The importance of the face plate as part of an engineered holistic ground support scheme in dynamic conditions

Ground support schemes consist of surface support systems that contact the excavation boundary and a reinforcement system embedded into the rock mass. Under static conditions, these systems can work relatively independently, as the requirement for them to work together is minimal, as demonstrated by practices such as shotcreting over reinforcement bolts, preventing significant interaction. However, the compatibility of these components with each other determines the overall effectiveness and capacity of the overall ground support scheme under quasi-static and dynamic conditions. Under loading from a seismic event, the support scheme installed to prevent a rockfall, is only as good as the weakest link. As the load transfer from the rock mass to the surface support occurs, engaging the reinforcement, it is critical that the connections are given special attention to prevent premature failure of the scheme. Traditionally, the connection, or the plate, is designed to be stronger than the reinforcement element as a plate failure usually renders the entire support scheme inoperable. There is, however, a difference between yielding and failing, and this is crucial in dynamic conditions. The individual elements each have unique characteristics in terms of load/displacement but when combined in a scheme they react differently. A well designed connection bearing or face plate between the surface support and the reinforcement acts as a system load indicator allowing for exclusion or rehabilitation to occur prior to a complete support system failure and resultant fall of ground. During dynamic testing, it has been shown that plates designed to deform in the yielding zone of a rockbolt can increase the total deformation and energy capacity of the bolt by 3–5 kJ compared to other plates. A plate should begin to deform in the yielding zone of the bolt and ultimately fail after the reinforcement element fails. This plate failure is a controlled release of energy (dissipation) as opposed to a reinforcement failure creating a projectile nut (thread) or a fall of ground.