# General Features

3DEC is primarily intended for analysis in rock engineering projects, ranging from studies of the progressive failure of rock slopes to evaluations of the influence of rock joints, faults, bedding planes, etc. on underground excavations and rock foundations. 3DEC is ideally suited to study potential modes of failure directly related to the presence of discontinuous features.

The program can best be used when the geologic structure is fairly well-defined (for example, from observation or geologic mapping). Both manual and automatic joint generators are built into 3DEC to create individual (and sets of) discontinuities that represent jointed structure in a rock mass. A wide variety of joint patterns can be generated in the model. There are also two tunnel generators to set up models with long, regularly shaped excavations. In addition, a discrete fracture network (DFN) generator is included in 3DEC.

Different representations of joint material behavior are available. The basic model is the Coulomb slip criteria, which assigns elastic stiffness, frictional, cohesive and tensile strengths, and dilation characteristics to a joint. Other, more complicated models are also provided including a creep model and a continuously yielding model. Joint models and properties can be assigned separately to individual, or sets of, discontinuities in a 3DEC model. Note that the geometric roughness of a joint is represented via the joint material model, even though the plot of discontinuities shows the joint as a planar segment.

Blocks in 3DEC can be either rigid or deformable. There are 20 built-in material models for deformable blocks, ranging from the “null” block material (which represents holes (excavations)), to the shear yielding models (which include strain-hardening/softening behavior and represent nonlinear, irreversible shear failure). Thus, blocks can be used to simulate backfill and soil materials, as well as intact rock. (Purchasers of the User-Defined Model (UDM) option may write their own models.) An effective stress analysis can be performed by assigning a pore-pressure distribution that acts on both the blocks and the contacts.

The automatic zone generator in 3DEC allows the user to divide deformable blocks into finitedifference tetrahedral zones. A block zone generate command allows the user to specify as fine a discretization as needed, and to vary the discretization throughout the model. Thus, a fine tetrahedral mesh can be prescribed for blocks in the region of interest, and a coarser mesh can be used for blocks farther out. 3DEC also has automatic radially graded mesh generation within polyhedra for modeling “infinite domain” problems. For block plasticity analysis, a special zone generator can be used to create “mixed-discretization” blocks, for improved accuracy when modeling plastic collapse. The user may also use high-order tetrahedral elements or nodal mixed discretization for plasticity problems.

The explicit solution algorithm in 3DEC permits either static or dynamic analysis. Static analysis is the default solution mode. Dynamic analysis is provided as an optional feature, and is discussed in Dynamic Analysis.

Both stress (force) and fixed-displacement (zero velocity) boundary conditions are available for static analysis. Boundary conditions may be different at different locations.

3DEC includes the ability to model steady-state or transient-fracture fluid flow. The flow logic includes a system of flow planes, flow pipes and flow knots. Flow between the joints (through the blocks) is also possible and fluid may flow from the fractures into the blocks and vice versa.

Structural element logic is implemented to simulate rock reinforcement including liners and bolts as described in Structural Elements.

3DEC contains a powerful built-in programming language, FISH, that enables the user to define new variables and functions. FISH is a compiler: programs entered via a 3DEC data file are translated into a list of instructions stored in 3DEC ’s memory space; these are executed whenever a FISH function is invoked. FISH permits:

• user-prescribed property variations in the block structure (e.g., nonlinear increase in modulus with depth);
• plotting and printing of user-defined variables (custom-designed plots);
• implementation of special joint generators;
• servo-control of numerical tests;
• specification of unusual boundary conditions with variations in time and space; and
• automation of parameter studies.

Interactive manipulation of screen images is built directly into 3DEC. This allows the user to generate shaded perspective views, wire-frames, vectors, tensors, contours, time histories, etc. The history plots are especially helpful in ascertaining when an equilibrium state or failure state has been reached. 3DEC also has the facility to create two-dimensional “windows” through the 3D model. On these windows, output can be presented in the form of principal stress plots, stress contour plots, relative shear plots and vector plots. All plots can be created in screen mode by single keystrokes that move and rotate the 3D model, orient the window and produce the required output (vectors, contours, etc.). The output can then be directed to a hardcopy device for incorporation into reports.