Environmental Engineering Reference
In-Depth Information
Note that altered rock is classified in the same way as weathered rock except that “weath-
ered” is replaced by “altered” and in the abbreviation, “W” is replaced by “A”.
In some granitic and similar rocks which are very strong when fresh it is commonly pos-
sible and useful to subdivide the distinctly weathered category further, on the basis of dry
strength of 50 mm diamond drill cores. The terms used are based on those of Moye (1955)
and Hosking (1990), defined as in Table 2.6. The rock fabric will be apparent in all
weathering grades.
The distribution of rock substance at various weathering grades can be shown on borehole
logs, and on plans, sections or elevation views of rock exposures, either by the abbreviations,
or graphically by suitable colour or black and white overprinting of the rock type symbols.
Table 2.4.
Recommended descriptive terms for strength of rock substance.
Point load strength
Approximate unconfined compressive
Rock strength (1)
Symbol
index (2)(3) I s(50)
strength Qu(MPa) (2)(3)
Very weak
VW
0.2
5
Weak
W
0.2-1
5-25
Medium strong
MS
1-2
25-50
Strong
S
2-4
50-100
Very strong
VS
4-10
100-250
Extremely strong
ES
10
250
(1) Based on ISRM (1985): Suggested methods for the quantitative description of discontinuities
in rock masses. (2) In rocks with planar anisotropy, the strength terms and numbers refer to the
strengths when tested normal to the anisotropy. (3) The numbers assume that Qu is 25 times I s(50) .
In practice the ratio ranges typically from 10 to 25, with the weaker rocks towards the lower end.
Table 2.5.
Description of weathered or altered rock substance: Examples.
Rock type term
Rock condition term
Rock strength term
Abbreviated form
Granite
Fresh
Very strong
Granite FR (VS)
Granite
Extremely weathered
Soil properties (GW-GC)
Granite XW (GW-GC)
Granite
Distinctly weathered
Medium strong
Granite DW (MS)
Sandstone
Fresh
Weak
Sandstone FR (W)
Basalt
Distinctly altered
Medium strong
Basalt DA (MS)
Table 2.6.
Weathered rock substance classification for granite and similar rocks. Modified from Moye
(1955) & Hosking (1990).
Term
Abbreviation
Description
Fresh
FR
Rock shows no evidence of chemical weathering
Slightly weathered
SW
Rock is slightly discoloured but rings when struck by a
hammer; not noticeably weaker than fresh rock
Moderately weathered
MW
Rock is discoloured, producess only a dull thud when struck
by a hammer; noticeably weaker than fresh or slightly
weathered rock but dry samples about 50 mm across cannot
be broken across the fabric by unaided hands
Highly weathered
HW
Rock is discoloured, can be broken and crumbled by hand,
but does not readily disintegrate in water
Extremely weathered
XW
Material disintegrates when gently shaken in water, i.e. has
soil properties
 
 
Search WWH ::




Custom Search