Planes of discontinuity in rock masses

Discontinuities are planes of weakness or breaks in a rock mass across which the rock has little or no tensile strength. They separate the rock material into blocks and control the strength, deformability and permeability of the mass.

Types of discontinuities

• Bedding planes: surfaces between sedimentary layers.
• Joints: fractures with no visible displacement; they occur in sets (joint systems).
• Faults: fractures with displacement; they often have gouge, breccia or crushed rock.
• Foliation and cleavage planes: planar fabric in metamorphic rocks (slate, schist, gneiss).
• Shear zones and fracture zones, and unconformities or contacts between rock types.

Characteristics (ISRM)

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.