Rock support is associated with the rock mass quality as well as the in the state of situ stresses. A safety factor, defined by the strength of the support system and the load exerted on it, is usually used for rock support design. This design method is based on the principle of structure mechanics. It is appropriate to do so in a load-controlled situation, such as rock falls under gravity, but it is not valid to use the traditional strength-load safety factor for support design in high stress rock conditions under which instability usually involves stress-induced rock failure. In this case, the principle of rock support should be to absorb the energy released from the rock instead to equilibrate the weight of falling blocks. In this paper, the failure modes of rock under different stress conditions are first viewed. The philosophy and principles of rock support design are then presented for different rock conditions. Finally, yield support elements used so far in civil and mining engineering are briefly reviewed.
Instability is always a concern in underground excavations. The stability of an under-ground opening, such as a tunnel, cavern or mine stope, is mainly governed by three factors: the quality of the rock mass, the in situ stress state and the size and geometry of the opening. Of these three factors, the in situ stress state is the factor that plays the crucial role in the stability of a rock mass. The type of excavation disturbance is mainly associated with the magnitudes of the in situ stresses with respect to the quality of the rock mass. Too low rock stresses would lead to de-stressing in the country rock after excavation, while too high stresses would lead to rock failure due to stress concentration.
