Evaluation of Geotechnical Hazards

Transcription

1 Evaluation of Geotechnical Hazards by Geoffrey R. Martin Appendix B: Evaluation of Geotechnical Hazards Describes Evaluation Procedures Soil Liquefaction Soil Settlement Surface Fault Rupture Flooding 1

2 Hazard Procedures based on three primary source documents Field Exploration Location of Liquefiable Soils Location of Groundwater Level Depth of Liquefaction Field Exploration Methods Standard Penetration Test (SPT) Cone Penetration Test (CPT) 2

3 X CPT Data Evaluation CPT-1 CPT-2 CPT-3 B-1 Backfill +20 X' 8 4 Friction Ratio (%) Sand to (Hydraulic Tip Resistance (tsf) Marine 8 4 Friction Ratio (%) Silty Sand Fill) Sand Tip Resistance (tsf) 4 Friction Ratio (%) SP- SP- CL CL ML MH MH SC SC SP Tip Resistance (tsf) (22) 4 (13) 7 (31) 40 (31) 44 (>60) 34 (7) 7 (13) 11 (13) >50 32 >50 >50 >50 >50 Quarry Run Fill Clay / Silt (Lagoonal Clay) Sand to Silty Sand (Lakewood-San Pedro Formation) Harbor Bottom Sediments 0 Rock -20 Rip-Rap Elevation (ft) Scale feet Soil Classification (USCS) SPT Blowcounts (N) Key Zone of Liquefaction CPT Data Evaluation Field Measured Blowcount N 60(bpf) Ground El. = +13 ft Interpreted/Observed Soil Description B-1 CPT-3 0 ML ML -20 Elevation (ft) -40 SP- CL SP CL -60 ML ML -80 MH SP-SC SC SP CL SP/SC ML/ SP- Recorded SPT Blowcounts Interpreted SPT Blowcounts from CPT Sounding 3

4 Evaluation of Liquefaction Potential Simplified procedure based on empirical observations Used for most routine projects More rigorous numerical modeling Sites where liquefiable soils extend to significant depths Sites that have significant interlayering Sites where ground remediation costs are high Simplified procedure for Evaluating Liquefaction Potential Factor of Safety = Cyclic Resistance Ratio (CRR) Earthquake Cyclic Stress Ratio (CSR) CRR determined from empirical charts based on SPT or CPT data CSR determined from design peak ground accelerations depth CRR CSR Liquefiable zone 4

5 Simplified Procedure: CRR from SPT Data Simplified Procedure: CRR from CPT Data 5

6 Simplified Procedure: CSR Determination Liquefaction Hazard Assessment Flow Failures Limited Lateral Spreads Ground Settlement Choice of Factor of Safety Depends on: Vulnerability of Structure Acceptable Level of Risk Damage Potential Design Earthquake Magnitude Values to Warrant Hazard Assessment could range from

7 Flow Failures Potential massive translational failure when static factor of safety <1 where post-liquefaction undrained residual strength mobilized. Lateral Spreading Progressive down slope deformation under cyclic inertial loading during time intervals when F.S.<1 Four approaches to assess the magnitude of lateral spread displacement: 1. Youd et. al empirical approach 2. Newmark time history analyses 3. Simplified Newmark Charts 4. Numerical Modeling 7

8 Youd et. al empirical approach Newmark time history analyses 8

9 Newmark time history analyses HM LP HM- LP- 4.0 IMP IMP- Ground Displacement (ft) DUZ- ERZ- IMPV- KOBEupperbound DUZ ERZ IMPV KOBE 0.5 lowerbound ky (g) Newmark time history analysis Simplified Charts 9

10 Evaluation of Soil Settlement Hazard Tokimatsu and Seed (1987) Methodology Evaluation of Surface Fault Rupture Hazard Studies to Refine Definition of Hazard Location of fault traces Timing of most recent slip actively Ground rupture characteristics 10

11 Evaluation of Surface Fault Rupture Fault Displacement Evaluation Observation from past earthquakes Empirical relationship to earthquake magnitude Values based on fault slip rate 11