Page 471 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

BLASTING AND TUNNELING 9.71

FIGURE 9.61 Roof bolts in a tunnel arch.

load of 22,500 pounds in regular strength, and 32,000 in high-strength steel. Lengths are 2 to 8
feet, in 6-inch steps.
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The bolt threads into the plug of an expansion shell that fits into a 1 ⁄ 8 -inch-diameter hole. Ears
on the bolt prevent it from sliding too far into the shell, so that tightening pulls the plug down into
the shell, expanding it against the sides of the hole. (See Fig. 9.63.)
Roof ties may be used to support the roof between bolts. Wire mesh can be used in addition to
the ties, or instead of them, where the problem is separation of small pieces.
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Flat or dished (reinforced) plates of ⁄ 8 - or ⁄ 4 -inch thickness and 6-inch diameter are used where
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ties are not needed. They are usually made with a 1 ⁄ 8 -inch hole. A hard steel washer prevents the
bolt head from pulling through.
The drilled hole must be at least as deep as the bolt is long, and may be deeper. The bolt is usu-
ally provided with a hard steel washer and assembled to the shell at the factory. The shell is
pushed through a plate and as far as it will go into the hole, and the bolt head is tightened with an
impact air wrench, usually to a torque of 150 to 200 pounds-feet.
The bolt head is held outside by the plate and washer, so the threaded plug is pulled outward
as it is tightened, squeezing the shell against and into the walls of the hole. The grip of the shell
in hard shale, sandstone, or limestone is usually greater than the breaking load of the bolt. In soft
shale, grip is usually less than bolt strength. In rotten rock, grip may not be adequate.