Parameters needed for rock support design are discussed. Underground loading conditions and the methodology of rock support are covered first, followed by the parameters related to rock support design; that is, the factor of safety, the maximum allowable displacement, the ultimate displacement of support elements and the released strain energy, and their application in support design. The factor of safety needs to be calculated in different ways depending on whether the loading condition is gravitational, squeezing, or a dynamic rockburst.
The major instability issue related to high in-situ stresses is either rockbursting in hard and strong rock or time-dependent squeezing in soft and weak rock. The ‘driving force’ behind both rockbursting and rock squeezing is the strain energy released from the rock mass after excavation. In the case of rockbursting, the energy released in a seismic event has to be absorbed by the rock support system in order to avoid rock collapse. Therefore, it requires a dynamic support system capable of absorbing a good amount of energy prior to failure. This concept was proposed as early as in the 1960s, and has been well acknowledged by others to date (Cook and Ortlepp, 1968; Ortlepp, 1969, 1992). A good number of energy-absorbing rockbolts have been invented (Ortlepp Bornman, and Erasmus, 2001; Charette and Plouffe, 2007; Varden , 2008; Li, 2010; Cai and Champaigne, 2010; Wu and Oldsen, 2010; Galler, Geschwandtner, and Doucet, 2011; He et al., 2014; Darlington 2014). On the other hand, there exists a maximum allowable displacement from the point of view of engineering practice. Therefore, two parameters, the amount of energy to be absorbed and the maximum allowable displacement, need to be taken into account in dynamic rock support design. In addition, the displacement capacity of support elements, such as rockbolts, needs to be also taken into account. A satisfactory dynamic rock support system should be capable of absorbing a good amount of energy, but should also be capable of constraining the rock displacement within the maximum allowable limit. In principle, a strong support system is desired so that it does not need to displace much to absorb a given amount of released energy.
In this paper, the methodology of rock support is discussed first and then the parameters used for rock support design, such as the factor of safety and the maximum allowable displacement, are presented for the following three types of rock conditions: rockfall under gravity, rock squeezing, and rockburst.
