A rock bolt is a long anchor bolt

Anchor bolt

An anchor bolt is used to attach objects or structures to concrete. There are many types of anchor bolts, consisting of designs that are mostly proprietary to the manufacturing companies. All consist of a threaded end, to which a nut and washer can be attached for the external load...

, 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.

Rock bolts were first used in mining starting in the 1890s, with systematic use documented at the St Joseph Lead Mine in the US in the 1920s. Rock bolts were applied to civil tunneling support in the US and in Australia, starting in the late 40s. Rock bolts were used and further developed, starting in 1947, by Australian engineers who began experimenting with four metre long expanding anchor rock bolts while working on the Snowy Mountains Scheme

Snowy Mountains Scheme

The Snowy Mountains scheme is a hydroelectricity and irrigation complex in south-east Australia. It consists of sixteen major dams; seven power stations; a pumping station; and 225 kilometres of tunnels, pipelines and aqueducts and was constructed between 1949 and 1974. The Chief engineer was Sir...

.

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. Rock bolts are an essential component of the New Austrian Tunneling method. As with anchor bolts, there are many types of proprietary

Property

Property is any physical or intangible entity that is owned by a person or jointly by a group of people or a legal entity like a corporation...

 rock bolt designs, with either a mechanical or epoxy

Epoxy

Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....

 means of establishing the set. There are also fiberglass

Fiberglass

Glass fiber is a material consisting of numerous extremely fine fibers of glass.Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the invention of finer machine tooling...

 bolts which can be cut through again by subsequent excavation. Many papers have been written on methods of rock bolt design.

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 may be 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.