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Version: 3.4.8 (unreleased)

Block Size

Block Size turns a fracture intensity map into the block-size measures a rock mass classification asks for. These are the volumetric joint count Jv, RQD derived from it, and block volume, each written as a layer over the outcrop.

Before you start​

Block Size reads a per-point fracture intensity layer named P32 (fracture area per volume) or P21 (trace length per area). On a mesh, make them with Attributes & Analysis → Statistical → P21 Map, which writes both; see Fracture Intensity Mapping. A point cloud has no P21 Map command, so it needs a P32 or P21 layer brought in another way.

Without either layer the command stops and the messages panel tells you to run P21 Map first.

Running it​

  1. Select one or more loaded meshes or point clouds in the Data tree.
  2. Right-click and choose Attributes & Analysis → Block Size (Jv, RQD)....
  3. Set the options in the dialog and press OK. One dialog serves every selected object.

The dialog​

The line at the top says whether project Sets can weigh the P21 conversion: N project Sets with M planes weigh the P21 conversion by their angle to the face, or, with none, that P21 converts as for isotropic joints.

FieldDescription
Jv fromP32, fracture area per volume (Jv = P32) or P21, trace length per area (P32 = C21 x P21). A source no selected object has is marked (no P32 layer) or (no P21 layer). Default P32.
P21 to P32For P21 only. C21 from the project Sets and the local face, or C21 = 4/pi, as for isotropic joints. Available only when project Sets hold planes.
Face radius (model units)The radius over which the local face is fitted for the conversion by sets. Default 1.
Shape factor betaβ in the block volume, 27 for cubic blocks and more for flat or long ones. Default 27.

The dialog notes that Jv is in joints per m³, with model units taken as metres, and that RQD from Jv is derived, not measured on core. It remembers your choices for the rest of the session.

What it writes​

LayerValues
JvVolumetric joint count, joints per m³. Shown when the run finishes.
RQD from JvRQD in per cent, 0–100, derived from Jv.
Block Volumem³. No value where Jv is 0.
C21The P21 to P32 factor used at each point. Only from a P21 run; a P32 run removes an old one.

The messages panel gives the median Jv, RQD and block volume. It reminds you that RQD from Jv is derived (Palmström 2005), and that Jv from P32 is a lower bound where joints end inside the rock.

Each run is recorded under Analysis Runs.

How it is worked out​

  • From P32: Jv = P32. For persistent joints the area of joints per unit volume is the number of joints crossing a metre in every direction (Palmström 2005).
  • From P21: Jv = C21 × P21. Traces on a face under-count the joints at a low angle to it.
    • With C21 from the project Sets and the local face, the face is fitted at each point over the face radius. Each set's mean plane is weighted by 1/sin of its angle to the face, capped at 7, and the weights are averaged by the sets' numbers of planes. A point with no fitted face uses 4/π.
    • With C21 = 4/pi, the joints are taken as isotropic.
  • RQD = 110 − 2.5 Jv, kept between 0 and 100 (Palmström 2005).
  • Block volume Vb = β · Jv⁻³ (Palmström 2005).
Derived, not logged

RQD from Jv is an estimate. Label it as derived wherever you quote it, and prefer RQD logged on core or a scanline when you have it. A geotechnical domain can take either and says which it used.

Using the result​

  • Read the median Jv of the layer over a geotechnical domain and enter it as the domain's Jv (joints/m3). See Rock Mass Classification.
  • Colour the outcrop by Block Volume to see where blocks are large enough to matter for rockfall or excavation.

References​

  • Palmström, A. (2005). Measurements of and correlations between block size and rock quality designation (RQD). Tunnelling and Underground Space Technology, 20, 362–377.

See also​