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Dafalias & Taiebat (NTUA, UCD, UBC) Democritus , July 2011 1Earthquake Liquefaction Hazards Mitigation at NPP Sites

Earthquake Liquefaction Hazards Assessments at Nuclear Power Plant Sites

Yannis F. Dafalias, Ph.D.Department of Mechanics, National Technical University of Athens

Department of Civil and Environmental Engineering, University of California, Davis

Mahdi Taiebat, Ph.D., P.Eng.Department of Civil Engineering, The University of British Columbia


Recent Research Solicitation from NRC of the USA

• Investigation and Modeling of Element-Level Soil Behavior under Multi-Dimensional Loading- Solicitation Number: 04-10-147

- Agency: Nuclear Regulatory Commission

- Office: Office of Administration

- Location: Division of ContractsAppendix A: "General Design Criteria for Nuclear Power Plants," to 10CFR Part 50, General Design Criterion (GDC) 2, "Design Bases for Protection Against Natural Phenomena," requires, in part, that nuclear power plant structures, systems, and components (SSCs) important to safety must be designed to withstand the effects of natural phenomena (such as earthquakes) without loss of capability to perform their safety functions. This includes not only the effects of shaking, but also possible shear or volumetric deformations in the underlying geologic foundation materials.Regulatory Guide 1.198, "Procedures and Criteria for Assessing

Seismic Soil Liquefaction at Nuclear Power Plant Sites,"provides guidance on seismic assessments, but currently provides minimal guidance on how to conduct deformation assessments.


Outline

• Simple explanation of liquefaction

• Extensive photographic exposition of liquefaction catastrophes

• Fundamentals of liquefaction behavior

• Physical modeling (full scale - shake table - centrifuge)

• Numerical modeling (constitutive – fluid/solid interaction - dynamics)

• Conclusion


Liquefaction


1989 Loma Prieta earthquake


1964 Niigata earthquake (photo: NISEE)


1964 Niigata earthquake (photo: NISEE)


Surface rupture: Taiwan



• Surface ruptures during the 1999 Chi-Chi earthquake caused extensive damage to civil infrastructure in Taiwan.








Adapazari - Liquefaction Effects on Shallow Foundations

• Widespread damage to buildings occurred throughout Adapazari, Turkey, during the 1999 Kocaeli earthquake. A major cause of damage was liquefaction of the recent alluvial deposits that underlaid large portions of the city. The result was excessive settlements and bearing capacity failures for countless buildings, most of which were supported on shallow foundations.


Adapazari - Liquefaction Effects on Shallow Foundations


Tanks on Liquefied Ground

• The 1995 Kobe earthquake in Japan in which various liquefaction effects on tanks and their foundations were observed.


Dakai Subway Collapse

• The collapse of the Dakai subway during the 1995 Kobe EQ was the first case of a subway station collapsing due to an earthquake. This case history has since been widely analyzed for its lessons on seismic soil-structure interaction effects.


Liquefaction Damage to Ports in Kobe, Japan

• Port and waterfront facilities in Kobe, Japan, suffered extensive damage due to liquefaction during the 1995 Kobe earthquake. The problem was pervasive because the majority of the waterfront facilities were reclaimed lands consisting of loose to medium-dense cohesionless fills.






Lower San Fernando Dam - Liquefaction-induced Failure of the Upstream Slope

• The upstream slope of the Lower San Fernando Dam, in California, failed due to liquefaction during the 1971 San Fernando earthquake. The dam was constructed by "hydraulic filling," which involves mixing the fill soil with a large amount of water, transporting it to the dam site by pipeline, depositing the soil and water on the embankment in stages, and allowing the excess water to drain away. The fill that remains is loose, and is subject to liquefaction as the result of earthquake shaking.






Consequences of liquefaction

• Loss of bearing support

• Settlements – can be uniform in some cases, but are mostly abrupt and non-uniform

• Floatation of buried structures, such as underground tanks

• Loss of lateral support [piles extending to or through the liquefied soil layer(s)]

• Increased lateral pressures against retaining structures, such as quay walls

• Lateral spreads (limited lateral movements)

• Lateral flows (extensive lateral movements)


Fundamentals of liquefaction behavior


Basics

• Triaxial setting:

- At critical state

Loading of saturated sands

Stress paths for monotonic drained loading with constant p' and undrained loading (constant volume shearing) of saturated loose-

of-critical and dense-of-critical sands


Undrained loading (ICU – Isotropically Consolidated Unrained)Monotonic loading of saturated sands

Undrained triaxial compression tests (after Ishihara 1993).


Undrained cyclic loadingCyclic loading behavior of saturated sands

Undrained cyclic triaxial test (test from Boulanger & Truman 1996).


