Components, systems and materials › Slope protection and rockfall › Rock bolt and rock anchor
Rock bolt and rock anchor
What it is
A rock bolt is a steel bar installed in a drilled hole to reinforce a rock mass; a rock anchor is a longer, usually prestressed, tendon that ties an unstable block or a structure back to sound rock deep behind it.
Where it is used
Tunnels and caverns, rock cuts, dam abutments, retaining walls on rock, transmission tower foundations and uplift-resisting foundations.
How it works
Rock is often strong between joints but weak across them. A bolt crossing the joints adds shear and tensile resistance and knits the blocks into a self-supporting arch around an opening. A prestressed anchor is locked off at a design load, so it clamps a block against its sliding plane before any movement happens. Its load is transferred into rock along a grouted bond length, behind an unbonded free length.
Types
- Fully grouted bars (dowels)
- resin-anchored bolts
- mechanically anchored expansion-shell bolts
- self-drilling hollow bars
- friction (split-set) bolts
- prestressed bar or strand anchors
Materials
Deformed steel bars or hollow bars; cement grout or resin capsules; plates and nuts; strand tendons with sheathing for the free length; corrosion protection such as galvanising, epoxy or double protection for permanent anchors.
Bolt length, pattern and capacity come from rock mass classification, wedge analysis or numerical modelling. Anchors are designed for the driving force of the block with a factor of safety, checking tendon strength, grout-to-rock bond, grout-to-tendon bond and the rock cone. Permanent anchors need a defined corrosion protection class.
Installation and QA steps
- Drill to the design length and angle, and flush the hole
- Insert the bar or tendon with centralisers
- Grout from the bottom up or install resin cartridges
- Fit plate and nut, then tension prestressed anchors to the lock-off load
- Test: pull-out tests on trial anchors, proof or acceptance tests on a share or all production anchors
- Record drilling log, grout volumes and test results
- Grout voids leaving the bar unbonded
- Corrosion of the free length on permanent anchors
- Loss of prestress from creep in weak rock
- Bolts too short to reach beyond the loosened zone
Codes cited
- EN 1997 (Eurocode 7) (Europe): Geotechnical design.
- BS 8006-1 (UK): Code of practice for strengthened/reinforced soils and other fills.
Cited by number only. We do not reproduce code text, tables or figures.
Related
Road tunnel through a mountain: drill, blast, NATM
Excavate in rounds and let the rock help support itself.
Generic description for education. We do not name, endorse or compare manufacturers or products.
Found an error? Write to indiacongroupofengineers [at] gmail [dot] com. For corrections and feedback. We never ask for payment or personal details.