Field Observations of Rock Bolts in High Stress Rock Masses

Abstract

1 IntroductionRock bolting has been widely used for rock reinforcementin civil and mining engineering for a long period. Initiallysporadic bolting was used to secure loose blocks in shallowunderground openings. Bolts function as hanging elementsto fasten loose blocks to stable formations behind. A simpledemand, in this case, is that bolts have to be strong enoughto sustain the dead weight of the unstable blocks. There-fore, the strength of the bolt is important for rock support inshallow locations. In rock masses at depth in which the insitu stresses are usually high, loose blocks seldom existfollowing excavations, but rather stress-induced rock fail-ure occurs in many cases. In general, the rock bolts used inunderground openings at depth are still the same types asthose used in shallow openings. The main difference maybe in the manner of bolting. Systematic, rather sporadic,bolting is used more than often today for rock support indeeply located openings. The loading condition of a rockbolt at depth is more complicated than a bolt for securing aloose block. It has been often experienced that conven-tional rock bolts do not provide satisfactory rock rein-forcement because of their premature failure in high stressrock conditions. New types of rock bolts are desired inorder to deal with the harsh rock conditions at depth. Inorder to figure out the desired performance of such rockbolts, it is necessary to have a good knowledge of how rockbolts behave in high stress rock conditions. In this articlethe loading conditions and the failure modes of rock boltsare presented through field observations.The aim of the presentation of the field observations isto show the shortcomings of the conventional rock boltsand to figure out the features rock bolts should possess forrock support in high stress rock masses.2 Rock Bolts Observed in the FieldIt is an obvious fact that the in situ rock stresses becomehigh at depth. The consequence of the high in situ stressesis that rock fails either in the form of time-dependentdeformation (creeping) in weak and soft rocks, or in theform of dynamic (rock burst) or quasi-dynamic (slabbing)ejection in hard rocks. The failure mode of rock has a directimpact on the design of ground support systems.The field observations presented in this paper wereconducted in cut-and-fill mines. With the cut-and-fillmining method, stopes are mined in 3–5 m high slicesalong the strike of the ore body by breasting using drilljumbos. After a slice has been mined out it is backfilledwith waste rock and tailing. Because of the nature of themining method, the roof rock of a previous slice cut will beexposed on the advance face of the current slice cut.Therefore, the cut-and-fill mining method provides aunique opportunity to observe the interaction between boltsand the rock in situ. The observations on the advance faceof mine stopes will reveal the loading conditions as well asthe failure modes of rock bolts in high stress rockconditions.Figure 1 shows a part of an advance face in a cut-and-fillmine stope. A number of sub-horizontal stress fracturestogether with a rock bolt are exposed on the face. The boltwas one of the roof bolts that had been installed in theprevious slice of the stope. The fractures have a domedform with the dome peak at the position of the bolt. This