# Introduction

3DEC is a three-dimensional numerical program based on the distinct element method for discontinuum modeling. The basis for this program is the extensively tested numerical formulation used by the two-dimensional version, UDEC (Itasca 2014). 3DEC simulates the response of discontinuous media (such as a jointed rock mass) subjected to either static or dynamic loading. The discontinuous medium is represented as an assemblage of discrete blocks. The discontinuities are treated as boundary conditions between blocks; large displacements along discontinuities and rotations of blocks are allowed. Individual blocks behave as either rigid or deformable material. Deformable blocks are subdivided into a mesh of finite difference elements, and each element responds according to a prescribed linear or nonlinear stress-strain law. The relative motion of the discontinuities is also governed by linear or nonlinear force-displacement relations for movement in both the normal and shear directions. 3DEC has several built-in material behavior models, for both the intact blocks and the discontinuities, that permit the simulation of response representative of discontinuous geologic, or similar, materials. 3DEC is based on a Lagrangian calculation scheme that is well-suited to model the large movements and deformations of a blocky system.

The distinguishing features of 3DEC are summarized:

- The rock mass is modeled as a 3D assemblage of rigid or deformable blocks.
- Discontinuities are regarded as distinct boundary interactions between these blocks; joint behavior is prescribed for these interactions.
- Continuous and discontinuous joint patterns can be generated on a statistical basis. A joint structure can be built into the model directly from the geologic mapping.
- 3DEC employs an explicit in-time solution algorithm that accommodates both large displacement and rotation, and permits time-domain calculations.
- The graphics facility permits interactive manipulation of 3D objects. In the graphics screen mode, the user can “move” into the model and make regions invisible, which allows for better viewing. This allows the user to build the model for a geotechnical analysis, and instantly view the 3D representation. This greatly facilitates the generation of 3D models and interpretation of results.

3DEC also contains the powerful built-in programming language FISH (short for FLAC-ish; FISH was originally developed for our two-dimensional, finite-difference continuum program, FLAC). With FISH, you can write your own functions to extend 3DEC’s usefulness. FISH offers a unique capability to 3DEC users who wish to tailor analyses to suit specific needs.

With the exception of the graphics mode, 3DEC is a command-driven (rather than menu-driven) computer program. Although a menu-driven program is easier to learn for the first time, the command-driven structure in 3DEC offers several advantages when applied in engineering studies:

- The input “language” is based upon recognizable word commands that allow you to identify the application of each command easily and in a logical fashion (e.g., the BOUNDARY command applies boundary conditions to the model).
- Engineering simulations usually consist of a lengthy sequence of operations (e.g., establish in-situ stress, apply loads, excavate tunnel, install support, etc.). A series of input commands (from a file or from the keyboard) corresponds closely with the physical sequence that it represents.
- A 3DEC data file can easily be modified with a text editor. Several data files can be linked to run a number of 3DEC analyses in sequence. This is ideal for performing parameter sensitivity studies.
- The word-oriented input files provide an excellent means by which to keep a documented record of the analyses performed for an engineering study. Often, it is convenient to include these files as an appendix to the engineering report, for the purpose of quality assurance.
- The command-driven structure allows you to develop preprocessing and postprocessing programs to manipulate 3DEC input/output as desired. For example, you may wish to write a joint-generation function to create a special joint structure for a series of 3DEC simulations. This can readily be accomplished with the FISH programming language, and incorporated directly in the input data file.

The formulation and development of the distinct element method embodied in 3DEC has progressed for a period of over 40 years, beginning with the initial presentation by Cundall (1971). In 1988, Dr. Cundall and Itasca staff adapted 3DEC specifically to perform engineering calculations on a PC. The software is designed for high-speed computation of models containing several thousand blocks. With the advancements in floating-point operation speed and multi-core computers, and the ability to install additional RAM at low cost, increasingly larger problems can be solved with 3DEC.

A comparison of 3DEC to other numerical methods, a description of general features and new updates in 3DEC Version 5.0, and a discussion of fields of application are provided in the following sections. If you wish to try 3DEC right away, the program installation instructions and a simple tutorial are provided in Section 2.2.

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