Soil Nailing

Soil nailing is a ground improvement technique which is used to reinforce soil to make it more stable. Soil nailing is used for slopes, excavations, retaining walls etc. to make it more stable. The faces of steep slopes are usually protected by shotcrete and mesh for temporary works and by either cast-in-place concrete or prefabricated panels for permanent applications.

For shallower reinforced slopes, the surface can be protected by mesh reinforcement. Soil nailing is used to stabilize the slopes or excavations where required slopes for excavation cannot be provided due to space constraints and construction of retaining wall is not feasible. It is just an alternate to retaining wall structures.

Principle

The rods are passive reinforcement which, when the excavated face begins to strain, generate tensile forces and shearing resistance to counter the yield of the soil. Faces of steep slopes are usually protected by application of shotcrete, concrete so as to grout the reinforcing steel or nails. These provide stability to the steep soil slope.

Description

Soil nail walls are generally constructed from the top down. The soil is excavated in 3 to 6 feet deep stages. Nearly-horizontal holes are drilled in the centers into the exposed face after every excavation stage. Tension-resisting steel bars are inserted into the holes and grouted in place.

A drainage system is installed on the exposed face, followed by the application of reinforced shotcrete facing. Bearing plates are then fixed to the heads of the soil nails. This installation process explained above is repeated until the design wall depth is obtained. The finished soil nails create a zone of reinforced ground which is stable.

Merits of soil nailing

• It is suitable for confined sites with difficult access because the construction equipment required for soil nail installation is small and mobile.
• The soil nails can be installed from crane-suspended working platforms for already existing steep slopes such as bluffs or existing retaining walls.
• It can easily manage the site constraints and variations in ground conditions encountered during construction.
• During construction, it causes less environmental impact when compared to cutting back and retaining wall construction as no major earthworks and tree felling are needed.
• There could be time and cost savings compared to conventional techniques.
• The failure mode of a soil-nailed system is of ductile nature hence providing warning signs before failure.

 Limitations

• Metal soil nails will corrode.
• The presence of utilities, underground structures or other buried obstructions poses restrictions to the length and layout of soil nails.
• The zone occupied by soil nails is sterilized and the site poses constraints to future development.