Table 8.

Summary of characteristics of iron precipitation in sandstone rock mass

Type of iron precipitationWeathering zoneRock mass weathering*,†DescriptionIron content (%)Discontinuity shear strength
Discontinuities
 StainedSlightly to fresh rockFresh–slightly (grades 0 and 1)Staining only (discoloured surfaces)1.1–2.9 (1.9)Controlled by the roughness of the discontinuity walls and infill (if present)
 CoatedOxidationModerately–high (grades 2 and 3)Thin layers (0.5–2 mm) of iron oxide along the discontinuity walls; may be present on both opposite discontinuity walls; locally iron oxide bridges3.3–9.7 (5.1)Increased due to hardened and rougher discontinuity walls and bridges
 CementedOxidationModerately–highly (grades 2 and 3)Healed discontinuities or welded opposing discontinuity walls (0.5–>5 mm)18.8–23.8 (21.3)Equivalent to or higher than the surrounding intact rock
 Intact rockIntact rock blocks discoloured to brownish or rusty in appearance; iron precipitation on and between rock-forming material grains; iron concentration is high along the discontinuity surfaces and gradually decreases inwards in intact rock
  • Notes: *Rock mass weathering follows Table 2.

  • Relict discontinuities of any type often present in highly and completely weathered rock mass (grades 3 and 4).

  • Average in brackets.