With the plethora of cable bolts on the market, geotechnical engineers can be baffled by the differences in performance claims from the manufacturers but perhaps more importantly they are sometimes at a loss in making an optimal selection matching behaviour with a particular site condition. To this end a new testing methodology has been developed that can cater for the varying types of cable bolt behaviours. The methodology is based on the laboratory short encapsulation pull test (LSEPT) as recommended in the British Standard and overcomes several deficiencies such as cable rotation. Of most significance however, is its inability to measure the post-peak residual performance of high load transfer capacity cables and consequently, there is an inference that these cables may be susceptible to sudden anchorage failure with relatively little yield. This paper details the results of anchorage performance tests of a range of cable bolts and the differences in performance with rock strength and drill hole diameter.
A research project was undertaken to develop a new laboratory-scale test apparatus to study the axial performance of fully grouted cable bolts that are used in the Australian underground coal mining industry. Work by Thomas (2012) and others in recent years found there were a number of deficiencies in the testing methodologies making it difficult to determine and compare the performance of the dozen or more different types of cable bolts on the market. This has meant that geotechnical practioners have been hamstrung in optimizing the design of ground support systems in underground environments that are increasingly more arduous.
A cable bolt is a flexible tendon consisting of a quantity of wound wires that are grouted in boreholes at defined distances between holes in order to provide ground reinforcement of excavations (Hutchinson & Diederichs, 1996). They were initially introduced into the underground hard rock mining industry in the 1960s (Thorne & Muller, 1964) and since the early 1970s have been brought to coal mining operations.
Originally, cables were only used as a temporary reinforcement element. One reason for this being many earlier of the earlier cables were made from discarded steel ropes that had very poor load transfer properties due to their smooth surface profile, lacking the equivalent to ribs found on rockbolts. Over subsequent years a number of modifications have been made to the basic plain strand cable, such as buttoned strand (Schmuck, 1979), double plain strand (Matthews, Tillmann & Worotnicki, 1983), epoxycoated strand (Dorsten, Hunt & Kent, 1984), fiberglass cable bolt (Mah, 1990), birdcaged strand (Hutchins et al, 1990), bulbed strand (Garford, 1990), and nutcaged strand cable bolts (Hyett et al, 1993). These changes to the cable surface geometry have been undertaken in an effort to improve the load transfer efficiency and anchorage capacity that has resulted in the more widespread use of cable bolts for permanent reinforcement.