1. PEER Jonathan P. Stewart University of California, Los Angeles May 22, 2002 Geotechnical Uncertainties for PBEE

2. 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)

3. 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

4. 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

5. 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

6. Site Response Uncertainty IM pdf from attenuation –IM dispersion is dependent on site condition –Estimated empirically

7. 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

8. 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)

9. 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

10. 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

11. 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

12. 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)

13. One-Dimensional Site Response Hydraulic fill 3 m 6 m 3% ground slope Ref: Jones et al. 2001

14. Monte Carlo Results Ref: Jones et al. 2001