Rock
bolt
A rock bolt is a long anchor bolt, for stabilizing rock excavations, which may be
used in 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.[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. There are also fiberglassbolts which can be cut through again by subsequent
excavation. Many papers have been written on methods of 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
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.
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