Page 166 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

134 Engineering Rock Mass Classification

site, although rock properties in many cases are downgraded by joints. Keep in mind
that the properties of rocks have a profound influence on the formation and development
of joints. Petrological data can make an important contribution toward the prediction of
mechanical performance, provided that one looks beyond the rock names at the
observations on which they are based (Franklin, Broch, & Walton, 1970). Therefore,
it is important to retain the names for the different rock types because they show relative
indications of their inherent properties (Piteau, 1970).

These considerations and the study of more than 15 different classification systems
have been used by Palmstrom (1995) when selecting the following input parameters
for RMi:

1. Size of the blocks delineated by joints—measured as block volume, Vb
2. Strength of the block material—measured as UCS, qc
3. Shear strength of the block faces—characterized by factors for the joint char-

acteristics, jR and jA (Tables 10.1 and 10.3)
4. Size and termination of the joints—given as their length and continuity factor, jL

(Table 10.2)

CALIBRATION OF RMi FROM KNOWN ROCK MASS
STRENGTH DATA

It is practically impossible to carry out triaxial or shear tests on rock masses at a scale
that is the same size as the underground excavations (Hoek & Brown, 1988). As the
RMi is meant to express the compressive strength of a rock mass, a calibration of
the same is necessary.

The UCS of intact rock, qc, is defined and can be determined within a reasonable
accuracy. The jointing parameter (JP), however, is a combined parameter made up

TABLE 10.1 The Joint Roughness Found from Smoothness and Waviness

Small-scale Large-scale waviness of joint plane
smoothness* of
joint surface

(The ratings in bold are Slightly Strongly Stepped or
similar to Jr in the
Q-system) Planar undulating Undulating undulating interlocking

Very rough 23 4 66

Rough 1.5 2 3 4.5 6

Smooth 1 1.5 2 3 4

Polished or slickensided* 0.5 1 1.5 2 3

For filled joints: jR ¼ 1; for irregular joints a rating of jR ¼ 6 is suggested

*For slickensided surfaces the ratings given cover possible movement along the lineation. (For movements
across lineation, a rough or very rough rating should be applied for the surface.)

Source: Palmstrom, 2000.