Soil nailing reinforces an existing soil mass in place by installing closely spaced steel bars into the ground and connecting them to a facing. The nails are passive: they carry no load when installed and mobilise tension only as the ground begins to deform. The result is a coherent reinforced block that behaves as a gravity structure, and it is created without excavating and rebuilding the slope.
How soil nails work
Bars are drilled or driven into the face on a grid, typically at one to two metre centres, inclined ten to twenty degrees below horizontal, and grouted along their length. As the unreinforced ground behind the face tends towards a slip, the nails crossing the potential failure surface pick up tension through friction between the grout and the soil. Design checks the pull-out resistance of the bond length beyond the slip surface, the tensile capacity of the bar, the face connection, and the overall and internal stability of the reinforced block. The governing UK references are CIRIA C637 for design and BS EN 14490 for execution.
Where soil nailing is the right answer
Nailing suits cuttings in stiff cohesive soils and weak rock, where the face will stand unsupported for the height of one construction lift. It is the standard method for stabilising an existing cutting in service, because the works are carried out from the face without removing the ground behind it — often the only option beside a live carriageway or railway. It is much less suited to soft or loose soils that will not stand for a lift, to running sand, and to ground with high groundwater flow, where grout cannot be placed reliably. Nails also occupy a substantial volume of land behind the face, so third-party land or easements are frequently the binding constraint rather than the engineering.
Facing options
The facing spans between nail heads and holds the surface material. A hard facing of sprayed concrete on mesh is used where durability and impact resistance matter. A flexible facing of high-tensile steel mesh or geogrid, sometimes over an erosion control mat, is chosen where a vegetated finish is wanted and the surface loading is modest — it is the common choice on highway cuttings for both ecological and visual reasons. Where nails support a granular fill or an eroding surface between heads, geocell or a mattress can be used to confine it. The facing is a structural element, not a cladding, and its connection to the nail head governs the local capacity.
Soil nails, ground anchors and retaining walls
Ground anchors are pre-stressed and act actively from installation, are widely spaced and carry high individual loads, and require testing of every anchor. Soil nails are passive, closely spaced and individually low-capacity, and testing is on a sacrificial sample. Compared with a conventional retaining wall, nailing avoids the excavation, the temporary works and the imported fill, which is why it is usually cheaper on an existing slope; a new-build level change with a hard architectural face is normally better served by a wall or a reinforced soil structure.
Related ViaCon solutions
ViaCon supplies geogrid, geocell and facing systems used with soil nail schemes. See our geotechnical solutions range. Related glossary entries: soil stabilisation, ground reinforcement, slope stabilisation and retaining wall.
Frequently asked questions about soil nailing
What is soil nailing?
Soil nailing reinforces an existing soil mass in place by installing closely spaced grouted steel bars into the ground and connecting them to a facing. The nails mobilise tension as the ground tends to move, forming a coherent reinforced block that acts as a gravity structure.
How far apart are soil nails spaced?
Typical spacing is one to two metres in both directions, with nails inclined around ten to twenty degrees below horizontal. Actual spacing, length and inclination follow from the stability analysis and the pull-out resistance available in the ground.
What is the difference between soil nails and ground anchors?
Ground anchors are pre-stressed and act from installation, are widely spaced and carry high individual loads. Soil nails are passive, closely spaced and individually low-capacity, and only mobilise load as the ground deforms.
Can soil nailing be used in soft soils?
Generally no. Nailing requires ground that will stand unsupported for the height of a construction lift and that allows grout to be placed reliably. Soft or loose soils, running sand and high groundwater flow are all poor candidates.