Civil Engineering Reference
In-Depth Information
a traffi c tunnel with a diameter of 10 m located in a very poor rock mass characterized
by Q = 0.2 support class 31 is recommended (Fig. 12.3).
A temporary support requires a smaller safety margin compared with permanent sup-
port. In the Q system this is accounted for by taking the values of the input parameters
Q and ESR in the diagram represented in Fig. 12.3 as 5
ESR, respectively,
(Barton et al. 1974). By these means, the rock mass quality is virtually increased and
safety is reduced to achieve a smaller amount of support.
⋅
Q and 1.5
⋅
Rock mass rating (RMR)
The RMR system originally introduced for tunneling (Bieniawski 1974) was later mod-
ifi ed and adapted several times with respect to other applications in rock engineering
(Bieniawski 1976, Bieniawski 1979, Bieniawski 1989).
The rock mass rating index RMR is defi ned as sum of the ratings of six input param-
eters. These are positive ratings R
1
to R
5
for the unconfi ned compressive strength
σ
cIR
of the intact rock, the RQD, the spacing and appearance of discontinuities and the
groundwater or joint water conditions, respectively. In addition, a negative rating R
6
was introduced to account for the orientation of discontinuities with respect to the
structure, depending on the type of structure (Fig. 12.5):
(12.3)
Figure 12.5
Rock mass rating RMR
(Bieniawski 1974)
Table 12.3
Rock mass classifi cation on the basis of the RMR (Bieniawski 1974)
RMR
Class
81 - 100
I (very good)
61 - 80
II (good)
41 - 60
III (fair)
21 - 40
IV (poor)
≤ 20
V (very poor)
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