What are Soil Nails?

Soil nailing evolved from the New Austrian Tunneling method, which is a system for underground excavations in rock. This system consists of passive steel reinforcement in the rock followed by the application of reinforced shotcrete. Since the early 1960s, the concept of combining passive steel reinforcement and shotcrete has also been applied to the stabilisation of rock slopes.

The first application of soil nailing was implemented in 1972 for a railroad widening project near Versailles, France. Soil nails were used to stabilise an 18-meter (59 ft) high slope consisting of sandy soil. This method proved to be more cost-effective, while at the same time cutting down the construction time when compared to other conventional support methods.

The United States first used soil nailing in 1976 for the support of a 13.7m deep foundation excavation in dense silty sands. Soil nailing was implemented in the expansion of The Good Samaritan Hospital in Portland, Oregon. This retaining system was produced in approximately half the time at about 85% of the cost of conventional retaining systems. [3]

First, the slope or wall which needs to be stabilised is predrilled. Soil nails are inserted and grouted firmly to hold the soil in place. If the soil contains corrosive elements, then the soil nails need to be coated in anti-corrosives to protect them. For optimum slope stability, a covering later fixes the soil nails in place.

After the soil nailing process has been completed, shotcrete is commonly used to secure the reinforcing rods. There are also other options used, such as creating a green or living wall. The type of retaining wall used is influenced by the environment of the construction. In some cases, structural steel mesh cladded with a facing mesh is used and filled with a graded hardwearing local stone.

Soil nails are a durable solution, with an expected minimum lifespan of 100 years. This is an advantage of the solution, as anchoring and retaining wall alternatives may have similar lifespans, but will require more maintenance and repair.

A budget plan for pricing soil nailing work would include preparatory work, drainage system, soil nailing, and shotcrete (facing).

A cost estimation for soil nailing considers the four variables:

1. Length
2. Horizontal distance
3. Safety factor (SF)
4. Time

By analysing the required work, a method for calculating the unit price for the construction can be done. This includes the cost for the materials needed, the labour of installation, and the equipment used to complete the soil nail construction works.

The advantages of soil nailing include:

• A cost-effective solution for permanent or temporary retaining walls.
• Soil nail walls are ideal for sites which have a limited amount of space, as there is little need for room to manoeuvre.
• Soil nailing can follow irregular and tight corners, as well as provide stability to the wall from the top to the bottom. This means that the soil can be secured whilst workers continue to dig down deeper.
• It is a rapid and economical method
• Grouting is only required once
• Soil nails are not prestressed

For more information on the advantages of soil nailing, watch our dedicated video: