ABSTRACT
In highly stressed rock masses, rock failure is an unavoidable matter after excavation. In certain circumstances, rockburst events occur, causing casualties and equipment damage. In shallow tunnels where rock stresses are low, the main objective of rock support is to stabilize the loosened rock blocks after excavation. The task of support elements, such as rock bolts and steel sets, is to prevent the loosened blocks from falling. This means that in shallow locations, the loading condition for the support element is the dead weight of the blocks. In this case, the strength of the bolts has to be larger than the dead weight of the potentially falling blocks. In other words, strong rock bolts have to be used in shallow locations. This obeys the principle of structural mechanics which states that a structure fails when it yields. In highly stressed rock masses, rock blocks seldom become loosened in underground openings. Instead, rock fails because of elevated stresses that are beyond the strength of the rock. In this case, the loading for the support system is no longer dead-weight controlled, but rather a displacement controlled process. The well-known Ground Response Curve (GRC) describes such a loading process (Carranza-Torres and Fairhurst, 2000). The more displacement is allowed, the less the need for support pressure to stabilize the ground. In high stress rock conditions, the rock support should be strong and also ductile, i.e. it should be able to absorb a good amount of deformation energy prior to failure.
