Rock mass, rock material, discontinuities

Rock material

Rock material (intact rock) is the solid rock between the discontinuities, a block of rock without any fractures, as in a laboratory core specimen. It is described by its rock type, grain size, mineral composition and the strength found from laboratory tests (UCS, point load).

Rock mass

Rock mass is the rock in the field as a whole: the intact rock plus the system of discontinuities that cut it. Its behaviour is controlled mostly by the discontinuities, so its strength is much lower than that of the intact rock.

   rock mass = intact rock blocks + discontinuities
   ____ ____ ____
  |    |    |    |   each block = rock material
  |____|____|____|   gaps/lines   = discontinuities

Characteristics of discontinuities

1. Orientation: the attitude, given by dip and dip direction (or strike). Together with the slope it decides if a block can slide.
2. Spacing: the distance between adjacent discontinuities of one set. Close spacing gives small blocks and a weak mass.
3. Persistence (continuity): the length of the discontinuity trace; a persistent plane makes a more continuous failure surface.
4. Roughness: unevenness of the surface; rough surfaces give higher shear strength.
5. Aperture: the open distance between the walls. A wide aperture means low strength and high permeability.
6. Filling (infill): material between the walls (clay, sand, calcite); clay filling reduces shear strength sharply.
7. Wall strength (weathering): strength of the rock at the wall surface.
8. Seepage (water flow): water in the discontinuities.
9. Number of sets and block size.

These characteristics are logged in the field by scanline or window mapping and are the input for rock mass classification (RMR, Q) and slope and tunnel design.