Bonded Block Model in 3DEC

Continuum approaches are very effective in revealing important metrics such as stress (and strain) directions and their relative magnitudes as excavations progress. However, they are inherently limited when the rock behavior involves mechanisms such as spalling and bulking. In such situations, discontinuum approaches such as discrete element method (DEM) can realistically simulate the initiation and propagation of cracks leading to extension and shear fracturing, as well as the rock mass strength dependency on confinement.

New tools have been added to 3DEC so that it can be used to simulate a massive rock as bonded polyhedral elements (e.g., tetrahedra) that can break at their subcontacts as a result of stress concentrations.

  • POLY TET command allows the creation of tetrahedral blocks based on 4 corner coordinates.
  • LIST ZONE POLY command outputs tetrahedrals as blocks from block zones for easy bonded block model generation.
  • TET keyword added to the DENSIFY command allows for more intelligent densifying of tetrahedral blocks.
  • FRAGMENT commands have been added to enable the tracking and plotting of fragments (i.e., disconnected groups of blocks).

The Bonded Block Model (BBM) approach simulates the initiation of cracks that can coalesce and/or propagate to fracture the rock (e.g., intact, jointed, or veined). This results in an emergent damage pattern with associated bulking. The 3DEC approach differs from the Bonded Particle Method (BPM) used in PFC as it can:

  • simulate an initial zero-porosity condition, and
  • simulate interlocked, irregular block shapes which provides resistance to block rotation (i.e., moments) after contact breakage.

The above processes tend to dominate the rock mass behavior in regions of low confinement near excavations and thus are relevant for both stability assessment and support design. For example, below is a cross-sectional view of rock mass damage as the support pressure around a tunnel is reduced (Garza-Cruz et al., 2014). The various colors indicate distinct rock fragments.

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