Components, systems and materials › Slope protection and rockfall › Soil nail
Soil nail
What it is
A soil nail is a passive steel bar grouted into a drilled hole in a soil slope or excavation face, used in rows to reinforce the ground in place. The face is protected with shotcrete.
Where it is used
Road widening cuts, basements near existing buildings, landslide repairs, tunnel portals and retaining walls built top-down.
How it works
Soil nailing builds a reinforced block of the existing ground. Excavation proceeds in shallow lifts; after each lift a row of nails is installed and the face is sprayed. When the soil behind starts to move, the nails pick up tension through friction along their grouted length and hold the active wedge to the stable soil behind. Unlike anchors, nails are not prestressed; they work as the ground deforms slightly.
Types
- Drilled and grouted nails
- self-drilling hollow-bar nails
- driven nails for temporary works
- nails with hard shotcrete facing
- nails with soft facing such as mesh and vegetation
Materials
Deformed steel bars, or hollow bars; neat cement grout; centralisers; face plates and nuts; shotcrete with mesh or fibres; drainage strips and weep holes.
Design checks external stability (sliding, overturning, bearing and global stability through and around the nailed block) and internal stability (nail pull-out, nail tensile strength and facing strength at the head). Common practice uses nail lengths of roughly 0.6 to 1 times the wall height, inclined 10 to 20 degrees below horizontal, at about 1 to 2 m spacing; the FHWA circular GEC No. 7 is widely referred to.
Installation and QA steps
- Excavate a lift of about 1 to 2 m that will stand unsupported for a short time
- Drill, insert bar with centralisers, and grout the hole
- Place drainage and mesh, then spray the shotcrete facing
- Fix bearing plates and nuts
- Repeat lift by lift
- Carry out verification and proof tests on nails
- Water building up behind the facing
- Excavating a lift too deep before nailing
- Facing too thin at the heads
- Nails in soft clay or loose saturated sand where they cannot develop friction
Codes cited
- FHWA GEC No. 7 (USA): Geotechnical Engineering Circular on soil nail walls.
- BS 8006-1 (UK): Code of practice for strengthened/reinforced soils and other fills.
- EN 1997 (Eurocode 7) (Europe): Geotechnical design.
- EN 14487 (series) (Europe): Sprayed concrete: definitions, specification and execution.
Cited by number only. We do not reproduce code text, tables or figures.
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