Selasa, 02 Desember 2014

DESIGN OF SHOTCRETE SUPPORT IN UNDERGROUND MINING

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)
Table 4: Summary of Recommended Shotcrete Application in Underground Mining
(continuing Table 1,Table 2 and Table 3)

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