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Chapter 8
Rock Mass Quality Q-System
Genius is 99 per cent perspiration and 1 per cent inspiration.
Bernard Shaw
THE Q-SYSTEM
Barton, Lien, and Lunde (1974) at the Norwegian Geotechnical Institute (NGI) originally
proposed the Q-system of rock mass classification on the basis of approximately 200 case
histories of tunnels and caverns. They defined the rock mass quality (Q) by the following
causative factors:
Q ¼ ½RQD=Jn½Jr=Ja½Jw=SRF ð8:1aÞ
where RQD ¼ Deere’s Rock Quality Designation ! 10,
¼ 115 À 3:3 Jv 100 ð8:1bÞ
Jn ¼ joint set number, Jr ¼ joint roughness number for critically oriented joint set,
Ja ¼ joint alteration number for critically oriented joint set, Jw ¼ joint water reduction
factor, SRF ¼ stress reduction factor to consider in situ stresses and according to the
observed tunneling conditions, and Jv ¼ volumetric joint count per m3 (see Chapter 4
for details).
For various rock conditions, the ratings (numerical value) of these six parameters are
assigned. The six parameters given in Eq. (8.1a) are defined in the next section. The goal
of the Q-system is to characterize the rock mass and preliminary empirical design of the
support system for tunnels and caverns (see the section Design on Supports later in this
chapter). There are 1260 case records to prove the efficacy of this design approach; it is
the best classification system for tunnel supports (Kumar, 2002).
Rock Quality Designation
RQD is discussed in Chapter 6 and in more detail in Chapter 4. The RQD value in
percentage is also the rating of RQD for the Q-system. In a poor rock mass where RQD
is less than 10%, a minimum value of 10 should be used to evaluate Q (Table 8.1). If
the rock cores are unavailable, the RQD can be estimated by the volumetric joint count
(Jv) from Eq. (8.1b). The RQD estimated from Jv is usually conservative. The Jv is the
sum of frequencies of all joint sets per meter in a pit of 1 m  1 m  1 m.
Engineering Rock Mass Classification 85
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