Abstract: Describe the applications of expansion shell/resin on the bolts having some contributed to the roof falls. And the basic guide of mine exploratory industry.
A mine experienced roof falls on a working section panel.
Several roof bolts were recovered from one of the falls and inspected.
The bolts were expansion shell/resin assisted type bolts using a jam nut.
The bolts recovered had severely twisted shell with the leaves buckled inward onto the threads.
Although some geologic conditions had an impact on the deteriorated roof conditions in the area, the distorted shells on the bolts may have significantly contributed to the roof falls.
The roof bolts used a standard expansion shell with a jam nut and were installed with 2 feet of resin.
Test has shown that when the jam nut is too strong, the nut will not strip off and the expansion shell will then twist and deform during tensioning of the bolt.
The results are low tension/torque ratio, shell distortion, high thread tension, or some combination of these.
For expansion shell/resin assisted bolts, the jam nut should have a breaking torque less than or equal to the installed torque of the bolt.
Best Pratices
Make sure products received from manufacturers are certified as meeting all applicable ASTM specifications. Jam nuts should not interfere with the proper tensioning of the bolt.
Check roof bolting machines for proper torque settings. Abnormally high torque may also add to the problem of distorted expansion shells.
Re-instruct roof bolting machine operators as to proper installation procedures, such as proper drill hole depth, resin cartridge length, and installed torque. Follow the manufacturer's installation procedures.