Unstable slopes can quickly become a safety risk, a compliance issue, and a costly delay if not stabilised correctly. Soil nails offer a proven slope stabilisation method that works with the ground rather than against it, but they are not suitable for every site. This guide explains what soil nails are, how soil nail slope stabilisation works, and when they are the right solution compared with traditional retaining walls.
Soil nails are steel reinforcing elements installed into existing soil or rock to stabilise slopes and excavations. They are typically ribbed steel bars or hollow sections that are drilled into the ground and fully grouted in place.
Rather than acting as a freestanding structure, soil nails reinforce the soil itself. Once installed, the soil and nails behave as a composite mass, forming a gravity-type retaining system that resists movement. Soil nails are commonly used for both temporary and permanent slope stabilisation, slope excavation support, and retaining wall reinforcement. In New Zealand, they are widely applied on highway cuttings, embankments, hillside house developments in Queenstown or anywhere with restricted access.
Soil nail slope stabilisation works by increasing the shear strength of the ground and preventing failure along potential slip planes. The nails function as passive tension elements, meaning they only mobilise strength when the soil begins to move.
Construction is carried out top-down in controlled stages:
Typical drill hole diameters range from 100 mm to 150 mm in New Zealand conditions, providing sufficient grout cover and bond strength. Larger diameters may be used for longer nails or difficult ground.
Soil nails stabilise slopes through several interacting mechanisms:
Correct nail length, spacing, and inclination are critical to achieving the required factor of safety, particularly in New Zealand’s variable soils.
Choosing between soil nails and retaining walls depends on site conditions, design requirements, and construction constraints.
Soil nails are often preferred when:
Because soil nails reinforce existing ground, they typically require less excavation and material than retaining walls, reducing cost and programme impacts.
Traditional retaining walls may be a better choice when:
In some projects, a hybrid approach combining soil nails with retaining wall elements delivers the best outcome.
The lifespan of soil nails depends on design, materials, and environmental exposure. Permanent soil nail systems in New Zealand are commonly designed for a service life of 75 to 100 years.
Longevity is achieved through:
Regular inspections and maintenance of surface facings further extend performance, particularly in exposed slope environments.
At Tough Terrain, we specialise in working on slopes and sites that don’t fit the textbook. From steep embankments and remote hillsides to constrained civil and commercial projects, we combine specialist equipment with real-world ground experience to deliver practical, safe solutions. If your site has been labelled “too hard” or needs a second opinion, contact our team to discuss the right approach for your slope stabilisation project.