Table 22.1. (Continued)
Grade
Degree of decomposition
Field recognition
Engineering properties
III
Moderately weathered
Slightly weathered
Fresh rock
Considerably weathered throughout.
Possessing some strength
- large pieces (e.g. NX
drill core) cannot be
broken by hand. Often
limonite-stained. 50 to 90%
rock
Distinctly weathered through
much of the rock fabric
with slight limonite staining.
Some decomposed feldspar in
granites. Strength approaching
that of fresh rock. More
than 90% rock
Fresh rock may have
some limonite-stained joints
immediately beneath
weathered rock
Excavated with difficulty without
use of explosives. Mostly
crushed under bulldozer tracks.
Suitable for foundations of
small concrete structures and
rockfill dams. May be
suitable for semipervious fill.
Stability in cuttings depends
on structural features, especially
joint attitudes
Requires explosives for excavation.
Suitable for concrete dam
foundations. Highly permeable
through open joints. Often more
permeable than the zones
above or below. Questionable
as concrete aggregate
Staining indicates water percolation
along joints; individual pieces
may be loosened by
blasting or stress relief
and support may be
required in tunnels and
shafts
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