Tunnel Deformation Monitoring "Action Levels" In Coal Mines For Support/Reinforcement Design

Mine tunnel support using rockbolts was introduced in UK coal mines over the period 1987- 1992 and is now used in over 70% of all mine tunnels. In every case the reinforcement design has been based on the observational approach. After the initial reinforcement design has been established appropriate action levels are defined for predetermined amounts of movement indicating the onset of instability. If the action levels are exceeded appropriate remedial action is taken usually in the form of additional support or reinforcement to maintain stability. This paper outlines the observational design method as used in UK coal mine tunnels. It discusses the low cost instrumentation which has been developed to monitor roof deformation, dual height telltales, how action levels are defined based on the geotechnical environment and the types of remedial measures typically taken. An alternative approach for dealing with areas of increased deformation involves reacting to early roof movement trends and modifying the support installed at the face of the development accordingly. Installation of additional support at the face of the heading improves ground control in poor geotechnical conditions and has the advantage of potentially reducing later additional support costs. A case study is described to illustrate how 'early' action levels can be incorporated into a roof support plan enabling a flexible roof support strategy. Such a system will be site specific and continued analysis of monitored information will be required to confirm the action levels or to adjust them according to any change in geotechnical environment. More accurate roof deformation monitoring systems, such as the Remote Reading Telltale system recently developed by Rock Mechanics Technology Ltd. (RMT) and currently undergoing Mines Safety and Health Administration (MSHA) approval, are ideally suited to this application. The Remote Reading Telltale system provides accurate mine wide monitoring of roof condition on a surface PC.