Underground » Strata Control and Windblasts
Active rib bolting has been evaluated as a means of improving rib support effectiveness and reducing bolt densities in high gateroads. This project (Stage 3) has demonstrated that pre-loading rib bolts does provide tangible benefits. The perception that coal ribs could not be reinforced by active bolting because of inherent inhomogeneity has been discounted. Active bolting will:
- Increase the shear strength of the coal by increasing coal confinement and angle of internal friction.
- Increase the load transfer ability of the coal.
- Create stronger columns of coal by clamping dilated cleat and mining induced fractures (mif) together.
- Reduce bolt cantilevering for which they have poor resistance.
The monitoring data from this study as well as the previous Syndicated Project has shown trends that are remarkably similar to those found in roof stability studies. In particular, the tendency of the immediate rib to fracture and dilate progressing to higher levels of total deformation. In addition, the increased extent of rib softening (yielding) with both a higher rib and less support is identical to that measured in the roof studies.
At one trial site, active rib bolts at half the density of standard bolts actually improved the level of reinforcement. Results of experiments at Ellalong and North Goonyella collieries indicated that bolt pre-loading provides confinement / reinforcement over greater distances than previously anticipated.
The confining stresses induced by point anchored bolts was observed to reduce when tightening adjacent bolts. This interaction was not fully investigated but full encapsulation or staged tightening of the bolts will reduce the problem.
A technique for quantifying the extent and magnitude of bolt pre-tensioning has been developed and will assist in optimising rib bolting densities.