Geology Reference
In-Depth Information
C.5.4.2. Mass weathering classi
cations
Formal mass schemes are commonly prescribed in different standards
because they are readily drawn on paper (percentages of soil and rock)
but in practice are often dif
exible and not particu-
larly useful. The scheme used in Hong Kong is presented in
Table C8.
This was prepared following the work of Martin & Hencher (1986)
and was largely a response to the perceived failings of the scheme used
in the then BS 5930:1981 and similar schemes adopted by the
ISRM. The zones have some sense to them and it seems to work
reasonably well in practice, although conditions are found where the
application is simply impossible and/or unhelpful. Describing pro
cult to apply, in
les
according to material grade distributions is the preferred method of the
author. Elsewhere, the ISRM recommendation is still to use a mass
scheme by preference
despite it
being impossible to use this in logging (though some people try). The
-
the old scheme of BS 5930:1981
-
Comment: largely similar to scheme recommended in Anon (1995) and BS5930: 1999. This
classi
cation was developed for classifying thick, heterogeneous weathered pro
les. The zone
boundaries make good sense in terms of engineering signi
cance. Principles are often useful starting
points for differentiating engineering units in a ground model.
ZONE
SYMBOL
TERM
TYPICAL CHARACTERISTICS
Residual soil derived from
in situ
weathering; mass
structure and material texture/fabric completely
destroyed: 100% soil.
RESIDUAL SOIL
RS
Less than 30% rock.
Soil retains original mass structure and material
texture/fabric (i.e. saprolite).
Rock content does not affect shear behaviour of
mass, but relict discontinuities in soil may do so.
Rock content may be signi
PW 0/30
cant for investigation
and construction.
PARTIALLY
WEATHERED
ROCK MASS (PW)
30% to 50% rock.
Both rock content and relict discontinuities may
affect shear behaviour of mass.
PW 30/50
50% to 90% rock.
Interlocked structure.
PW 50/90
Greater than 90% rock.
Small amount of the material converted to soil
along discontinuities.
PW 90/100
100% rock.
May show slight discolouration along
discontinuities.
UNWEATHERED
ROCK
UW
