Field Observations of Rock Bolts in High Stress Rock Masses

Excerpt

Rock bolting has been widely used for rock reinforcement in civil and mining engineering for a long period. Initially sporadic bolting was used to secure loose blocks in shallow underground openings. Bolts function as hanging elements to fasten loose blocks to stable formations behind. A simple demand, in this case, is that bolts have to be strong enough to sustain the dead weight of the unstable blocks. Therefore, the strength of the bolt is important for rock support in shallow locations. In rock masses at depth in which the in situ stresses are usually high, loose blocks seldom exist following excavations, but rather stress-induced rock failure occurs in many cases. In general, the rock bolts used in underground openings at depth are still the same types as those used in shallow openings. The main difference may be in the manner of bolting. Systematic, rather sporadic, bolting is used more than often today for rock support in deeply located openings. The loading condition of a rock bolt at depth is more complicated than a bolt for securing a loose block. It has been often experienced that conventional rock bolts do not provide satisfactory rock reinforcement because of their premature failure in high stress rock conditions. New types of rock bolts are desired in order to deal with the harsh rock conditions at depth. In order to figure out the desired performance of such rock bolts, it is necessary to have a good knowledge of how rock bolts behave in high stress rock conditions. In this article the loading conditions and the failure modes of rock bolts are presented through field observations. …