Table 7.

Frictional component of rock mass strength (FC) per iron-precipitation type and rock mass weathering grade for Jr and Ja values of discontinuities typically found in the weathering profile of an area with iron-dissolution and precipitation

Jr*Rock mass weathering gradeJa*FC (°)
ValueDescriptionValueDescription
Cemented
 4Discontinuous joint2, 3, 40.75Tightly healed, hard, non-softening, impermeable filling79
Coated
 3§Rough or irregular, undulating22Slightly altered joint walls, non-softening mineral coatings56
33Silty- or sandy-clay coating, small clay-fraction (non-softening)45
 1.5Rough or irregular, planar22Slightly altered joint walls, non-softening mineral coatings37
33Silty- or sandy-clay coating, small clay-fraction (non-softening)27
Stained
 3Rough or irregular, undulating0, 11Unaltered joint walls, surface staining only72
12Slightly altered joint walls, non-softening mineral coatings, sandy particles, clay-free disintegrated rock, etc.56
23Silty- or sandy-clay coatings, small clay fraction (non-softening)45
 2Smooth undulating0, 11Unaltered joint walls, surface staining only63
12Slightly altered joint walls, non-softening mineral coatings, sandy particles, clay-free disintegrated rock, etc.45
23Silty- or sandy-clay coatings, small clay fraction (non-softening)34
 1.5Rough or irregular, planar0, 11Unaltered joint walls, surface staining only56
12Slightly altered joint walls, non-softening mineral coatings, sandy particles, clay-free disintegrated rock, etc.37
23Silty- or sandy-clay coatings, small clay fraction (non-softening)27
 1Smooth planar0, 11Unaltered joint walls, surface staining only45
12Slightly altered joint walls, non-softening mineral coatings, sandy particles, clay-free disintegrated rock, etc.27
23Silty- or sandy-clay coatings, small clay fraction (non-softening)18
 0.5Slickensided planar0, 11Unaltered joint walls, surface staining only27
12Slightly altered joint walls, non-softening mineral coatings, sandy particles, clay-free disintegrated rock, etc.14
23Silty- or sandy-clay coatings, small clay fraction (non-softening)9
  • * Jr and Ja values and descriptions follow Barton (2002).

  • Rock mass weathering grades follow Table 2.

  • The authors consider a Ja value of 0.75 or higher appropriate, although Barton (2002) recommends this only for coated discontinuities. Whether the resulting FC is realistic is questionable, as tightly healed discontinuities will have to break through the cementation before shear can occur along the discontinuity. This may not happen because the shear strength of the cementation may be equal to or stronger than the shear strength of intact rock and breaking through intact rock may occur. The shear properties will then be those of intact rock.

  • § A Jr with a value of 3 can also include the presence of iron-bridges along discontinuities.