Optional Features

The dynamic analysis option permits two-dimensional, plane-strain or axisymmetric, fully dynamic analysis with FLAC. The calculation is based on the explicit finite difference scheme to solve the full equations of motion, using lumped gridpoint masses derived from the real density of surrounding zones (rather than fictitious masses used for static solution). This formulation can be coupled to the structural element model, thus permitting analysis of soil-structure interaction brought about by ground shaking. The dynamic feature can also be coupled to the groundwater flow model. This allows, for example, analyses involving time-dependent pore pressure change associated with liquefaction. The dynamic model can likewise be coupled to the optional thermal model in order to calculate the combined effect of thermal and dynamic loading. The dynamic option expands FLAC's analysis capability to a wide range of dynamic problems in disciplines such as earthquake engineering, seismology and mine rockbursts.

The creep option can be used to simulate the behavior of materials that exhibit creep - i.e., time-dependent material behavior. Six creep models have been implemented in FLAC. These are:

• a classical viscoelastic model;
• a two-component power law;
• a reference creep formulation (the WIPP model) for nuclear-waste isolation studies;
• a Burger-creep viscoplastic model combining the Burger's creep model and the Mohr-Coulomb model;
• a WIPP-creep viscoplastic model combining the WIPP model and the Drucker- Prager model; and
• a crushed-salt constitutive model.

The first model is the classical formulation known as the Maxwell substance. The second model can be used for mining applications (e.g., salt or potash mining), and the third model is commonly used in thermomechanical analyses associated with studies for the underground isolation of nuclearwaste in salt. The fourth model expands on the first model and also includes aKelvin and a Mohr-Coulomb component. The fifth model is a variation of the third model and includes a Drucker-Prager plasticity component. The sixth model is also a variation of the third and includes volumetric and deviatoric compaction behavior.

In addition, it is possible for users to write their own creep constitutive models using FISH.

The two-phase flow option in FLAC allows numerical modeling of the flow of two immiscible fluids (with optional capillary pressure) through porous media. The formulation applies to problems, such as those encountered in reservoir simulation, in which a fluid displaces another and simultaneous flow of the two fluids takes place in the porous medium with no mass transfer between them. The option uses built-in pressure and permeability laws (of the van Genuchten form), and an explicit finite difference solution scheme with upstream weighting.

The thermal option of FLAC incorporates both conduction and advection models. The conduction models allow simulation of transient heat conduction in materials, and the development of thermally induced displacements and stresses. The advection model takes the transport of heat by convection into account; it can simulate temperature-dependent fluid density and thermal advection in the fluid.

The explicit formulation used in FLAC may be slow in some circumstances, but it is very tolerant of extreme nonlinearities and offers a straightforward framework in which to implement complex physics. The method provides a flexible way to simulate complex, 2D systems involving transient, nonlinear heat, fluid and solid coupling.

A model using this option may be run independently or coupled to the mechanical stress calculation or pore pressure calculation, either in static or dynamic mode.

Written in C++ and compiled as DLL (dynamic link library) files that can be loaded whenever needed in a FLAC simulation. The main function of the model is to return new stresses, given strain increments. However, the model must also provide other information, such as names, and perform operations such as writing and reading save files. The methodology is similar to that for developing a user-defined model with FISH.

User-defined models may be utilized by all Itasca codes. New DLL models can be obtained from the Itasca web site devoted specifically to model development and exchange: www.itasca-udm.com.