Design Methods - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 337

discontinuity properties by the joint roughness number J r and the joint alteration number
J a that describe the appearance of the discontinuity surfaces. The rating of J a includes an
estimation of the residual friction angle of the discontinuities. According to Barton et al.
(1974) and Grimstad & Barton (1993), the second quotient in (12.2) serves as a measure
of shear strength between blocks of intact rock. The third factor accounts for the effects
of groundwater, weak zones and in-situ stresses, which are described by the joint water
reduction factor J w and the stress reduction factor SRF. The numerical value of Q calcu-
lated according to (12.2) can take on values from 0.001 to 1000. This range is subdivided
into nine classes, according to Table 12.2 (Barton et al. 1974).

Table 12.2 Rock mass classification on the basis of the Q value (Barton et al. 1974)

Q Class

0.001 - 0.01 exceptionally poor

0.01 - 0.1 extremely poor

0.1 - 1 very poor

1 - 4 poor

4 - 10 fair

10 - 40 good

40 - 100 very good

100 - 400 extraordinarily good

400 - 1000 exceptionally good

It should be noted that the Q system takes into account neither orientations of the discon-
tinuities nor deformability, strength or permeability of the rock mass and their potential
anisotropy.
The diagram in Fig. 12.3 represents the first support chart according to the Q system,
which is based on 212 case histories of projects located in igneous, metamorphic and
sedimentary rocks (Barton et al. 1974). The more recent support chart from Barton &
Grimstad (1993) is based on 1050 case histories. However, the latter refers to projects that
are located in prevailing hard, massive, magmatic and metamorphic rocks in Scandinavia,
which are usually quite competent and not considered as very difficult with regard to tun-
neling. Furthermore, the basis of the chart of 1993 is not sufficiently documented, and
the selected support classes are formulated in terms of fiber-reinforced shotcrete that is
commercially used in Scandinavia but not in Germany and many other countries. In these
latter countries mesh-reinforced shotcrete is prevailingly used, on which the 1974 chart
is based. For these reasons the chart from 1974, instead of the updated chart from 1993,
is considered as standard for Q system support recommendations that are relevant for
engineers who are not working in Scandinavia. The 1993 chart is considered as not rep-
resentative for rocks located outside of Scandinavia, particularly for sedimentary rocks.

Using the chart from 1974 for underground openings, a support class is specified with the
aid of the input parameters Q and D e (Fig. 12.3). The so-called “equivalent dimension”
D e is the ratio of the relevant dimension of the excavation - the diameter, height or span
of the cross-sectional area - and the so-called “excavation support ratio” (ESR). ESR rep-
resents an inverted safety factor, that is, a higher value corresponds to a lower safety level
and vice versa, and is dependent on the purpose of the underground structure (Fig. 12.4).