4 Liquefaction Function of: Common Input Parameters: Earthquake magnitudeDistance from siteGroundwater conditions (current or ‘high water’?)Depth to ‘liquefiable’ strata (svo , rd)Common Input Parameters:Peak Ground Acceleration (PGA)Magnitude (M)
5 LiquefactionNational 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 CSRfieldFunction of:Geologic history (deposit type, age, OCR)Soil structure (relative density, clay content)Groundwater conditionsFactor of Safety = CRR/CSR
7 Liquefaction Evaluation of CRR (NCEER, 1997): SPT blow count (N)Corrected blow countNeed fines contentCorrected clean sand blow count – N1(60)CSCPT tip resistance (qc) and sleeve friction (fs)Shear wave velocity (Vs)Corrections for magnitude (M)Scaling factor (MSF) – apply to F.S.
15 Liquefaction – Cohesive Materials Strength loss – not technically liquefaction‘Seismic softening’‘Chinese’ Criteria (Seed et al. 1983)Function of wc, LL, clay contentNot 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 strengthsEven for cohesionless materials
17 Liquefaction-induced Settlement Zhang et al., 2002Tokimatsu and Seed, 1987Ishihara 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 massDecrease excess pore pressure quicklyReduce drainage path distance with tightly spaced drains
19 Mitigation - Densification Increase cyclic shear strength (CRR) by increasing relative density of cohesionless materialsAdvantages:Field Verifiable!Conduct field testing before and after treatmentEmployed 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.
24 Liquefaction Mitigation-Densification Sandy siteCompaction grouting for liquefaction mitigationUrban 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 massDecrease excess pore pressure quicklyReduce 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 EQsNot verifiablePost-installation CPT or SPT results will not differ from pre-installation.Vertical load testing of elements is not applicable.tsoiltinctsoil
27 GI for Large Earthquakes Large magnitude earthquakes:PGA ~ gM >7Typical CSR values ~High liquefaction potential for all soils N<30Densification has limited application
28 Reinforcement Original Design Methodology Shear stress reduction factor (KG) (Baez and Martin, 1993):GINC=Inclusion shear modulusGSoil=Soil shear modulusARR=Ainclusion/AtotalStrain compatibility and force equilibriumAssumes linear elastic soil and INC behaviorCSRapplied to soil = KG * CSRearthquake
29 Mitigation - Reinforcement 10% Area ReplacementGINC/GSOIL=5KG=0.7
30 Reinforcement Methods: Deep soil mixing Stone Columns (aggregate piers)New research indicates this reinforcement effect is limitedJet Grouting
31 Mitigation - Reinforcement Requires engineering judgment regarding input parametersIs 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 efficiencyVendor/contractor ‘white-papers’ do not qualify as design standards or peer-reviewed methodsState-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 ~ 2500W14x120 – A=0.23 ft2ARR = 0.01%50’ Spacing!!
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 ProfileDSM 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 analysesThe 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 DepositBatch PlantNPoint out maintenance building and storage facility, location of batch plant and intake canalIntake Canal
47 Liquefaction Mitigation Increase strength ( CRR)Ground improvement (densification or grouting)Decrease driving stress ( CSR)Shear reinforcement with ‘stiffer’ elements within soil massDecrease excess pore pressure quicklyReduce drainage path distance with tightly spaced drains
48 Mitigation - DrainageLimit 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 testingLimited peer-reviewed publications or design standards.Methods:EQ Drains – perforated pipe installed on tight gridStone columns – additional feature, but not relied on for designPermeability of stone column materialContamination with outside material.
49 EQ Drain TheoryReduce the excess pore pressure accumulation during earthquake
50 EQ Drain Details Typically 75-150 mm diameter Slotted PVC pipe with filter fabricTypical spacing 1-2 m triangularInstalled with large steel probe with wings (densification also intended)
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 cycleNL based on CSR of soil, SPT, FinesNeq, td are functions of earthquake, but there are correlations to magnitudeAssume periodic wave formDeAlba et al., 1975
53 DerivationsFactor of safety is inverse of RuSettlement
54 EQ Drain Design Graphical solutions to diff equation (JGS): Address drain size, well resistanceProvides Ru, but no settlement calculationsFEQDrain – Finite Element software programProvides Ru and settlement calculationsBoth methods need the following:Soil permeability, khSoil compressibility, mv,Earthquake duration, tdNumber of earthquake cycles, NeqDrain spacing (trial values)