Geology Reference
In-Depth Information
necessarily equate with natural breaks in core (the incipient joint
problem, as discussed in
Chapter 3).
The engineering geologist must
beware of counting all visible traces in de
ning RQD, otherwise the
ground will be assigned a much lower quality rating than is really
justi
ed. This can have major consequences in assessing the potential
of tunnel boring machines or roadheaders to make progress when
cutting rock.
C.6.2 More sophisticated rock mass classi
cation
schemes
Various rock mass classi
cations have been developed, largely as
methods of estimating support requirements for underground excava-
tions linked to case histories, although GSI (covered below) is aimed
instead at predicting engineering parameters rather than engineering
performance and support requirements. As discussed in
Chapter 6,
these classi
cations really come into their own whilst tunnelling and
where a decision has to be made quickly, onmucking out, as to the level
of support that is required to stabilise the rock mass. In a drill and blast
operation, there is no time for the engineering geologist to take sophis-
ticated measurements and carefully weigh up the potential modes of
failure and rock loads whilst the drilling crew (no doubt on a bonus
linked to advance rate) wait patiently
…
However, not all tunnel engineers are great fans of rock mass classi-
Currently,
much time and effort tends to be wasted in assembling prescribed data,
often painstakingly acquired at the tunnel face, to enable calculation of
a RMC algorithm which is then
fications schemes. Sir Alan Muir Wood (2000) comments,
'
filed in a geological log book but not
applied to serve any further purpose.
'
And,
'
For weak rocks, the con-
tribution from RMC is more limited
as well as
discontinuities. Attempts to base support needs for weak rocks on
RMC have been notably unsuccessful.
…
based on material
…
RMC are inade-
quate to provide reliable information on failure modes
'
He continues,
'
the way
ahead must be to identify important parameters for a particular situa-
tion and to present these in a multidimensional way
'
,and
'
'
.
C.6.2.1 RMR
The Rock Mass Rating (RMR) of Bienawski (1976, 1989) has also
stood the test of time as a useful classi
cation system, despite many
five parameters for the RMR
system and their ranges of assigned points are as follows:
question marks over de
nitions. The
1. Uniaxial compressive strength of rock material (0
-
15)
2. Rock Quality Designation (3
-
20)
3. Spacing of discontinuities (5
-
20)
