Report

Ground Modification for Liquefaction Mitigation January 11, 2013 Kansas City, MO Tanner Blackburn, Ph.D., P.E. Assistant Chief Engineer www.HaywardBaker.com Presentation Summary Determining liquefaction susceptibility NCEER guidelines Mitigation methods Densification Reinforcement Drainage Geotechnical Seismic Hazards Liquefaction Bearing capacity Excessive settlement Lateral spreading Slope Stability Cyclic shear strength Kinematic loading of slopes/earth 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) Liquefaction National Center for Earthquake Engineering Research (NCEER) Summary Report (1997 Meeting, published in JGGE, 2001). Seed and Idriss (1971): cyclic 0 . 65 a max s vo rd g Normalized by vertical effective stress: CSR eq 0 . 65 a eq s vo g s ' vo rd 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 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. Liquefaction – SPT Analysis 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. Liquefaction – Shear Wave Liquefaction - MSF 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 Example (cont’d) CSR eq 0 . 65 a eq s vo g s ' vo rd 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! Liquefaction - FS 0 qt [tsf] 40 80 120 160 200 Rf [%] 0 0.4 0.8 1.2 1.6 2 0 CSR and CRR 0.5 1 0 0 0 0 10 10 10 10 20 20 20 20 0 Depth [ft] 30 Depth [ft] 30 Depth [ft] Depth [ft] CPT-9 30 40 40 40 50 50 50 60 60 60 30 40 CSR CRR 50 60 Factor of Safety 0.5 1 1.5 2 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 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 Liquefaction-induced Settlement Tokimatsu and Seed, 1987 Ishihara and Yoshimine, 1992 Zhang et al., 2002 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 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. Mitigation - Densification Methods: Dynamic compaction Vibro-compaction Vibro-replacement Blast densification Compaction grouting Liquefaction Mitigation-Densification Loose sand zone 0 Hospital site 2 Vibroreplacement to 45 ft. 0 qt [MPa] 10 20 0 0 CSR and CRR 0.5 1 0 1.5 Factor of Safety 1 Seismic Settlement [mm] 0 20 40 2 0 0 2 2 4 4 6 6 8 8 10 10 12 12 10 4 Depth [m] 6 20 8 30 10 12 40 Post Treatment Pre-Treatment Treatment Depth 14 14 50 14 Liquefaction Mitigation-Densification qt [tsf] 0 40 80 120 160 200 Sandy site CSR and CRR 0.5 1 0 0 0 10 10 10 10 20 20 20 20 30 Depth [ft] 30 Depth [ft] Depth [ft] 30 Depth [ft] CPT-9 grouting for liquefaction mitigation vibrations 0 0 0 Compaction Urban site, no Rf [%] 0 0.4 0.8 1.2 1.6 2 30 40 40 40 40 50 50 50 50 CSR CRR Pre CRR Post 60 60 60 60 Factor of Safety 0.5 1 1.5 2 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 Mitigation - Reinforcement Reduce cyclic shear stress applied to liquefiable soil by installing ‘stiffer’ elements within soil matrix that attract stress. Can be used in nondensifiable 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 soil 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 Reinforcement Original Design Methodology Shear stress reduction factor (KG) (Baez and Martin, 1993): KG 1 G 1 ARR INC 1 G Soil 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 Mitigation - Reinforcement 10% Area 0 qt [tsf] 40 80 120 160 200 0 0.5 0 0 0 0 10 10 10 10 20 20 20 20 0 Replacement CSR and CRR Rf [%] 0 0.4 0.8 1.2 1.6 2 CPT-9 GINC/GSOIL=5 30 30 Depth [ft] G 1 ARR INC 1 G Soil 30 Depth [ft] 1 Depth [ft] KG Depth [ft] KG=0.7 30 40 40 40 40 50 50 50 50 CSR Pre CSR Post CRR Pre 60 60 60 60 Factor of Safety 0.5 1 1.5 2 Reinforcement Methods: Deep soil mixing Stone Columns (aggregate piers) – New research indicates this reinforcement effect is limited Jet Grouting 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 Liquefaction Mitigation-Reinforcement Example of required judgment: KG 1 G INC 1 ARR 1 G Soil 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) Liquefaction Mitigation-Reinforcement Example of required judgment: KG 1 G INC 1 ARR 1 G Soil 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!! Current research by Boulanger, Elgamal, et al. 34 Spatial distribution Rrd 35 Reinforcement – Panels and Grids Figure : Basic Treatment Patterns (Bruce 2003) Linear Elastic FE DSM Model Boulanger, Elgamal, et al. Linear Elastic Soil Profile DSM Half Unit Cell Shear reduction - panels Ratio of shear stress reduction coefficients; (a) Gr = 13.5, (b) Gr = 50 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 noncompatible 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. Thanks to Masaki Kitazume, Tokyo Institute of Technology Provided images to HBI. Thanks to Masaki Kitazume, Tokyo Institute of Technology Provided images to HBI. Thanks to Masaki Kitazume, Tokyo Institute of Technology Provided images to HBI. Brunswick Nuclear Plant Southport, NC Spoil Deposit Batch Plant Intake Canal N Ventura Cancer Center, CA 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 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. EQ Drain Theory Reduce the excess pore pressure 1 1 0.8 0.8 Pore pressure ratio Pore pressure ratio accumulation during earthquake 0.6 0.4 0.2 0.6 0.4 0.2 0 0 0 5 10 15 Shear stress cycles 20 25 0 5 10 15 Shear stress cycles 20 25 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) EQ Drain Installation EQ Drain Design Concept Based on radial dissipation theory (just like vertical consolidation, but radial geometry) k h 1 u r w r r ch ch u u g mv t t t N t • Change in PP per cycle depends on PP of previous cycle kh m v w 1 u u u g r r r t t u g • NL based on CSR of soil, SPT, Fines u g N N t N eq Assume periodic wave form td 1 ug so ' N 2 arcsin Nl 2 DeAlba et al., 1975 ~ 0 .7 N l Number of uniform stress cycles causing r tan u N s eq 2 r N N l t d 2 sin u 2 u g ' o liquefacti on in undrained • Neq, td are functions of earthquake, but there are correlations to magnitude test Derivations Factor of safety is inverse of Ru Settlement layer T layer m ' s v New v m ' R s v New u vo s v u ' u R u s vo ' layer T layer 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) EQ Drain with Stone Column Installation Stone Column Installation with EQ Drains 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 Questions