Civil Engineering Reference
In-Depth Information
view of the most common empirical classifi cation systems and their fi elds of applica-
tion. In the following, some widely used classifi cation systems are briefl y described.
Rock quality designation (RQD)
The “rock quality designation” RQD is the rock mass rating index of the earliest empir-
ical classifi cation system introduced by Deere et al. (1967). RQD was originally defi ned
as the percentage of intact rock lengths greater than 10 cm along any direction of a rock
exposure. By means of the RQD the rock mass is subdivided into fi ve classes, ranging
from “very poor” to “excellent” (Table 12.1).
Table 12.1
Rock mass classifi cation on the basis of the RQD (Deere et al. 1967)
RQD [%]
Class
< 25
very poor
25-50
poor
50-75
fair
75-90
good
90-100
excellent
For a drill core, RQD is the percentage of intact core pieces exhibiting a length of more
than 10 cm divided by the total length of the core:
(12.1)
The lengths of core pieces should be measured along the center line of the core (ISRM
1978e, Goodman 1993). Fractures that are caused by drilling, if recognizable, are to be
ignored when determining the lengths of core pieces.
The RQD was not intended to be a design method but in some cases is used as a input
parameter for later-developed more refi ned rock mass classifi cation systems such as the
subsequently described Q and RMR systems.
Q system
The Q system was developed for conventional tunneling (Barton et al. 1974) and later extended
to other applications in the fi eld of rock mechanics (Grimstad & Barton 1993, Barton 2002).
The rock mass rating index of the Q system denoted as the “rock quality index” is de-
fi ned as a product of three factors:
(12.2)
The fi rst factor, consisting of the ratio of RQD and the joint set number J
n
, considers the
infl uence of the discontinuity system and, according to Barton et al. (1974) and Grimstad
& Barton (1993), is a measure of rock block size. The second factor characterizes the
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