Considering deformations when specifying roof support

Support elements can be loaded on installation (pre tensioned) or are subsequently loaded by movements in the rock mass. In the immediate vicinity of excavations in coal measures the movements are along either bedding or fractures induced by brittle failure. At mining depths less than about 300 m, movements applied to the support elements are associated with the deformation of jointed rock beams and are relatively small (less than about 5 mm) and a reinforcement strategy is appropriate. At greater depths and at the maingate corner, brittle failure of the rock mass can develop, and the associated movements are much greater. Horizontal closure of the failure zone leads to vertical dilation and also shear movements that may be in excess of the capacity of grouted tendon systems. In such cases a hold and retain support strategy is required that seeks to maximise the deformation capacity of the system. There is a possibility that the required deformation capacity is beyond the elongation of current cable systems: an alternative approach is suggested but will require a major rethink of how we approach roof support in laminated rock masses. Compression of the tailgate pillar can lead to a relaxation of the confinement in tailgate roofs and a hold and retain strategy may be required if the pillar is designed to yield at the tailgate corner.