DFN Overview

In DIPS, the Baecher DFN Wizard allows you to generate statistical sets of joint planes (fractures) in 3D. DFNs require the following input parameters:

• Spatial model
• Fracture intensity
• Fracture size
• Fracture orientation
• Fracture termination percentage (Elmo et al., 2014).

Note that DFNs are meant for joints and not major structures such as faults. Faults can be added separately as a deterministic structure in other Rocscience programs (for example: RocSlope3, RocTunnel3, etc.).

Joint planes are assumed to be planar and are represented by thin polygons.

Spatial Model

The spatial model controls how the joint planes are spatially distributed in a generation region.

Fracture Intensity

Fracture intensity describes how dense the fracture network is. It is commonly described using Pij notation, where i represents the dimension of the sample and j represents the dimension of the measurements (Elmo, 2006). Commonly used fracture intensity parameters include:

• P10. P10 represents the number of fractures per unit length of a linear scanline or borehole and is equivalent to fracture frequency.
• P21. P21 represents the length of fractures per unit area.
• P32. P32 represents the area of fractures per unit volume.

Fracture Size

In the context of DFNs, fracture size is defined as the radius of a circle with an area equal to the area of the polygon that represents the fracture (Elmo, 2006; Elmo et al., 2014). It is not the same as trace length or fracture persistence.

Fracture Orientation

Fracture orientation controls the orientation (dip and dip direction) of the fractures in a DFN. It can be described deterministically (by using a constant orientation), stochastically (by using distributions such as a Fisher distribution), and with bootstrapping (sampling from data).

Fracture Termination Percentage

Fracture termination percentage specifies what percentage of fractures terminate against previously generated fractures.

References

Elmo D. 2006. Evaluation of a hybrid FEM/DEM approach for determination of rock mass strength using a combination of discontinuity mapping and fracture mechanics modelling, with particular emphasis on modelling of jointed pillars. Ph.D. Thesis, University of Exeter, United Kingdom.

Elmo, D., Liu, Y., & Rogers, S. 2014. Principles of discrete fracture network modelling for geotechnical applications. DFNE 2014, Vancouver, Canada.