Underground » Strata Control and Windblasts
This report contains details of four small scale field trials of pretensioned roof bolting, achieved by the incorporation of a thrust bearing as a fraction washer between the dome washer and nut of a roof bolt head assembly.
The data shows that the thrust bearing results in an increase in bolt loading at the instant of drill rig stalling and also allows the nut to rotate longer so that it closes any delaminations in the immediate roof present at the time of bolt installation. The impact of these two effects on roof behaviour has been measured in the trials and a conceptual model for the various phases of roof behaviour developed around the measured results, observed roof conditions as well as some established engineering principles.
Three fundamental mechanisms of roof behaviour have been identified, namely beam buckling (Euler Beam), low angle shearing of thin beams (Euler Beam breaking down) and guttering (visible form of low angle shearing).
In the terminology of the New Austrian Tunnelling Method (from where this work originated in 1991), the roof has been made to "support itself" through the maintenance of beam action by the effect of pretensioned bolting, as opposed to the "holding of the roof" scenario of the sheared mass which occurs at times with standard bolting systems. The civil engineering tunnellers have used this concept to great economic benefit in the past twenty years or so. The potential to do so in the coal mining tunnelling section is now a real possibility given the results of this project.