Rock bolt

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A rock bolt is a long anchor bolt, for stabilizing rock excavations, which may be tunnels or rock cuts. It transfers load from the unstable exterior, to the confined (and much stronger) interior of the rock mass.

The device was invented in 1947 by Australian engineers who began experimenting with four metre long expanding anchor rock bolts while working on the Snowy Mountains scheme. Rock bolts had previously been used by coal miners in the 1900s but only as a temporary measure. The device perfected by the engineers was never patented.[1]

As shown in the figure, rock bolts are almost always installed in a pattern, the design of which depends on the Rock Quality Designation, and the type of excavation [2]. Rock bolts are an essential component of the New Austrian Tunneling method. As with anchor bolts, there are many types of proprietary rock bolt designs, with either a mechanical or epoxy means of establishing the set. As well, there are fiberglass bolts which can be cut through again by subsequent excavation. Many papers have been written on proper rock bolt design [3]

Rock bolts work by 'knitting' the rock mass together sufficiently before it can move enough to loosen and fail by unravelling (piece by piece). As in the photo, rock bolts are used to support wire mesh, but this is usually a small part of their function. Unlike common anchor bolts, rock bolts can become 'seized' throughout their length by small shears in the rock mass, so they are not fully dependent on their pull-out strength. This has become an item of controversy in the Big Dig project, which used the much lighter pull-out tests for rock bolts, rather than the proper tests for concrete anchor bolts.