Fast Lagrangian Analysis of Continua in 3 Dimensions
Itasca Consulting Group, Inc.
111 Third Avenue South, Suite 450
Minneapolis, Minnesota 55401 USA
First Edition (FLAC3D Version 2.1) April 2002
Second Edition (FLAC3D Version 3.0) September 2005
FLAC3D is a three-dimensional explicit finite-difference program for engineering mechanics computation.The basis for this program is the well-established numerical formulation used by our two-dimensional program, FLAC.* FLAC3D extends the analysis capability of FLAC into three dimensions, simulating the behavior of three-dimensional structures built of soil, rock or other materials that undergo plastic flow when their yield limits are reached. The explicit, Lagrangian, calculation scheme and the mixed-discretization zoning technique used in FLAC3D ensure that plastic collapse and flow are modeled very accurately.
FLAC3D is designed specifically to operate on IBM-compatible microcomputers runningWindows 98 and later operating systems.
1.2 Comparison with Other Methods
1. The “mixed discretization” scheme (Marti and Cundall, 1982) is used for accurate modeling of plastic collapse loads and plastic flow.
2. The full dynamic equations of motion are used, even when modeling systems are essentially static. This enables FLAC3D to follow physically unstable processes without numerical distress.
3. An “explicit” solution scheme is used (in contrast to the more usual implicit methods).
4. FLAC3D is robust in the sense that it can handle any constitutive model with no adjustment to the solution algorithm.
These differences are mainly in FLAC3D’s favor, but there are two disadvantages.
1. Linear simulations run slower with FLAC3D than with equivalent finite element programs.
2. The solution time with FLAC3D is determined by the ratio of the longest natural period to the shortest natural period in the system being modeled.
1.3 General Features
1.3.1 Basic Features
FLAC3D offers a wide range of capabilities to solve complex problems in mechanics, and especially in geomechanics. Additionally, an interface, or slip-plane, model is available to represent distinct interfaces between two or more portions of the grid. FLAC3D contains an automatic 3D grid generator in which grids are created by manipulating and connecting pre-defined shapes.* Boundary conditions and initial conditions are specified in much the same way as in FLAC. FLAC3D incorporates the facility to model groundwater flow and pore-pressure dissipation, and the full coupling between a deformable porous solid and a viscous fluid flowing within the pore space. Structures, such as tunnel liners, piles, sheet piles, cables, rock bolts or geotextiles, that interact with the surrounding rock or soil, may be modeled with the structural element logic in FLAC3D. A factor of safety can be calculated automatically for any FLAC3D model composed of Mohr-Coulomb material.
FLAC3D also contains a powerful built-in programming language, FISH, that enables the user to define new variables and functions.
• user-prescribed property variations in the grid (e.g., nonlinear increase in modulus with depth);
• plotting and printing of user-defined variables (i.e., custom-designed plots);
• implementation of special grid generators;
• servo-control of numerical tests;
• specification of unusual boundary conditions; variations in time and space; and
• automation of parameter studies.
FLAC3D contains extensive graphics facilities for generating plots of virtually any problem variable.
1.3.2 Optional Features
Four optional features (for dynamic analysis, thermal analysis, modeling creep-material behavior,and writing user-defined constitutive models) are available as separate modules that can be included in FLAC3D at an additional cost per module. Also, a fifth optional feature, a hexahedral-meshing preprocessor (3DShop), is available as a separate program.*
There are eight optional material models available that simulate time-dependent (creep) material behavior (All creep models are described in Section 2 in Optional Features.):
(1) the classical viscoelastic (Maxwell) model;
(2) a Burger’s substance viscoelastic model;
(3) a two-component power law;
(4) a reference creep formulation (the WIPP model) implemented for nuclear waste isolation studies;
(5) a Burger-creep viscoplastic model combining the Burger’s model with the Mohr-Coulomb model;
(6) a power-law viscoplastic model combining the two-component power law and the Mohr-Coulomb model;
(7) a WIPP-creep viscoplastic model combining the reference creep formulation with the Drucker-Prager plasticity model; and
(8) a “crushed-salt” model that simulates both volumetric and deviatoric creep compaction
User-defined constitutive models can be written in C++ and compiled as DLL (dynamic link library) files that can be loaded whenever needed with this optional feature.