Engineering geology aspects for design and classification - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 49

Classification procedure. The following six parameters are used for the rock mass clas-
sification in the RMR system:
1. Uniaxial compression strength of the rock material.
2. Determination of the rock mass quality ( RQD ).
3. Discontinuity spacing.
4. Condition of the interfaces.
5. Water ingress.
6. Discontinuity orientation.
In order to used the geomechanical classification, the rock mass is split into sections, in
which the condition of the rock has nearly the same properties. Although the rock mass as
a natural material is not homogeneous, individual sections can be delineated according to
the already mentioned aspects and used for the investigation (homogeneous sections). The
characteristic properties of section are entered in a data sheet and evaluated with the aid
of Table 2-16 and Table 2-17. It is important that Table 2-16 can be used independently of
the orientation of any faults and the results are then corrected using Table 2-17 for their
orientation and for the structure to be constructed.

Table 2-16 Classification parameters and their evaluation.

Parameter Range of values

1 Strength of

the intact
rock (Mpa)

Point
load
strength
index

> 10 4 - 10 2 - 4 1 - 2 -

Uniaxial
compres-
sion
strength

> 250 100 - 250 50 -100 25 - 50 5 - 25 1 - 5 < 1

Rating 15 12 7 4 2 1 0

2 Drill core

quality RQD
[%]

90 - 100 75 - 90 50 - 75 25 - 50 < 25

Rating 20 17 13 8 3

3 Spacing of

discontinui-
ties

> 2 0.6 - 2 0.2 - 0.6 0.06 - 0.2 < 0.06

Rating 20 15 10 8 5

4 Condition

of disconti-
nuities

very rough
surface, not
continuous,
no separa-
tion, un-
weathered
wall rock

slightly
rough
surface,
separa-
tions
< 1 mm,
weath-
ered walls

slightly
rough
surface,
separa-
tions
< 1 mm,
highly
weathered
walls

slickenside
surface or
slip < 5 mm
or sepa-
rations
1 - 5 mm
continuous

soft slips > 5 mm or

separations > 5 mm
continuous

Rating 30 25 20 10 0