Collar fittings such as bearing plates, domed or cupped washers, B&W anchors and nuts are an integral part of the reinforcing system especially where point anchoring is employed. When a cable is point anchored the full load is taken by the collar fittings, unlike a full column grouted bolt where the load transfer occurs along the length of the bolt.
There has been much discussion about the ability of the B&W anchor to take the tensile capacity of a cable strand. The earlier two-leaf, flat B&W anchor has now largely been replaced by a longer, domed, three leaf variety that has a greater hold on the cable. However, most manufacturers agree that the B&W anchor does not allow for the full strength of the cable to be achieved with anchor strengths of 75-90% of cable strength being obtained.
The load transfer from the leaves of the B&W anchor to the outer wires of the cable and on to the inner wires is also an issue. Instances of inner wires, particularly the “king wire”, receding into the cable have occurred.
A loss of 50-80% in the pretension being imparted to a cable occurs during the “bedding in” of the leaves of the B&W anchor after the load is released from the ram. This is exacerbated by wedge misalignment if the cable is not installed at right angles to the face of the rock. DSIArnall claim to have reduced this loss to 20% by having a centralising rim on their barrel. The advantage of the B&W anchor is that it is simple, relatively cheap and easy to use.
In the case of the Megabolt, the bolt head is attached to the cable by forged button heads on the end of each wire. These forged heads are at least as strong as the wires. The nut and thread of the head assembly is rated above that of the cable. The collar fittings of the Megabolt allow for the full strength of the cable to be achieved. This type of arrangement means that the Megabolt collar fittings are a more expensive than the simple B&W anchor, however, the pay off is improved performance.
Probably the biggest impact on the performance of cable tendons is the quality of the workmanship during installation. It is important that people doing the installation are properly trained in all aspects. That is, installation, tensioning and grouting, and understanding why they are doing the work. After all, once ground support is installed we all work under it believing we are safe. Roof falls not only endanger life, but severely disrupt production.
* Ron McKenzie (email@example.com) is a mining engineer who has specialised in ground support for the past 16 years
Originally published in the March 2001 edition of Australia's Longwalls.