Itasca International, Inc

- Version 8.1 - Numerical Modeling Software



FLAC, Fast Lagrangian Analysis of Continua, is numerical modeling software for advanced geotechnical analysis of soil, rock, groundwater, and ground support in two dimensions. FLAC is used for analysis, testing, and design by geotechnical, civil, and mining engineers. It is designed to accommodate any kind of geotechnical engineering project that requires continuum analysis. FLAC utilizes an explicit finite difference formulation that can model complex behaviors, such as problems that consist of several stages, large displacements and strains, non-linear material behavior, or unstable systems (even cases of yield/failure over large areas, or total collapse).

FLAC has been developed primarily for geotechnical engineering applications in the fields of civil, mining, oil and gas, and power generation. FLAC is also a valuable tool used for research in rock- and soil-mechanics, particularly of localization and evolution of shear bands in frictional materials. FLAC has also been used in the manufacturing field where the analysis of highly deformable materials is needed.

The explicit, time-marching solution of the full equations of motion (including inertial terms) permits the analysis of progressive failure and collapse, which are particularly important phenomena for mine design and geotechnical construction.

Options in FLAC are sold separately from the code license, allowing users to extend the program’s capabilities as meets their own analysis needs.

Dynamic Analysis: Can be performed with FLAC using the optional dynamic calculation module. User-specified acceleration, velocity, or stress waves can be input directly to the model either as an exterior boundary condition or an interior excitation to the model.

Creep Analysis: This option can be used to simulate the behavior of materials that exhibit time-dependent material behavior.

Two-phase Flow: The two-phase flow option in FLAC allows numerical modeling of both fluid-flow and fully coupled simulations (with optional capillary pressure) of two immiscible fluids through porous media.

Thermal Analysis: The thermal analysis option in FLAC permits both conduction and advection to be incorporated into models.

FLAC offers large strain simulation of continua using interfaces that simulate faults, joints, or boundaries. It utilizes an explicit solution scheme that can model unstable physical processes.

The program provides 19 built-in material models, groundwater flow, coupled mechanical-flow calculation, inclusion of structural elements, plotting statistical distribution of any property, optional automatic remeshing during solution, and a built-in scripting language (FISH) that can customize or automate virtually all aspects of program operation, including user-defined properties and other variables.

The program can be extended with options that are offered separately from the base program (see Options for more information).

FLAC offers a fully integrated development environment that includes: project management facilities, built-in libraries of materials, pre-defined meshes, movies, extensive plotting capabilities, and run-time monitoring of results.

  • 64-bit version
  • Large-strain simulation of continua
  • Explicit solution scheme
  • Extensive solution controls and options
  • Multi-physics modeling
  • Track histories of model properties and results throughout the model to allow for comparison to actual monitoring and instrumentation data
  • Built-in scripting language (FISH) permits access (read) and control (write) of most model parameters, even while cycling
  • Multi-threaded solutions with no CPU locks or additional cost
  • Built-in project management tools
  • All operations are interchangeably performed via user interface (UI), written commands, or FISH scripting
  • Coupled hydro-mechanical-thermal effects using socket commands and FISH scripting
  • DOS console can be hidden or visible
  • Plots can be set to refresh automatically or manually during model cycling
  • Built-in material properties database
  • Option to add user-defined models via C++
  • Specify property statistical distributions
  • Groundwater fluid flow analyses are included
    • Effective stress (water table)
    • Steady-state
    • Transient
  • Constitutive model properties provide both basic and advanced inputs
  • Includes 19 standard, built-in material models:
    • Null
    • Elastic, isotropic
    • Elastic, transversely isotropic
    • Drucker-Prager
    • Mohr-Coulomb
    • Ubiquitous-joint (UBJ)
    • Caniso
    • Strain hardening/softening
    • Bilinear strain hardening/softening UBJ
    • Double yield
    • Modified Cam-clay
    • Hoek-Brown
    • Modified Hoek-Brown
    • Cysoil
    • Simplified Cysoil
    • Mohr-Coulomb swelling
    • Plastic Hardening UPDATED (small strain option)
    • NorSand NEW
    • Soft-Soil NEW
  • Automatic, fast solutions using the shear strength reduction (SSR) method and a converging bracket approach
  • Includes groundwater, structural support elements, and material strength properties for zones and interfaces
  • Applicable for Mohr-Coulomb, Ubiquitous-Joints, and Hoek-Brown constitutive models
  • Associated plastic flow rule (i.e., dilation) can be enabled
  • Automatic safety map contouring to indicate the distribution of factors of safety throughout the model
  • Multiple graphical output formats (PNG, JPG, BMP, EMF, DXF, and PostScript)
  • Copy plots directly to clipboard to paste into reports or slides
  • Easily export history results to spreadsheet-compatible CSV files
  • Export tables, histories, and model variable data to ASCII files
  • View-data option for TABLE and PROFILE charts that displays a table of the charted data NEW
  • Extensive visual plotting facilities (large number of plot items)Automatically export a series of PNG images at regular cycle intervals to create a video-ready series (third party software required for video assembly)
  • Up to 20 frame sets can be created during cycling and saved to different folders for video assembly NEW
  • Color ramps for contour plots Record pane and plot legend font size can be scaled using the mouse wheel
  • Multiple histories can be exported to a file with data written either vertically (columns) or horizontally (rows)
  • Liner moment-thrust and shear-thrust diagrams can be plotted
  • Visual model geometry creation and manipulation tools
  • Library of generic, pre-built geometries (slopes, tunnels, walls, footings, etc.)
  • Import grid geometry from CAD or table data (ASCII)
  • Re-meshing tools for highly deformed grids during cycling
  • Export extruded FLAC3D models
  • Mouse position relays model (x,y) and (i,j) coordinates
  • Displacement and stress boundaries
  • Artificial boundaries
  • Structural elements for ground support include: beams, liners, cables, piles, rock bolts, strips, support members, and shells
  • Axisymmetrical shells for simulating 3D effects of shaft linings, pressure vessels, and circular plates, etc.
  • The length and angle of structural elements can be specified
  • Water table for effective stresses
  • Interfaces allow inclusion of faults, joints, and boundaries that permit slip, separation, and closure
  • Boundary relaxation permits:
    • Simulating 3D tunnel advance, including ground reaction curves (GRC)
    • Avoiding numerical artifacts when excavating in weak materials and/or high stress fields
  • Boundary conditions are easy to apply across attached grids
  • Project tree and clone model tools make for easy construction, sequencing, and parameterizing of analyses at any construction phase
  • Rapid batch processing using data files
  • Groundwater flow calculations are included as a standard feature
  • Coupled calculations between standard (mechanical and fluid flow) and options (thermal, creep, C++ UDM, and dynamic)
  • Mechanical and fluid flow calculations are multithreaded
  • FLAC/Slope is included for simple and fast factor-of-safety analysis of soil and rock slopes and earth dams
  • SOLVE elastic command for initial elastic equilibrium state extended to most plasticity constitutive models
  • Seismic wizard for pre-processing ground waves for dynamic analysis
  • FISH is a scripting is a programming language embedded within FLAC that enables the user to define new variables and functions.
  • These functions may be used to extend FLAC’s usefulness or add user defined features (e.g., servo-control boundaries may be applied to a numerical test, unusual property distributions specified, and parameter studies automated).
  • Built-in text editor and FISH management pane
  • Integer, floating-point, and string data types that are automatically assigned and reassigned based on value
  • Control statements include CASE, IF-ELSE-ENDIF, EXIT, LOOP, LOOP WHILE, SECTION, and COMMAND
  • Built-in error handling
  • Use FISHCALL to access information at any point during the solution cycle (e.g., before any structure calculations or after the motion calculations) for each solution cycle or step
  • Built-in general scalar variables and intrinsic logic, mathematics, text, and parsing functions
  • Access FLAC solution variables, zone, and gridpoint variables (including zone strains and strain rates), and constitutive model variables
  • Access data tables, arrays, ASCII/binary data, I/O routines, data structures, and memory
  • Access socket I/O routines to exchange data between two or more Itasca software (FLAC or PFC2D) running as separate processes
  • FISH line statements can now contain up to 200 characters
  • Includes FISH Library function that can automatically vary the slope angle iteratively to reach a given factor of safety