Outline

• Modeling in Geomechanics- Physical modeling Full scale

Shake Table

Centrifuge

- Numerical modeling Constitutive models

Solid-pore fluid interaction

Dynamic analysis


Modeling in Geomechanics

1. Physical Modeling- Full scale

- Shake Table

- Centrifuge

UCSD shake table (photo credit: nees@UCSD) UC Davis centrifuge

(photo credit: nees@UCD)

UT Austin full scale tests (photo credit: UT Austin)


How does the centrifuge work?





About 2 g, 14 RPM

SPIN UP



About 5 g, 22 RPM

SPIN UP



About 75 g, 90 RPM

The centrifugal force increases the “weight” of the model to simulate weight of full scale Civil Structures

SPIN UP



2 ft of soil spinning at 75 g represents 150 ft of soil at 1g

A building or other structure may be placed on the sand

S

A

N

D

SHAKING



SLOW DOWN



SLOW DOWN



Vertical actuators

S A N D


Scaling factors

• Scaling factors for converting the measured data to prototype units under a gravitation acceleration of N g in centrifuge modeling


Modeling in Geomechanics

1. Physical Modeling…2. Numerical Modeling…

• Elements of numerical modeling


Numerical modeling in geomechanics

• Modeling approaches- Microscopic: particulate approach

- Macroscopic: continuum hypothesis

• Governing physical laws: System of differential equations of motion with complex boundary/initial conditions- Numerical solution Methods: FDM, FEM, BEM, Mesh-free Method, DDA, NMM, …

Tools: FLAC, PLAXIS, DIANA, ABAQUS, OPENSEES, ANSYS, …

• Complexities and difficulties- Constitutive model

- Dynamic

- Fully coupled solid-fluid interaction

- 3-D (geometry and loading)

Fill

Loose soil

Dense soil


Theoretical modeling

• Constitutive Modeling - Small Deformation Elasto-Plasticity- Elasticity

- Elasto–plasticity

- Elastic–Plastic Material Models Tresca, von Mises

Drucker-Prager, Mohr-Coulomb

Modified Cam-Clay Model

Advanced plasticity models

Severn-Ternt, Norsand, UBCSAND, UBCHYST, Nested surface models (Elgamal–Yang’s model), Bounding surface models (SANISAND & SANICLAY), …

• Modeling of porous solid-pore fluid systems


A Constitutive Model for Sands

SANISANDSimple ANIsotropic SAND plasticity model

Dafalias, Manzari, Li, Papadimitriou, Bouckovalas,Taiebat

(1997-2008)

Powerful, yet simple constitutive model with systematic and relatively simple calibration process.

A single soil-specific set of constants for different levels of density and confining pressure.


Constitutive Model Validation

Undrained triaxial compression tests (CIUC) Toyoura sand

Drained triaxial compression tests (CIDC) Toyoura sand

Data: Verdugo & Ishihara (1996)Data: Verdugo & Ishihara (1996)

Experiment Simulation Experiment Simulation


Constitutive Model ValidationConstant-p cyclic triaxial tests - Toyoura sand

Experiment Simulation Experiment Simulation

Data: Pradhan et al. (1989)


Fully Coupled u−p−U Finite Element

• Formulation: Zienkiewicz and Shiomi (1984), Argyris and Mlejnek (1991)- u – displacement of solid skeleton (ux,uy,uz)- p – pore pressure in the fluid- U – displacement of fluid (Ux,Uy,Uz)

• Equations:- Mixture Equilibrium Equation:

- Fluid Equilibrium Equation:

- Flow Conservation Equation:

• Features:- Takes into account the physical velocity proportional damping- Takes into account acceleration of fluid- Is stable for nearly incompressible pore fluid


Liquefaction-Induced Isolation of Shear Waves

10m soil column – level ground

Permeability=10-4 m/s

Finite element model

Free drainage from surface

Analysis:

Self-weight & shaking the base


Void ration vs. Time Acceleration vs. Time

Uniform Layered Uniform Layered


Centrifuge test data Finite Element (FE) analysis

(mathematical model)

Analysis

• Match the results of “centrifuge test” with “FE analysis”

• Parametric study on the calibrated FE model for different EQ, soil properties, etc.


Conclusion

• Nuclear Power Plant construction is of enormous complexity• Increased safety design is required • Earthquake induced liquefaction is a catastrophic natural hazard• Assessment and remediation requires advanced methods of analysis • Predominant methods are

- centrifuge physical modeling

- laboratory sample experimental data

- inelastic constitutive modeling of soil

- numerical dynamic analysis of earthquake event

• Final hazard assessment can be made within degrees of approximation• Even the most advanced methods cannot predict the unexpected• Choice of site is of paramount safety importance

PowerPoint Presentation - CIVL598C - Geotechnical Earthquake ...

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