Cable bolt application in mining
For several decades now, cable bolting systems have been used for ground stabilisation and reinforcement in mines. There are around 19 types of cable bolts used in Australian mines, classified into main five categories. These are; (a) smooth or plain surface cable bolt; b) Bulbed; c) Nut caged; d) Spiral and indented cable bolts; e) a mix of plain and spiral cable bolts. Figure 1 above shows a selection of cable bolt types used in Australian underground coal mines.
Most cable bolts used in Australian mines are of seven or 19 wire construction. The seven wire cables has six outer wires wrapped around the central core wire, which is known as centre or king wire. However, the 19 wire cable has two layers of wires consisting of nine 5 mm diameter outer wires; nine 3 mm inner layer wires, all wrapped or laid around 7 mm inner or king wire. The lay of the cable can be determined by the direction in which the outer strands of the cable are wrapped or laid around the centre king wire. The king-wire can be slightly larger in diameter to ensure the outer wires contact each other. For further information on the cable components and construction methodology, the reader is referred to Hutchinson and Diederichs ( 1996)[CLICK], [http://icgcm.conferenceacademy.com/papers/detail.aspx?subdomain=icgcm&iid=965].
For a cable bolt support system to be effective, the loads have to be successfully transferred from the rock to the cable through the grouting materials. These axial forces can be applied via the bearing plate, or as a result of horizontal movement of the rock mass at shear planes and bed separations. Thus the anchorage applied at the top of the hole, can be enhanced by buttons, opening the strand- called birdcages or bulbs.
Cable bolts are normally evaluated for strength and load transfer properties. The strength of cable can be carried out by tensile failure tests, while the load transfer strength is evaluated by pull and shear strength tests as well as cable stiffness.
The pull test and / or ultimate tensile strength test is carried out using a double embedment pull testing method in accordance with the British Standard BS7861: Part 2: 2009. [Specification for Flexible systems for roof reinforcement]
Failure of a grouted anchorage by load transfer mechanism will occur either by; failure at the cable-to-grout interface; failure at the grout-to-rock interface; failure through the grout column; or by failure through the rock around the borehole wall. Thomas (2012) [Click] [http://icgcm.conferenceacademy.com/papers/detail.aspx?subdomain=icgcm&iid=1011][Click]. documented a total of 19 cable bolts, which were subjected for load transfer studies by pull testing.
The tested cable included, plain-strand, bulbed, nut-caged and spiral or indented cables (see opposite). Figure 2 (a,b) opposite shows load-displacement results of various cables. Each tested cable ends are encapsulated in steel tubes and subjected to pulling. Alternatively the cable end can be encapsulated in concrete cylinders in the laboratory or in overhead concrete blocks as show if Figure 2c. Alternatively tests can also be carried out underground in what is now known short encapsulated pull test (SEPT).