Engineering geology aspects for design and classification - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 51

Table 2-19 Evaluation of rock mass classes.

Rock mass class I II III IV V

Average free
stand-up time

self-
supporting
over 15 m for
20 years

self-
supporting
over 10 m for
1 year

self-
supporting
over 5 m for 1
week

self-
supporting
over 2.5 m for
10 hours

self-
supporting
over 1 m for
30 minutes

Cohesion of the
rock mass [kPa]

> 400 300 to 400 200 to 300 100 to 200 < 100

Friction angle of
the rock mass [°]

> 45 35 to 45 25 to 35 15 to 25 < 15

Strengths and limits. The RMR system is very simple to use; the classification parameters
can be gained from analysis of drill cores or from geomechanical records. This procedure
is applicable and adaptable to many situations in mining, for the stability of foundations
and slopes and in tunnelling. The geomechanical classification is very well suitable for use
in expert systems. On the other hand, the results of the RMR classification method tend to
be conservative, which mostly leads to an over-dimensioning of support measures. This
can be compensated by continuous monitoring during the construction period, with the
evaluation system being adapted to local conditions.

2.4.2.5 Relationship between Q and RMR systems
Working from over 100 cases studies, it proved possible to establish an originally unin-
tended, empirical relationship between the RMR and Q systems [23, 230, 22]. For tunnels,
this can be given as:

( RMR −44

9

)

RMR ≈ 9 · In Q + 44 or Q ≈ e

Barton sees the relationship as given by the following formula:

( RMR −50

50

)

RMR ≈ 15 · log Q + 50 or Q ≈ 10

The relationship between Q and RMR is also very well visible in Fig 2.8.

Figure 2-8 Relation-
ship between RMR and Q
systems.