In a detailed and valuable review paper, Windsor (2004) concluded that "the quality and performance of cable bolts used to stabilize transient production, non-entry, has increased over the past 20 years to the point where they are now an important part of modern mining practices Cable bolts have provided the industry with increased production, increased safety and increased flexibility in the extraction process.
However, with the development of a wider range of transportation and other larger mine openings, cable bolts are now also used to secure longer lifespan, excavating infrastructure. "Windsor (2004) recommends" that greater care and attention to detail be invested during the selection and installation of cable bolts for extracting mine infrastructure than is provided for extracting mine production ". He identified, in particular, the importance of geometry control, material quality, installation and testing of barrel and wedge fittings used as cable handles.
It is also important to realize that the use and effectiveness of rock and cable bolts in Australia's underground coal mines has grown rapidly in the past. Hebblewhite et al. (2004) show that significant trends over the past decade include:
- - longer use of bolts;
- - use of full partial and dominant polyester resin anchor bolts;
- - use of threaded bolt mounting systems;
- - the adoption of pre-tension bolts in a growing number of applications;
- - adoption of various grades of steel to achieve harder and stronger bolts; and
- - variations of defective bolt patterns and ribbing systems to improve belay and load transfer performance.
A problem that has long existed, but is often overlooked, is the corrosion resistance and longevity of stone bolts and cables. The initial installation of the Snowy Mountains which is generally considered to have pioneered the systematic use of rock bolting in Australia (eg Brown 1999b) is now more than 50 years old. Therefore, it is inevitable that this issue will consider the importance of this given by the paper presented to this symposium (eg Bertuzzi 2004, Hassell et al. 2004, Hebblewhite et al. 2004, Satola & Aromaa 2004, Windsor 2004). As noted by Hassell et al. (2004) and Potvin & Nedin (2004), long-term corrosion resistance from popular friction rock stabilizers, remains a problem. Corrosion protection is one of the advantages offered by fully encapsulated bolts and cables.
However, there are suggestions that cement installation alone does not provide long-term corrosion protection (eg 100 years) (Bertuzzi 2004). For long-term protection, two independent corrosion barriers are usually needed. Depending on the atmosphere and mineralogical conditions and groundwater in rock masses, corrosion can also affect surface equipment such as plates and nuts and bolts and cables themselves. Of course, galvanization provides protection to the steel underneath but does not have to be for long periods of time (Hassell et al. 2004, Windsor 2004).
Interestingly, in a 50 km detailed tunnel examination aged 35-40 years in the Snowy Mountains Scheme, Rosin & Sundaram (2003) found that lightweight hollow steel bolts that were fully cemented and hollow were in excellent condition, showing little evidence of corrosion. The around 5 mm layer of grout or protective bitumen applied to the screw and face plate seems to have worked very well. Carefully controlled installation and grouting are the prerequisites needed for achieving this performance (Windsor 2004).
With the increasing knowledge, experience and availability of a variety of analytical and numerical tools, bolt and stone installations are now being designed to increasingly demand operational conditions in the field of civil engineering and underground mining. However, the most successful installations are usually those whose performance is monitored by well-designed instrumentation systems as part of a systematic observation approach (eg Moosavi et al. 2004, Thibodeau 2004, Thin et al. 2004, Tyler & Werner 2004, Yumlu & Bawden , 2004).
Shotcrete
Over the past decade, more and more shotcrete has been used for support and control in the field of infrastructure, production and excavation of production in underground mines in Australia and elsewhere. Clements (2003) reports that nearly 100,000 m3 of shotcrete is applied annually in around 20 underground mines in Australia. Progress has been made in mixed design, testing, spraying and blending technologies that have been combined to increase the effectiveness of shotcrete. Wet-mixed fiber reinforced shotcrete is now an industry standard.
Of course, shotcrete has long been an important part of support systems and reinforcement in underground civil construction where its use has been established even for soils that are softer than those commonly found in underground mining (Kovari 2001). In underground mining, shotcrete is now used for good effects not only for infrastructure excavation, in weak soils (eg Yumlu & Bawden, 2004), for rehabilitation, and in heavy static or pseudo-static loading conditions (eg Tyler & Werner 2004), but as a component of support and reinforcement systems for dynamic or rockburst conditions (eg Li et al. 2003, 2004).
Toughness capacity or fiber-reinforced shotcrete absorption is very important in this application. The new toughness standard, the Round Determinate Panel test, has been developed in Australia and adopted in several other countries (Bernard 2000, 2003). The fiber-reinforced shotcrete performance measured in this test can vary greatly with its type (usually steel or structural synthetic fibers of polypropylene) and the dose of fiber used.
Mesh and sprayed liners
Another important change in the practice of support and strengthening in underground mining in recent years has been the increasing emphasis on mesh and sprayer liners of several types as the main soil control mechanism. Although, due to the large amount used and its importance as a supporting technique, shotcrete has been treated here as a special support category, but is often included with other techniques in the spray-on liners class (eg Spearing & Hague 2003). The overall subject of the mesh and sprayer liners has become so significant that it now has a series of international specialist meetings.
In some mining districts such as those in Western Australia and Ontario, Canada, mining regulations and codes of practice now require some form of surface support, usually mesh, to be used in all personnel entry excavations. In Western Australia, the Code of Practice applies to all posts higher than 3.5 m and requires that surface support be installed up to at least 3.5 m from the floor (Mines Occupational Safety and Health Advisory Board 1999). These provisions are part of the steps taken to understand and reduce the dangers of coral rock debris in Western Australia, and Australian underground mines (Lang & Stubley 2004, Potvin & Nedin 2004).