Engineering Geology

Chapter 9
Table 9.5. Guide for the selection of primary support at shallow depth, size 5–15 m,
construction by drilling and blasting (after Bieniawski, 1974). With kind permission of Balkema
Alternative support systems
Rock Mainly rock bolts (20 mm
mass diameter, length half of
class tunnel width, resin bonded) Mainly shotcrete Mainly steel ribs
I Generally no support required
II Rock bolts spaced 1.5–2 m, Shotcrete 50 mm Uneconomic
plus occasional
in crown.
wire mesh
III Rock bolts spaced 1.0–1.5 m Shotcrete 100 mm in Light sets spaced
plus wire mesh crown and 50 mm on 1.5–2 m
and 30 mm shotcrete in sides, plus occasional
crown where required wire mesh and rock
bolts where required
IV Rock bolts spaced 0.5–1.0 m Shotcrete 150 mm in Medium sets spaced
plus wire mesh crown and 100 mm on 0.7–1.5 m plus
and 30–50 mm shotcrete sides plus wire mesh 50 mm shotcrete in
in crown and sides and rock bolts, 3 m crown and sides.
long and spaced 1.5 m
V Not recommended Shotcrete 200 mm in Heavy sets spaced 0.7 m
crown and 150 mm on with lagging. Shotcrete
sides plus wire mesh, 80 mm thick to be
rock bolts and light applied immediately
steel sets. Seal face after blasting
close invert
primary or temporary support. Their purpose is to ensure tunnel stability until the secondary
or permanent support system, for example, a concrete lining, is installed. The form of primary
support depends on depth below the surface, tunnel size and shape, and method of excavation.
Table 9.5 indicates the primary support measures for shallow tunnels 5–12 m in diameter driven
by drilling and blasting.
Barton et al. (1975) pointed out that Bieniawski (1974), in his analysis of tunnel support, more
or less ignored the roughness of joints, the frictional strength of the joint fillings and the rock
load. They, therefore, proposed the concept of rock mass quality, Q, which could be used as
a means of rock classification for tunnel support (Table 9.6). They defined the rock mass
quality in terms of six parameters:
1. The RQD or an equivalent estimate of joint density.
2. The number of joint sets, J n , which is an important indication of the degree of freedom
of a rock mass. The RQD and the number of joint sets provide a crude measure of
relative block size.
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