Question 5 of 6: Rock Bolt Anchor Systems

Question 5: Rock Bolt Anchor Systems (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

5.1 — Mechanical point anchors

A mechanical point anchor concentrates the entire holding capacity of a rock bolt at a single point near the toe of the drillhole, leaving the rest of the bolt shaft free to elongate elastically under load. The most common form is a wedge-and-cone (barrel-and-wedge) expansion anchor: a tapered cone threaded onto the leading end of the bolt is drawn back into a split, expanding sleeve as the bolt is torqued or the nut is tightened, forcing the sleeve's segments outward against the borehole wall and gripping it by friction/mechanical interlock alone, with no bonding agent involved. Because the anchor is set and stressed immediately, the bolt provides its full design tension the moment installation is complete — the principal reason mechanical point anchors remain popular for temporary or rapid-cycle ground support (e.g. immediately behind an advancing face) even though resin- or cement-grouted systems now dominate for permanent support.

Several installation issues can compromise a point-anchored bolt's performance. Borehole diameter tolerance is critical — an oversized hole (worn bit, soft/broken ground enlarging the hole) prevents the expanding segments from achieving full bite, while an undersized hole can jam the anchor before it reaches design depth. Anchor-zone rock quality matters as much as the anchor hardware: if the toe of the hole lands in a fractured, weathered, or soft zone, the anchor can slip or pull through under load regardless of how well it was set, so anchor position should be chosen (or a hole re-drilled) to seat in the most competent rock available. Over-torquing during setting can shear the expanding segments or strip the anchor thread before the design tension is reached, silently leaving a bolt at far less than its intended pre-load; conversely under-setting leaves the anchor under-expanded and prone to slip and stress loss (load relaxation) over the following days to weeks, so a proportion of installed bolts should always be pull-tested to confirm design load is actually held. Finally, since only the toe is anchored, the entire free length is unprotected against corrosion unless a separate corrosion-protection sheath or grout is added after tensioning — a bare point-anchored bolt is not considered suitable for permanent, long-design-life support in a wet or corrosive environment.

5.2 — Friction bolts

A friction bolt develops its entire holding capacity by radial contact pressure distributed along its whole embedded length, rather than at a single anchor point, and needs no resin, cement or mechanical wedge. The most widely used type is the split-set stabiliser: a slotted, longitudinally split steel tube slightly larger in outside diameter than the drilled hole, driven in with a hydraulic or pneumatic installation rig; as it is forced in, the tube's slot compresses (reducing its diameter to enter the hole) and the tube's own spring-back generates a continuous radial force against the borehole wall along its full length, giving friction-based load capacity and immediate, full-length support the instant it is driven home.

Advantages. Installation is extremely fast (seconds per bolt, no mixing, no set-up/cure time) and full support capacity is available immediately, which suits rapid advance rates in development headings and squeezing or raveling ground needing support right at the face. The bolt tolerates a range of hole diameters reasonably well and is comparatively inexpensive per unit.

Limitations. Holding capacity is lower than a properly grouted rebar or cable bolt of similar diameter, and is highly sensitive to borehole diameter — an oversized hole (worn bit, soft ground) sharply reduces the radial contact pressure and can leave the bolt essentially unanchored. The thin-wall tube has no inherent corrosion protection and is not intended for long design-life or permanent support without a supplementary corrosion-protected system. In squeezing or highly stressed ground the friction grip can creep (slip slowly under sustained load) over weeks to months, so friction bolts are best regarded as fast, effective temporary/primary support to be supplemented or replaced by grouted, corrosion-protected bolting for the permanent support system.