Rib support design and the coal mass rating relationship

This paper aims to explain the relationship between rib conditions, abutment fracture and coal mass rating (CMR). Understanding the relationship between the three factors provides data for rib support design. The primary function of rib support is to control the failed coal within the abutment zones along the excavation boundary. Several factors influence the shape of the rib abutment zones, including the mining depth, abutment stress, coal strength, roadway orientation and CMR.

Borescoping, underground inspections, UCS testing and monitoring instrumentation provided the data needed to understand the rib conditions relating to the abutment fracture characteristics. This data was then used to establish a correlation between CMR and abutment fracture characteristics, offering a potential application of the findings in the design of rib support. The investigation's findings were numerically modelled, revealing the impact of changing CMR values on rib abutment fractures.

The data defined three rib abutment zones: primary, secondary and tertiary. The primary rib abutment zone forms the skin of the excavation boundary and is associated with the lateral movement of the fractured coal. The secondary abutment zone resides behind the primary abutment zone and is associated with fractured coal, lacking significant lateral movement in normal conditions. The tertiary abutment zone is defined by a diagonal fracture extending away from the toe or crest of the secondary abutment fracture.

The potential roadway development rib support solution is achieved by extending the length of the support element beyond the secondary abutment fracture, securing the primary and secondary abutment zones. The occurrence of the tertiary abutment zone was found to become a potential release plane in areas of high drivage or in response to abutment loads from secondary extraction, which plays a crucial role in the overall rib support solution.