Table 22.1. Classification of residual tropical soils by degrees of weathering (ISSMFE, 1985).
Grade
Degree of decomposition
Field recognition
~~
Engineering properties
O~
VI
Soil
V
IV
Completely weathered
Highly weathered
No recognisable rock texture;
surface layer contains humus
and plant roots
Rock completely decomposed by
weathering in place but
texture still recognisable. In
types of granitic origin,
original feldspars completely
decomposed
to clay minerals. Cannot
be recovered as cores
by ordinary rotary drilling
methods
Rock so weakened by
weathering that fairly large
pieces can be broken
and crumble in the
hands. Sometimes recovered
as core by careful
rotary drilling. Stained by
limonite. Less than 50%
rock
Unsuitable for important foundations.
~
Unstable on slopes when
cover is destroyed
Can be excavated by
hand or ripping without
~
use of explosives. Unsuitable
for foundations of concrete
dams or large structures.
May be suitable for
~"
foundations of Earth dams
and for fill. Unstable
in high cuttings at
E"
steep angles. Requires erosion
protection
Similar to grade V. Unlikely to
be suitable for foundations of
concrete dams. Erratic presence
of boulders makes it an
unreliable foundation stratum for
large structures
(Continued)