There are many examples
(very few of which are documented) where shotcrete has been used as a
last act of desperation in an effort to stabilize the failing rock
around a tunnel and, to most people's surprise, it has worked. The
complex interaction between the failing rock mass around an
underground opening, and a layer of shotcrete of varying thickness
with properties which change as it hardens, defies most attempts at
theoretical analysis. It is only in recent years, with the
development of powerful numerical tools, that it has been possible to
contemplate realistic analyses, which will explore the possible
support-interaction behaviour of shotcrete. A clear understanding of
shotcrete behaviour will require many more years of experience in the
use of and in the interpretation of the results obtained from these
programs. It is also important to recognize that shotcrete is very
seldom alone and its use in combination with rockbolts, cablebolts,
lattice girders, or steel sets further complicates the problem of
analyzing its contribution to support.
Figure 1: Rockbolt
Figure 2: Cablebolt
Figure 3: Lattice Girders
Current shotcrete 'design' methodology
relies very heavily upon rules of thumbs and precedent experience.
Wickham et al (1972) related the thickness of a shotcrete tunnel
lining to their Rock Structure Rating (RSR). Bieniawski (1989) gave
recommendations on shotcrete thickness (in conjunction with rockbolts
or steel sets) for different Rock Mass Ratings (RMR) for a 10 m span
opening. Grimstad and Barton (1993) have published an updated
relating different support systems, including shotcrete and fibre
reinforced shotcrete, to the Tunneling Quality Index Q. Vandewalle
(1993) collected various rules of thumb from a variety of sources and
included them in his monograph.
Table 1, Table 2, Table 3 and Table 4 are a compilation of current
shotcrete practice, combining all of empirical rules. The reader is warned that this table
can only be used as an approximate guide when deciding upon the type
and thickness of shotcrete to be applied in a specific application.
Modifications will almost certainly be required to deal with local
variations in rock conditions and shotcrete quality.
Table 1: Summary of Recommended Shotcrete Application in Underground Mining
Table 2: Summary of Recommended Shotcrete Application in Underground Mining
(continuing Table 1)
Table 3: Summary of Recommended Shotcrete Application in Underground Mining
(continuing Table 1 and Table 2)
(continuing Table 1 and Table 2)
Table 4: Summary of Recommended Shotcrete Application in Underground Mining
(continuing Table 1,Table 2 and Table 3)
(continuing Table 1,Table 2 and Table 3)
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