Geology Reference
In-Depth Information
C.5.4.1. Material weathering classi
cations
Having used weathering classi
cations a great deal in practice, the
author is convinced that material classi
cations such as that used in
Hong Kong and presented i
n Table C6
are the most useful for dealing
with rock that weathers from a strong condition progressively to a soil
so that thick pro
les of saprolite are sometimes found. It has been used
for logging many tropically weathered igneous and sedimentary rocks,
even in temperate climates, and is essentially a strength rating.
The Australian standard weathering scheme is presented in
Table
C7 a
nd is essentially the same as that in
Table C6 e
xcept that different
terminology is used and distinctly weathered (DW) includes a very
wide range of strengths (including grades III and IV from
Table C6).
In
Table C6
Material classi
cation: Geoguide 3 (GCO, 1988).
This classi
cation from GCO (1988) is applicable to uniform samples of weathered igneous and
volcanic rocks and other rocks of equivalent strength in fresh state. It is broadly compatible with
recommendations of Anon (1995), BS5930: 1999 and used in other standards such as CP4 (2003) in
Singapore. It has stood the test of time (from Moye, 1955) as a useful tool in logging core and
describing thick weathered pro
cation should be supplemented by other descriptive
terms, and it is often helpful to qualify with index tests such as Schmidt hammer readings on
exposures (not applicable for core logging).
les. The classi
DECOMPOSITION
TERM
GRADE SYMBOL
TYPICAL CHARACTERISTICS
RESIDUAL SOIL
VI
Original rock texture completely destroyed; can be
crumbled by hand and
finger pressure into constituent
grains.
COMPLETELY
DECOMPOSED
V
Original rock texture preserved; can be crumbled by
hand and
finger pressure into constituent grains; easily
completely discoloured compared with fresh rock.
HIGHLY
DECOMPOSED
IV
Can be broken by hand into smaller pieces; makes a
dull sound when struck by hammer; not easily indented
by point of pick; does not slake in water; completely
discoloured compared with fresh rock.
MODERATELY
DECOMPOSED
III
Cannot usually be broken by hand; easily broken by
hammer; makes a dull or slight ringing sound when
struck by hammer; completely stained throughout.
SLIGHTLY
DECOMPOSED
II
Not broken easily by hammer; makes a ringing sound
when struck by hammer; fresh rock colours generally
retained but stained near joint surfaces.
FRESH ROCK
I
Not broken easily by hammer; makes a ringing sound
when struck by hammer; no visible signs of
decomposition (i.e. no discolouration).
Notes:
1
A
sample placed in a container of water will lose strength and become a slurry. The rapidity and ease of slaking
can be used as a qualifying index test at site.
