FLAC3D Modeling • Problem Solving with FLAC3D

Problem Solving with FLAC

This section provides guidance in the use of FLAC3D in problem solving for static mechanical analysis.[1] It does so by breaking the modeling process down to a sequence from project start to project completion, as follows.

1. Project Planning and Setup

2. Grid Generation

3. Identifying Regions of the Model

4. Working with Geometric Data

5. Choice of Constitutive Model

6. Material Properties

7. Boundary Conditions

8. Initial Conditions

9. Stepping To Equilibrium

10. Loading And Sequential Modeling

11. Structural Support

12. Interfaces

13. Tips and Advice

14. Interpretation Of Results

15. Project Completion

Each of these modeling aspects is discussed in detail. The user who is familiar with the two-dimensional program FLAC will find that the modeling approach is very similar in FLAC3D. The major difference is the procedure for grid generation. We recommend that Grid Generation be studied carefully, and that the user grasps the techniques for grid generation presented there before creating their own model grids.

Note that FISH programs are used in this section to assist with model generation and problem solving. If new to the FISH language, it is advisable to first read FISH Tutorial.

The philosophy of modeling in the field of geomechanics was presented earlier in the topics discussed in the section Modeling Methodology. The novice modeler may wish to review that section first. The methodology of modeling in geomechanics can be significantly different from that of other engineering fields, such as structural engineering. It is important to keep this in mind when performing any geomechanics analysis.

Section Outline

• Approach and Project Setup
  • Modeling on a Spectrum
  • Recommended Steps
  • Start a Project
• Grid Generation in FLAC3D
  • Primitive-Based Grids
    • Overview of the Grid Primitives
    • Connecting Adjoining Primitive Shapes
    • Fitting the Grid to Simple Shapes
    • Grid Generation with FISH
  • Sketch-Based Grids
    • Create a Set
    • Define the 2D Geometry
    • Zoning and Other Operations
  • Building Blocks-Based Grids
    • Create a Building Blocks Set
    • Snap Blocks Together
    • Refinement Operations and Utilities
    • Zoning
  • Grids from Outside FLAC3D
    • Grids from Rhino/Griddle
  • Grid Generation: Additional Facilities
    • Densifying Grids
    • Geometry-Based Densification: Octree Meshing
    • Surface Topography and Layering
• Identifying Regions of the Model
  • Groups and Slots
  • Using the Group Range Element
  • Select and Hide
  • Using the Model Pane
    • Objects in the Model Pane
    • Visualizing Objects
    • Selection
    • Groups
    • Additional Commands
• Working with Geometric Data
  • Geometric Data
  • Geometry Visualization
  • Geometry Painting
  • Geometric Filtering - Geometry Range Elements
  • Geometry Data and Group Assignment
  • Geometry Data: Examples
    • Assign Group Names Based on Their Relationship to DXF Files
    • Ignore Existing Groups — Build New Ones Based on Surfaces
    • Partition Existing Groups Based on Surfaces
    • Zones below Surfaces become Separate Groups, Outside Groups Remain Unchanged
    • Zones above the Surfaces become a New Group, Everything below is Unchanged
    • Zones Intersecting Surfaces becomes Separate Groups
    • Split Zones below Surfaces
    • Split Large Zones Intersecting Surfaces
• Choice of Constitutive Model
  • Overview of Constitutive Models
  • The Constitutive Models in FLAC2D and FLAC3D
  • Selection of an Appropriate Model
  • The Effect of Water
  • Ways to Implement Constitutive Models
• Material Properties
  • Intrinsic Deformability Properties
  • Intrinsic Strength Properties
  • Post-Failure Properties
  • Extrapolation to Field-Scale Properties
  • Spatial Variation and Randomness of Property Distribution
• Boundary Conditions
  • Stress Boundary
    • Applied Stress Gradients
    • Changing Boundary Stress
    • Cautions and Advice
  • Displacement Boundary
    • Local System and Applied Velocities
    • Surface Corners and Velocity Boundaries
    • Fix vs Apply
    • Reaction Forces
    • Nonuniform Velocities
  • Real Boundaries — Choosing the Right Type
  • Artificial Boundaries
    • Symmetry Planes
    • Boundary Truncation
• Initial Conditions
  • Uniform Stresses — No Gravity
  • Stresses with Gradients — Uniform Material
  • Stresses with Gradients — Nonuniform Material
  • Stress Initialization in a Nonuniform Material
  • Compaction within a Nonuniform Grid
  • Initial Stresses following a Model Change
  • Stress and Pore-Pressure Initialization with a Phreatic Surface
  • Initialization of Velocities
• Reaching Equilibrium
  • Convergence Criteria
    • Maximum Out-of-Balance Force
    • Local Maximum Force Ratio
    • Average Force Ratio
    • Maximum Force Ratio
    • Convergence
    • Ratio
  • Choosing Convergence Criteria
  • Evaluating Equilibrium
    • Effects of Large Stiffness Differences
• Loading and Sequential Modeling in FLAC3D
  • Example: Loading on Three Tunnels
• Structural Support
• Interfaces
• Tips and Advice
  • 1. Check Model Runtime
  • 2. Effects on Runtime
  • 3. Considerations for Zoning Density
  • 4. Automatic Detection of an Equilibrium State
  • 5. Considerations for Selecting Damping
  • 6. Check Model Response
  • 7. Initializing Variables
  • 8. Minimizing Transient Effects on Static Analysis
  • 9. Changing Material Models
  • 10. Running Problems with In-Situ Field Stresses and Gravity
  • 11. Determining Collapse Loads
  • 12. Determining Factor of Safety
  • 13. Use Bulk and Shear Moduli
  • FLAC3D Runtime Benchmark
• Interpretation
  • Unbalanced Force and Convergence
  • Gridpoint Velocities
  • Plastic Indicators
  • Histories
• Project Completion
• References

Endnotes

[1]

Problem solving for coupled mechanical-groundwater analysis is discussed in Fluid-Mechanical Interaction, and for coupled mechanical-thermal analysis in Thermal Analysis. Problem solving for dynamic analysis is discussed in Dynamic Analysis. The fluid formulation is a standard feature of FLAC3D; thermal and dynamic analysis are available separately as FLAC3D Options.