ROCK BOLT

in tunnelling and underground mining, steel rod inserted in a hole drilled in the face of a rock formation to support the roof or sides of the cavity. The bolt may be provided with an expanding device to grip the rock at the far end of the hole or may be bonded to the rock by means of expanding cement. Rock bolts may be used singly or in rows and are increasingly supplanting timber as mine supports because they are more economical and easier to use and allow larger unobstructed areas; like timber, they can be used in any type of excavation.