Engineering geology aspects for design and classification - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 43

The value of Q (= rock quality) calculated according to this formula lies in a range from Q
= 0.001 to Q = 1000. The value for Q is entered in a diagram with a logarithmic scale and
the rock classes can be read off (Fig. 2-7).

Figure 2-7 Q System: Reference value for the support in comparison to the rock quality Q.

Classification procedure: Table 2-9 to Table 2-14 give the numerical values for each of
the factors listed above. They are interpreted as follows:
The first two factors, RQD and J n (joint set number), give an overview of the structure of
the rock mass and their quotient as a relative dimension for the joint body size in the drill
core. The RQD Index proposed as a rock characteristic by D. U. Deere and A. J. Hendron
[40] is used. It gives the ratio L 10 / L in percent, where L 10 denotes the number of pieces
more than 10 cm long contained in core length L .

Table 2-9 Q System: Determination of the rock quality - RQD [14, 22].

Description Range of values RQD Remark

very poor
poor
fair
good
excellent

0 to 25
25 to 50
50 to 75
75 to 90
90 to 100

(i) In case the measured value of RQD is less than
10, the value of RQD = 10 can be used in the last
formula.
(ii) The grading of the values of RQD in intervals of 5
is sufficient for further consideration.