1.2.1 History of rock bolts
The first use of tendons in rock reinforcement in the United Kingdom started in 1872 in a slate quarry located in North Wales (Schach et al., 1979). The use of rock bolts in a coal mine in Germany goes back to 1918, whereby the rock bolts were made of wood to prevent small rock pieces from falling into the excavation (Lang et al., 1979). In the late 1940’s, Norwegians developed rock bolting as a practical and economical technology (Bolstad and Hill, 1983).
Between 1949 and 1969, the Australian mining industry started to use rock bolting technology in the Snowy Mountains hydroelectric Scheme (Bolstad and Hill 1983). Gardner (1971) reported the use of timber support roof bolting in Australia in Elrington Colliery, New South Wales. Nowadays, roof bolting application is wide spread in Australia in almost every mine and tunnel construction. The annual usage of tendons in Australia is in the order of 7 M, consisting mainly of solid rebar bolts. Other bolts in use include split set, yieldable bolts and cable bolts. Solid bolts are normally anchored in rock using chemical resins, while cementitious grouts are largely associated with cable bolt encapsulation installation.
The installation of mechanical rock bolts in a metal mine in the United States began in 1972. The US Bureau of Mines (USBM) started roof bolting technology to decrease the number of fatal accidents in the mines. Tendon technology particularly rock bolting technology has been implemented in nearly all underground coal mines in the United States. 500 million rock bolts were installed all over the world in 2011. Among this, the
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number of used bolts, cable bolts and split sets in Australia was 5,000,000, 600,000 and 250,000 respectively. 100 million roof bolts were used in the US in 2011 (Peng, 1986) and 66% of that were fully grouted resin bolts (Chao, 2012). Figure 1-1 shows the details of bolt usage in US in 2011.
Chapter I Introduction
4 1.2.2 History of cable bolts
The application of cable bolts as a secondary support system has been a growing trend in underground coal mines worldwide (Fuller and O’Grady, 1993). The use of cable bolts as a long fully grouted steel element with high tensile strength started in the mid- 1960. The Willroy mine in Canada and the Free State Geduld Mines Ltd in South Africa were the first mines to use cable bolts. The prestressed wires were installed in long flexible lengths to provide deep anchorage in the rock mass (Windsor, 1992).
The first type of cable bolts consisted of seven smooth, prestressed, high tensile 7 mm diameter wires, which were arranged with plastic spacers. The plain strand cables of poor load transfer properties due to smooth and straight profile wires were initially introduced to mines as a temporary means of rock reinforcement. Over the years, a number of modifications have been introduced to the plain strand cable, such as strands surface profiling and indentations (Schmuck, 1979), double plain strand (Matthews et
al., 1983), epoxy-coated strand (Dorsten, 1984), fibreglass cable bolt (Mah, 1990),
birdcage strand (Hutchins et al., 1990), bulbed strand (Garford, 1990), and nutcage strand cable bolts (Hyett et al., 1993). These various types of cable bolt have been incorporated to improve the load transfer capacity as permanent ground reinforcement.
In the early 1970’s in Broken Hill, Australia, indented strand cable bolts were introduced for the first time. This change in the profile section from plain to indented cable bolts was a great success in improving productivity, adaptability, mechanical performance and higher load transfer ability. These new types of cable bolts have the ability to be pushed into the 30 metres holes because of their rigidity. The most popular cable bolt in the world is still the 7-wire, 15.2 mm nominal diameter strand. The central strand is called as “kingwire”, which is straight and the six others are slightly smaller bordering wires (Windsor, 1992).
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In the US in 2016, 1.32 million 15 mm diameter cable bolts and 25,000 18 mm vertical cable bolts were sold. The 15 mm strand cable bolt has the nominal strength of 27.22 t however the 18 mm strand cable bolt has nominal strength of 36.29 t. These two types of cable bolts are both 7 strand (King-wire and 6 outer wrap wires) (Tadolini, 2016).
In the late 1970’s and 1980’s, cable bolt technology was applied to the support and reinforcement of larger open stope spans in cut and fill mining. The reason for using cable bolts was to increase the length of support in poor ground conditions and to provide support in the early stage of mining excavations. Also, one of the main aspects of success in using cable bolts in cut and fill mining was the ability to pre-reinforcing the strata before excavation to stop any damage due to the stress re-distribution effects of mining.
Then, by 1981, the successful experiment of two revised designs including strand with additional anchors were trialled for the post-rock mass yield reinforcement of highly stressed crown pillars at Broken Hill in Australia. These anchors contained double- acting, barrel and wedge anchors or rectangular, steel swages and the intermissions between anchors were debonded by using polyethylene tubes or simply by painting them. Then, in 1983 at Mt Isa mines in Australia, the birdcage strand was introduced for cut and fill mining which had excellent load transfer characteristics. ‘Bulbed’ strand and ‘Ferruled’ strand are other types of the strands that were developed a few years later (Fuller, 1983; Windsor, 1992).