Options in FLAC are sold separately from the code license, allowing users to extend the program’s capabilities as meets their own analysis needs. Modules available as options for FLAC include: Dynamic, Creep, Two-Phase Flow, Thermal, and User-Defined C++ Constitutive Models.

Dynamic Option

Dynamic analysis can be performed with FLAC using the optional dynamic calculation module. User-specified acceleration, velocity, or stress waves can be input directly to the model either as an exterior boundary condition or an interior excitation to the model. FLAC contains absorbing and free-field boundary conditions to simulate the effect of an infinite elastic medium surrounding the model. The dynamic calculation can be coupled to the structural element model (soil-structure interaction), to the standard groundwater flow model (liquefaction), and to the optional thermal model.


This option can be used to simulate the behavior of materials that exhibit creep (i.e., time-dependent material behavior).

There are nine available optional material models that simulate viscoelastic and viscoplastic (creep) behavior: the classical viscoelastic (Maxwell) model; a two-component power law; a reference creep formulation (the WIPP model) implemented for nuclear waste isolation studies; a Burger's creep viscoplastic model; a WIPP-creep viscoplastic model; a ubiquitous viscoplastic creep model; a crushed-salt constitutive model; a power-law viscoplastic creep with ubiquitous joints, and a NEW soft-soil creep model.

All nine models are available with the creep option. A FLAC grid can be configured for both a creep calculation and a dynamic calculation. However, both modes cannot be active simultaneously because of the widely different timesteps.


The two-phase flow option in FLAC allows numerical modeling of both fluid-flow and fully coupled simulations (with optional capillary pressure) of two immiscible fluids through porous media. The formulation applies to problems 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. This optional feature extends the facility of the standard groundwater flow model.


The thermal analysis option in FLAC incorporates both the conduction and advection models (conduction: transient heat conduction and the development of thermally induced displacements and stresses; advection: transport of heat by convection — it can simulate temperature-dependent fluid density and thermal advection in the fluid). A thermal model can be run independently or coupled to the mechanical stress calculation or pore pressure calculation, either in static or dynamic mode.


New constitutive models can be added to FLAC as DLLs that are written and compiled in C++. The DLLs can be loaded in FLAC whenever needed, via the MODEL load command, or automatically if they are placed in the “exe64\plugins\models” folder.

By implementing this optional feature, users can access new constitutive models from Itasca’s online UDM Library. An advantage of these models is that they run at nearly the same speed as built-in models, and noticeably faster than FISH constitutive models.

This option is required to both load and run UDM models.

Geared toward new users, FLAC Basics is an introduction to the application of Itasca's FLAC software in geotechnical engineering. FLAC 8 Basics is based on the original FLAC Basics text first written in 1993 and extends the documentation to demonstrate the power and ease-of-use of FLAC's graphical user interface and new features in FLAC version 8.0 to facilitate the solution of complex geotechnical problems.

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