UCSD Nonlinear Soil Models in OpenSees:

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UCSD Nonlinear Soil Models in OpenSees
Capabilities and Input Parameters Zhaohui
Yang Benchmarking of Nonlinear Geotechnical
Ground Response Analysis Procedures
(PEER2G02) September 21, 2004
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• Topics
• Pressure insensitive material model (clays,
  silts) and input parameters
• Pressure sensitive material model (sands,
  gravels, silts) and input parameters
• Effect of dilation on nonlinear response
• Solid-fluid fully coupled elements, parameters
  related to dynamic analysis.
• Uncertainties in parameters.
• Manual and examples for soil models and fully
  coupled elements are available at
  http//cyclic.ucsd.edu/opensees

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• UCSD Geotechnical Simulation Capabilities in
  OpenSees
• Nonlinear model for pressure insensitive
  materials (Clays and Silts)

Cyclic response obeys Masing rule
Shear stress-strain from earthquake simulation
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Nonlinear shear stress-strain relation
automatic generation using hyperbolic
function

• Parameters Low-strain shear modulus (Gmax),
  
• Shear strength (tmax),
• Shear strain at which tmax
  is reached (gmax)
• Parameters may be selected based on empirical
  relations or experiments

Hyperbolic function
In which gr satisfies
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Nonlinear shear stress-strain relation
user defined modulus reduction
curve

• Parameters Low-strain shear modulus (Gmax),
  
• G/Gmax g pairs defining
  modulus reduction curve

Parameters may be selected based on empirical
relations (e.g., Seed and Idriss 1970) or
experiments

• Notes
• Shear strength (tmax) is the product of the last
  Gg pair
• Not accept modulus reduction curves that produce
  straining softening response.

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UCSD Geotechnical Simulation Capabilities in
OpenSees (2) Nonlinear model for pressure
sensitive materials (sands, gravels, and silts)
Undrained behavior
Undrained shear stress-strain from earthquake
simulation
w/o dilation
with dilation
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Nonlinear shear stress-strain relation
automatic generation or user defined
(same as for pressure insensitive materials)

• Parameters (specified at a reference pressure
  pr)
• Low-strain shear modulus
  (Gmax)
• Shear strength (tmaxf (f, tr,
  pr) )
• Shear strain at which tmax is
  reached (gmax)
• Bulk modulus (Bmax) determined
  based on desired k0
• Bmax2Gmax(12k0)/(3-3k0
  )

• Gmax and Bmax vary with confinement p by factor
  of (p/pr)n with n0.5 typically

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• Dilatancy/Liquefaction Parameters
• Dilation angle separates contractive and
  dilative zones
• Contraction parameters control rate of
  contraction or pore water pressure buildup
• Dilation parameters - control rate of dilation
  or pore pressure reduction
• Liquefaction parameters control shear strain
  accumulation during cyclic liquefaction
• Parameters may be selected based on empirical
  relations (e.g., Youd and Idriss 2001) or
  liquefaction experiments (e.g., Kammerer 2003)

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UCSD Geotechnical Simulation Capabilities in
OpenSees (3) Solid-fluid fully coupled elements
(u-p formulation)
Plane-strain elements
3D elements
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• Parameters related to dynamic analysis
• Mass density - may vary significantly near
  liquefaction
• Permeability may vary significantly near
  liquefaction
• Frequency dependent viscous (Rayleigh) damping

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Effect of dilation on shear stress-strain and
stress path in dense sands
Drained
Undrained
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Modulus reduction of undrained dense sand
(Dr100) Centrifuge experiments and numerical
simulation (Elgamal, Yang, Lai, Kutter and
Wilson 2004)

• Modulus increases at higher strain levels (due
  to dilation)

Experimental data vs. model simulation
Experimental data vs. empirical relations
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Damping ratio of undrained dense sand
(Dr100) Centrifuge experiments and numerical
simulation (Elgamal, Yang, Lai, Kutter and
Wilson 2004)

• 5 damping at low-strain levels
• Decreasing damping ratio at higher strain levels
  (due to dilation)

Experimental data vs. empirical relations
Experimental data vs. model simulation
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• Major Uncertain Parameters
• Non-liquefaction related
• How much (viscous) damping is needed at low
  strain levels?
• Should material damping at large strain obey
  Masing rule?
• Liquefaction related
• How much residual strength is needed during
  liquefaction?
• Can dilation be counted as a source of residual
  strength?
• How does effective permeability vary during
  liquefaction?