In underground excavation, the size of the failure zone in rock is dependent on the strength of the rock and the magnitude of the in-situ stress. A natural pressure arch would be developed in the country rock as long as an opening is excavated. In the case of a limited extension of rock failure, rock bolts should be anchored into the pressure arch to prevent the failed rock from disintegration. In the case of an extensive failure zone, an artificial pressure arch needs to be established in the near field of the opening, which could be done through systematic bolting. The philosophy of the artificial pressure arch was practiced in reinforcing a mine stope. In this paper, field observations of rock failure are presented first, including a case of core logging to localise the position of a naturally-formed pressure arch. Two philosophies for rock bolting are then introduced. Finally, two case studies are presented to demonstrate the practice of the supporting philosophies.
In underground excavation, weak and soft rocks usually subject to large deformation because of extensive rock failure in the case of high in-situ stresses in the rock mass. A good philosophy is required for rock support design to counteract the problem of instability in this type of rock. It is a well known knowledge that a basic principle for rock support is to help the rock to support itself, that is, to utilize the self-support capacity of the rock. In the practice of rock excavation, one has been aware of the phenomenon that an arched portion of rock becomes more stressed in the near field of the underground opening than other portions of the rock. This is the so-called pressure arch. The pressure arch can bear a considerable ground pressure and works actually as a protection shield over the opening. In case of a little rock failure in the country rock, a naturally formed pressure arch is located in the near field of the opening and it is competent enough to bear the ground pressure without need of support measures.
Weak and soft rock is characterised by its relatively low strength and low deformation modulus. Rocks containing soft minerals, such as mudstone, chloritic and talc-containing rocks, belong to this category of rock. Weak and soft rocks react to high stresses in the form of large deformation. The failure mode in weak rock is typically in shear, which can be reasonably described by the Mohr-Coulomb failure criterion or by the Hoek-Brown criterion. Figure 1 shows the pattern of fractures exposed on the excavation face of a mine stope in a weak chloritic quartzite. Two sets of conjugated fractures are formed in the rock under the high in-situ rock stresses. The failure mode is obviously in shear in this case. Shear failure could occur also in the roof andwalls. Figure 2 shows a shear failure in thewall of a deeplylocated opening.