9.7 Liquefaction

This section describes the input data for the constitutive models for liquefaction analysis that were developed on behalf of the Japanese Liquefaction User Group [15]. These models may be applied in a nonlinear analysis with Module NONLIN [Vol. Analysis Procedures]. For background theory see Chapter 24. See also Volume Geotechnical Analysis for some instructive examples of liquefaction analysis.

    (syntax)

LIQUEF

starts the input of a liquefaction model. The model specifier may be one of the following.

TOWHAT

for the Towhata-Iai model [§9.7.1].

NISHI

for the Nishi model [§9.7.2].

BOWL

for the Bowl model [§9.7.3].

USER

for a user-supplied liquefaction model. For this model you must specify some data items [§11.6.1] and the source code of subroutine USRLIQ [§11.6.2].

data

are additional data records to specify the properties of a specific liquefaction model.

Model choice.

With three dedicated constitutive models available for liquefaction analysis, your first question will be which model to choose. Besides your own preferences this choice is determined by the specific capabilities and limitations of the models.

Towhata-Iai

[§9.7.1]. This model is your choice if the analysis is two-dimensional and largely undrained, if you want a failure surface to be part of the model and if you want to include the effect of stress rotation on liquefaction occurrence. Also attractive is the robustness of the model.

Nishi

[§9.7.2]. This model is your choice in case of partially drained conditions, if the direction of shearing is arbitrary, and if you need a failure surface as part of the model. The model can be used with two- and three-dimensional elements. Also attractive is the limited number of model parameters.

Bowl

[§9.7.3]. This model is your choice in case of partially drained conditions, if the direction of shearing is predominantly horizontal, and if you do not require a failure surface as part of the model. The model can be used with two- and three-dimensional elements. Also attractive is the robustness of the model.

Element types.

All DIANA's relevant plane strain and axisymmetric continuum elements are available in combination with the liquefaction constitutive models. The Nishi and Bowl model are also available in combination with all DIANA's solid elements. See Volume Element Library for a description of the available plane strain, axisymmetric, and solid elements.

Pore pressure modeling.

All liquefaction constitutive models are effective stress models, which means that you can combine them with all DIANA's features for pore pressure modeling. However, the Towhata-Iai model intrinsically uses the assumption of undrained behavior, and should therefore not be used under drained conditions.

Large strain modeling.

All liquefaction constitutive models can be combined with Updated Lagrange large strain modeling [Vol. Analysis Procedures].

Combination with other constitutive models.

You may choose different constitutive models and different parameters for different sets of elements. Because the liquefaction models are total strain models you cannot apply them within one of these sets simultaneously with other material models like plasticity, cracking, temperature dependency, viscoelasticity, viscoplasticity etc.

However, you may deactivate the liquefaction models via a special LIQUEF OFF command in the preceding phased nonlinear analysis with Module NONLIN.^9.4 This allows you to determine the initial stress state prior to the actual liquefaction analysis.

    (file.dcf)

*NONLIN
^...   commands for initialization, output selection etc.
BEGIN EXECUT
  ^...   commands for time/load step execution
 BEGIN PHYSIC
  LIQUEF OFF
   ^...   other options
 END PHYSIC
END EXECUT
*END

Status output.

Depending on the applied liquefaction model, some status variables are optionally available for output at integration points.4