Geoscience Reference
In-Depth Information
•
Methods developed from the need to provide on site assessment empirical design
of ground support based on the exposed ground conditions.
•
Relationships exist between the various methods.
•
Only the 2 main classification systems in use are discussed further. These are the
Q and RMR Systems.
18.2 Rock mass rating system - RMR
•
The classes provided in the table below are the final output. The derivation of that
rating is provided in the subsequent tables.
•
This RMR class provides the basis for strength assessment and support
requirements.
Table 18.2
Rock mass classes (Bieniawski, 1989).
RMR class no.
Description
Rating
I
Very good rock
100-81
II
Good rock
80-61
III
Fair rock
60-41
IV
Poor rock
40-21
V
Very poor rock
20
<
18.3 RMR system - strength and RQD
•
The strength is assessed in terms of both the UCS and Point Load index strengths.
A conversion of 25 is assumed, however this relationship can vary significantly
for near surface and soft rock. Refer Chapter 6.
•
<
>
The RQD use the standard classification of poor (
25%) to excellent (
90%).
Table 18.3
Effect of strength and RQD (Bieniawski, 1989).
Parameter
Range of values
Strength of
Point - Load
>
10 MPa intact
4-10
2-4
1-2
For this low range - UCS
rock
strength
preferred
index, MPa
Uniaxial
250 MPa
100-250
50-100
25-50
5-25
1-5
1
>
<
compressive
strength
(UCS), MPa
Rating
15
10
7
4
2
1
0
Drill core quality RQD, %
90-100
75-90
50-75
25-50
25
<
Rating
20
17
13
8
3
18.4 RMR system - discontinuities
•
The discontinuity rating shows it to be the most more important parameter in
evaluating the rock rating.
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