The following are items to be aware of when installing mechanical rock bolts (forged head and threaded both end [TBE] bolts):

Type of Ground - The nature of the ground must be evaluated. Soft strata requires a larger plate washer and/or expansion shell bearing area. Greater expansion of the expansion shell leaves may be required. Soft ground results in larger hole sizes for a given bit size (bit rattling and reaming). Special anchors are available for soft ground. Excessively hard ground can be detrimental as well. Hard ground can result in poor anchorage as the shell is unable to “bite” into the rock.

Scaling - The ground should be thoroughly scaled i.e. barred down, before drilling and bolting. Periodic re-scaling may be required while drilling.

Strength and Yield Capacity of Bolt - The mechanical properties of the bolt should be appropriate for the ground conditions, bolt length and bolting pattern. Pull tests and torque-tension tests should be performed to determine yield strengths and anchorage capacities of the bolts being used.

Oversize Holes - Holes ¹/₈" oversized can reduce holding strength by 70%. Oversized holes can be caused using the wrong bit size, leaving the drill running while flushing the hole, soft ground (faults, gouge, etc.) and bent drill steel.

Undersized Holes - Undersized holes will not permit the plug in the expansion shell to seat properly. The leaves or bail can break and distort upon forced entry. Undersized holes are usually caused by worn bits and/or wrong bit sizes being used.

Friction Flair Expansion Shells - A recent improvement in expansion shell design is the addition of flairs or wings on the bail strap at the bottom of the leaves. Installation is improved as there is no need to set the anchor to keep the bolt from sliding out of the hole during installation. The anchor sets up faster as the plug engages the leaves freely. Spinning of the anchor in the hole is prevented, thereby eliminating the principal cause of bolt failure upon installation.

DSI - Mining’s T-Dome forged head rock bolts have a special concave head design which when used in conjunction with T-Dome plates, work like a ball-and-socket and correct for angularity without requiring extra washers.

Rock Bolt Dollies - Rock bolt installation dollies should only transfer rotational energy to the rock bolt during installation, not percussive energy, when stopers and jacklegs are used. The shank end of the dollie must be short enough to avoid contact with the drill piston. Typically, these rock bolt dollies have a 7/8" hex shank and are only 3-¹/₄"

Overtorquing - Overtorquing of the bolts must be avoided. With forged head bolts, the heads can be weakened or sheared off. Overtorquing may tension the bolt past it’s yield point which is undesirable.

Undertorquing - Undertorquing can result in poor anchorage. Under tensioned bolts are particularly susceptible to blasting damage. Bolts will not apply sufficient compressive force to the ground as per design requirements.

Monitoring - In practice, rock bolt tension often drops after installation. Expansion shells can slip due to blasting vibrations, shells can move as high stress concentrations along the leaves cause localized failure of the ground, and finally, the rock can fail under the plate washer.
The installation of any ground support system must be monitored to ensure proper procedures and installation quality are maintained. For rock bolts this requires verifying torque and the placement and spacing of bolts. Torque measurements can

Matching Bolt and Shell Threads - While North American mines typically use UNC thread specs, other countries can use different thread types. Care must be taken not to mismatch shells and bolts from various manufacturers. A minimum UNC class 1A thread spec should be met on all threads as per ASTM F432 guidelines. DSI - Mining exceeds ASTM specs and rolls threads to a class 2A specification.

Support Nuts - While they are sometimes used on TBE bolts, the practice of installing a nut below the anchor shell should be avoided on forged head bolts. If the support nut comes in contact with the shell, friction between these two surfaces will prevent the bolt from elongating properly and will give false torque readings as the bolt is tensioned. Headed bolts should be free to elongate into the hole through the expansion shell.

TBE Bolt Nuts - Do not run the nut on a TBE bolt to the end of the threads and onto the shank. Strength can be reduced by as much as 50%. Each revolution of the nut onto the shank reduces the thread engagement length.

Notching of TBE bolt threads - Notching threads on TBE bolts is a poor installation practice and must be avoided. This practice locks the nut onto the bolt which then makes the TBE bolt act as a forged head bolt. Deep gouging can reduce the bolts strength.

Proper Grade Plates - Thin or weak plates will deform at low bolt tension. High torques will produce lower tension values. The bolt could also rip through the plate during installation or by bolt loading. Plate should meet ASTM F432 specs. The minimum spec is a grade 2 plate which has less than 0.25" deflection when the load on the plate is increased from 6,000 to 20,000 lbs.