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PEER Jonathan P. Stewart University of California, Los Angeles May 22, 2002 Geotechnical Uncertainties for PBEE
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Definitions of Uncertainty Epistemic: uncertainty associated with incomplete or imperfect knowledge –Lack of information, e.g., insufficient soil sampling –Shortcomings in measurement, e.g., soil disturbance effects on modulus reduction/damping curves –Shortcoming of calculation, e.g., limitations of 1-D ground response model –Can be reduced with research (development of additional data, better models)
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Definitions of Uncertainty Aleatory: uncertainty inherent to a physical process or property –Spatial variability of soil properties –Dispersion of IM from source/path effects at high frequencies –Cannot be reduced with additional data/knowledge
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Context Where geotechnical uncertainty matters: Site response – IM EDP|IM for EDPs related to ground failure –Liquefaction and its effects (ground movement, instability) –Slope failure –Volume change in unsaturated soils Soil-structure interaction –Seismic demand imparted to structure from free-field –Flexibility/damping of foundation-soil interaction
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Information Resource Jones/Kramer/Arduino PEER report 2001/03 “Estimation of uncertainty in geotechnical properties for performance based earthquake engineering” Parameter variability from field/lab tests subdivided according to: –Inherent variabilty –Measurement variability –Spatial correlation
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Site Response Uncertainty IM pdf from attenuation –IM dispersion is dependent on site condition –Estimated empirically
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Site Response Uncertainty IM pdf from site-specific analysis –Uncertainty in nonlinear properties (G/G max, D) Epistemic from sample disturbance effects PEER Lifelines–developing models for depth, PI, % fines effects –V s Aleatory from spatial variability - e.g. Savannah River (Toro, Silva) Epistemic from measurement error, incomplete site testing Ref: Toro et al., 1997
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Site Response Uncertainty –Input motions Epistemic uncertainty in IM hazard results (target spectrum for ground motion scaling) Aleatory from phasing of input time histories Result: large uncertainty in calculated soil response – especially at short periods (e.g., T < 1 s)
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EDP|IM: Liquefaction Triggering: –Liq|(pene. resistance, IM) Epistemic from model minimized with recent PEER work (Seed et al.) Modest aleatory –Still large uncertainty in penetration resistance COV 50% (sand N-values); Ref. Phoon and Kulhawy, 1999 Effect on liquefaction can be of similar order to that of IM uncertainty
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Liquefaction Effects Ground/structure settlement –Correct form of model unknown –Epistemic from inadequate data –Aleatory uncertainty not quantified Undrained residual strength Lateral spread displacement Opportunity for PEER impact
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Soil-Structure Interaction Seismic demand – kinematic interaction –Rigorous analysis with incoherent wave field vs. simplified model with incoherence parameter –Epistemic model uncertainty –Aleatory uncertainty on incoherence parameters Soil-Foundation Interaction –Epistemic from model formulation (spring, continuum models from FE, FD) –Aleatory from material parameters
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Propagation of Uncertainties Evaluation of ground response effects on IMs – hazard analysis –Category-specific dispersion in PSHA –1-D response analysis procedures for randomized soil properties and input (RASCAL) –Must quantify epistemic uncertainty using logic trees –Methodology challenge: propagation of epistemic uncertainty through the framing equation Opensees simulations for dG[EDP|IM]d (IM) –Monte Carlo methods –Repeat for different IMs (epistemic)
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One-Dimensional Site Response Hydraulic fill 3 m 6 m 3% ground slope Ref: Jones et al. 2001
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Monte Carlo Results Ref: Jones et al. 2001
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