Pseudo-Static Support and Reinforcement Systems
It is perhaps remarkable to find that, although rock and cable bolts have been used in underground mining and construction for several decades (if not more than 100 years in the case of rock bolts), bolt elements and bolting systems continue to evolve and improve. The papers presented to this symposium detail advances made in fully encapsulated resin and cement grouted bolts (Mikula 2004, Mould et al. 2004, Neindorf 2004), one pass mechanized bolting (Mikula 2004, Neindorf 2004) and bulbed cables (Yumlu & Bawden 2004), for example.
The developments in ground support practices that have accompanied greater productivity, larger excavations and larger equipment are especially well-illustrated in the paper by Neindorf (2004) describing the evolution of ground support practices at the Mount Isa mine over the past 30 years.
In a detailed and valuable review paper, Windsor (2004) concludes that “the quality and performance of cable bolts used to stabilise temporary, non-entry, production excavations have improved over the last 20 years to the point where they are now an essential part of modern mining practice. Cable bolts have provided the industry with increased production, increased safety and increased flexibility in the extraction process.
However, with the development of wider span haulage and other larger mine openings, cable bolts are now also used to secure longer life, infrastructure excavations.” Windsor (2004) recommends “that greater care and attention to detail be invested during selection and installation of cable bolts for mine infrastructure excavations than that given to mine production excavations”. He identifies, in particular, the importance of the control of the geometry, material quality, installation and testing of the barrel and wedge fittings used as cable grips.
It is also important to recognize that the use and effectiveness of rock and cable bolts in Australia’s underground coal mines have developed considerably in the recent past. Hebblewhite et al. (2004) suggest that the significant trends over the last decade have included:
However, there are suggestions that cement grouting alone does not provide long-term (e.g. 100 year) corrosion protection (Bertuzzi 2004). For long-term protection, two independent corrosion barriers are usually required. Depending on the atmosphere and the mineralogy and groundwater conditions in the rock mass, corrosion may also affect surface fixtures such as plates and nuts as well as the bolts and cables themselves. Of course, galvanizing provides protection to the steel underneath but not necessarily for long periods of time (Hassell et al. 2004, Windsor 2004).
Interestingly, in a detailed inspection of 50 km of 35–40 year old tunnels in the Snowy Mountains Scheme, Rosin & Sundaram (2003) found the mainly fully cement grouted, hollow core mild steel bolts to be in excellent condition, showing little evidence of corrosion. An approximately 5 mm protective grout or bitumen coating applied to the bolt threads and face plates appeared to have worked very well. Carefully controlled installation and grouting is a necessary pre-condition for the achievement of such performance (Windsor 2004).
With increasing knowledge, experience and the availability of a range of analytical and numerical tools, rock and cable bolt installations are now being designed for increasingly demanding operational conditions in both civil engineering and underground mining. However, the most successful installations are usually those whose performance is monitored by a well-designed instrumentation system as part of a systematic observational approach (e.g. Moosavi et al. 2004, Thibodeau 2004, Thin et al. 2004, Tyler & Werner 2004, Yumlu & Bawden, 2004).
Oer the last decade, increasing use has been made of shotcrete for ground support and control in infrastructure, development and production excavations in underground mines in Australia and elsewhere. Clements (2003) reports that nearly 100,000 m3 of shotcrete is applied annually in some 20 underground mines in Australia. Advances have been made in mix design, testing, spraying technology and admixtures which have combined to improve the effectiveness of shotcrete. Wet-mix fibre-reinforced shotcrete is now the industry standard.
Of course, shotcrete has long been an essential part of support and reinforcement systems in underground civil construction where its use is well-established even for softer ground than that commonly met in underground mining (Kovari 2001). In underground mining, shotcrete is now used to good effect not only for infrastructure excavations, in weak ground (e.g. Yumlu & Bawden, 2004), for rehabilitation, and in heavy static or pseudo-static loading conditions (e.g. Tyler & Werner 2004), but as a component of support and reinforcement systems for dynamic or rockburst conditions (e.g. Li et al. 2003, 2004).
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