Geology Reference
In-Depth Information
4. Condition of discontinuities (0
-
30)
5. Groundwater conditions (0
-
15)
Points are assigned for each parameter and then summed to rate the
rock as very good to very poor. It is to be noted that the degree of
fracturing and condition of those fractures covers 70% of the Rock
Mass Rating and that there is a high level of double counting between
factors 2 and 3. There is also somewhat of a conundrum in that rock
with RQD of 90
100% is allocated the full 20 points but rock with
joint spacing of 60
-
200mm is only allocated 8 out of 20 points.
Orientation of discontinuities is used to adjust the summed rating
according to whether discontinuities are adverse relative to the engi-
neering project.
RMR is used (as is RQD by itself) to correlate with rock mass
parameters, including rock mass strength and deformability.
-
C.6.2.2 Q SYSTEM
The Q system of Barton
et al
. (1974) is commonly used to classify the
quality of rock mass
as a predictive tool in estimating tunnel support
requirements for a planned tunnel or cavern, for judging whether
ground conditions during tunnelling were as expected or not (contrac-
tual issues) and for making decisions on temporary and permanent
support requirements during tunnelling (
Chapter 6).
Barton (2000,
2005) also discusses ways of using the Q system in predicting TBM
performance. Q has a value range from 0.001 to 1,000.
-
J
r
J
a
J
w
SRF
QðqualityÞ¼
RQD
J
n
where:
RQD is Rock Quality Designation; J
n
is joint set number; J
r
is joint
roughness number; J
a
is joint alteration number; J
w
is joint water
reduction factor; and SRF is a Stress Reduction Factor (relating to
loosening of rock, rock stress in competent rock and squeezing condi-
tions in incompetent rock).
Ranges and descriptions for each parameter are given in the original
publications by Barton (2000, 2005) but also in Hoek & Brown
(1980) and at Hoek
'
s corner, as referred to at C6.
C.6.2.3 GSI
The Geological Strength Index (GSI) of Hoek (1999) provides a
means of estimating rock mass strength and deformability through
broad classi
cation based on rock type and quality of rock mass,
as illustrated in
Table C11
. Ways in which the GSI can be used to
estimate rock mass parameters are dealt with in
Chapter 5.
