UCSD1: 3D Nonlinear Hysteretic Soil Model
for Seismic Analysis (for FLAC/FLAC3D)


UCSD1 user-defined model (UDM) is an elastic-plastic material in which plasticity exhibits only in the deviatoric stress-strain response (Elgamal et al. 2008)*. The volumetric stress-strain response is linear-elastic and is independent of the deviatoric response. This material is implemented to simulate monotonic or cyclic response of materials whose shear behavior is insensitive to the confinement change. Such materials include, for example, organic soils or clay under fast (undrained) loading conditions.

For questions or remarks, please send email to Dr. Ahmed Elgamal (elgamal@ucsd.edu) and Dr. Jinchi Lu (jinlu@ucsd.edu).

*Elgamal, A., L. Yan, Z. Yang and J. P. Conte. (2008). "Three-Dimensional Seismic response of Bridge Foundation-Ground System," Journal of Structural Engineering, ASCE, Vol. 134, No. 7, 1165-1176.

UCSD1 Soil Model .DLL Files

There are 3 versions available (1D shearbeam, 2D and 3D versions). Please download a corresponding .dll file below and place it in the "exe32\plugins\models" folder under FLAC/FLAC3D installation directory (e.g., usually at "C:\Program Files (x86)\Itasca\FLAC700" for a 32-bit version of FLAC 7).

1) 1D Shearbeam Version:

This version allows for one-dimensional (1D) site response analysis in FLAC. No limit for the number of yield surfaces.

File: ucsd1demo.dll Download for FLAC 7(Coming soon!) Download for FLAC 8(Coming soon!)
Command to load the model in FLAC 7/FLAC 8: model dll ucsd1demo

Example File (230 MB): UCSD1_Example.rrr

2) 2D Version:

This version allows for two-dimensional (2D) analysis in FLAC. A dongle is necessary to operate the full version. Without a dongle, the number of yield surfaces is limited to 3.

File: ucsd1.dll Download for FLAC 7(Coming soon!) Download for FLAC 8(Coming soon!)
Command to load the model in FLAC 7/FLAC 8: model dll ucsd1
Click here for an example (Coming soon!)

3) 3D Version:

This version allows for three-dimensional (3D) analysis in FLAC3D. A dongle is also necessary to operate the full version. Without a dongle, the number of yield surfaces is limited to 3.

File: ucsd1.dll Download for FLAC3D 5 (Coming soon!)
Command to load the model in FLAC3D: model ucsd1
Click here for an example (Coming soon!)

Model Parameters

In the UCSD1 soil model, the shear stress-strain backbone curve is represented by the hyperbolic relationship, if a modulus reduction curve is not provided.

1) Hyperbolic Relationship for Stress-strain Backbone Curve

If a modulus reduction curve (G/Gmax) is not provided, the hyperbolic curve will be used. The syntax to use this soil model in FLAC/FLAC3D is as follows:

prop shearModul ... poissonRatio ... shearStrength ... peakShearStra ... &
noYieldSurf ...


in which:
    shearModul -- Low-strain shear modulus (G).

    poissonRatio -- Poisson's ratio.

    shearStrength -- Shear strength (τf).

    peakShearStra -- The shear strain at which the shear strength shearStrength is reached (γmax).

    noYieldSurf -- Number of yield surfaces (1 or larger).


The hyperbolic curve is defined by:


where:


For user convenience, a table is provided below as a quick reference for selecting parameter values. However, use of this table should be of great caution, and other information should be incorporated wherever possible.

Parameters Soft Clay Medium Clay Stiff Clay
shearModul 1.3e+07 Pa 6.0e+07 Pa 1.5e+08 Pa
poissonRatio 0.4 0.4 0.4
shearStrength 18,000 Pa 37,000 Pa 75,000 Pa
peakShearStra 0.1 0.1 0.1


2) User-defined Backbone Curve

If a modulus reduction curve is provided:

prop shearModul ... poissonRatio ... grTab ...

in which:
    grTab -- Table # for user-defined modulus reduction curve (G/Gmax). A separate FLAC/FLAC3D command table is needed to define the modulus reduction curve. In this case, the number of the points specified will be used as the Number of yield surfaces (Note: Please plot your own shear stress-strain curve based on the modulus reduction curve specified and make sure the modulus reduction curve you're using is reasonable).

    Click here for some sample modulus reduction curves.

Model Performance

Below are some sample shear stress-strain response output of the UCSD1 model in FLAC (vertical axis: shear stress in Pa; horizontal axis: shear strain; Number of yield surfaces = 20):