equilibrium Assumes linear elastic soil and INC behavior CSR applied to soil = K G * CSR earthquake

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Mitigation - Reinforcement 10% Area Replacement G INC /G SOIL =5 K G =0.7

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

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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

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Liquefaction Mitigation-Reinforcement Example of required judgment: Say we need K G =0.8, what ARR do we need? Stone columns? Typical G SC /G soil ~ 5 (Baez/Martin, Mitchell, FHWA) ARR = 6% (11’ grid spacing-36” columns)

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Liquefaction Mitigation-Reinforcement Example of required judgment: Say we need K G =0.8, what ARR do we need? Piles? Typical G Steel /G soil ~ 2500 W14x120 – A=0.23 ft 2 ARR = 0.01% 50’ Spacing!!

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Current research by Boulanger, Elgamal, et al. 34

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Spatial distribution R rd 35

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Reinforcement – Panels and Grids

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Figure : Basic Treatment Patterns (Bruce 2003)

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

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Shear reduction - panels Ratio of shear stress reduction coefficients; (a) G r = 13.5, (b) G r = 50

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Conclusion – Soilcrete Grid per Boulanger, Elgamal et. al DSM grids affect both: seismic site response (e.g., a max ) seismic shear stress distributions (e.g. spatially averaged R rd ) 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.

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

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

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

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Brunswick Nuclear Plant Southport, NC Batch Plant Intake Canal N Spoil Deposit

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Ventura Cancer Center, CA

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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

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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.

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EQ Drain Theory Reduce the excess pore pressure accumulation during earthquake

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

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EQ Drain Installation

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

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Derivations Factor of safety is inverse of Ru Settlement

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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, k h Soil compressibility, m v, Earthquake duration, t d Number of earthquake cycles, N eq Drain spacing (trial values)

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EQ Drain with Stone Column Installation

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Stone Column Installation with EQ Drains

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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

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Questions

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