peer jonathan p. stewart university of california, los angeles may 22, 2002 geotechnical...
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May 22, 2002
PPEEEERR
Jonathan P. StewartUniversity of California, Los Angeles
Geotechnical Uncertainties for PBEE
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)
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
Context
|)(||||| IMdIMEDPdGEDPDMdGDMDVGDV
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
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
Site Response Uncertainty
• IM pdf from attenuation– IM dispersion is
dependent on site condition
– Estimated empirically
0.01 0.1 1 10P e rio d (s)
0.3
0.4
0.5
0.6
0.7
Sta
nda
rd E
rro
r,
V = 530 - 760 m /sV = 310-530 m /sV = 180 - 310 m /sSadigh et a l.
Boore et a l.
m 7.5
m 6.5
PH A
}
F a
}
F v
Site Response Uncertainty
• IM pdf from site-specific analysis– Uncertainty in nonlinear
properties (G/Gmax, D)• Epistemic from sample
disturbance effects• PEER Lifelines–developing
models for depth, PI, % fines effects
– Vs
• Aleatory from spatial variability - e.g. Savannah River (Toro, Silva)
• Epistemic from measurement error, incomplete site testing
0 0.2 0.4 0.6 0.8 1ln(V ) - m /s
200
160
120
80
40
0
De
pth
(m
)
S ite Sp e cificS td. D ev. (s )
Corr. C oeff. (r )
Ref: Toro et al., 1997
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) 0.01 0.1 1 10
P e rio d (s)
0
0.3
0.6
0.9
(l
n u
nits
)
RR S from 1-D ground response
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 0
0.1
0.2
0.3
0.4
0.5
0 10 20 30 40N1,60,cs
CSR
50% 5%Mw=7.5 v' =1300 psf
__ _ Seed et al., (1984)
__ _ Yoshimi et al. (1994)
95%20%80%
P L
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
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
90% C onfidence in terva ls
0 200 400 600
V s (m /s)
0.00
0.20
0.40
0.60
a
Surface foundations w ith Q uaternaryShallow ly em bedded w ith Q uaternarySurface foundations w ith Tertiary and older
= 0 .57
a= 0.017 + 5.0E-04 V s (m /s)
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)