Corrosion Protection Methods for Soil Nailing Bars

Corrosion protection in soil nailed retaining walls can be provided by physical, chemical, or combined protective systems. Physical protection involves placing a continuous barrier between the steel nail (or soil nail reinforcement) and the surrounding soil. Chemical protection refers to the use of dielectric (insulating) materials that block electrical current flow.

Currently, a combination of these mechanisms is commonly employed for corrosion protection in soil nailed walls. When stray electrical currents are a concern, electrical insulation of nails (or reinforcements) is necessary. Common systems for corrosion protection of soil nails include:

Grout Coating

This method involves a complete coating of the steel bar with clean cement grout. After placing the bar at the center of the drilled hole, clean grout is injected to fill the annulus around the steel. The grout capsule provides both physical and chemical corrosion protection. A minimum grout cover delays the ingress of aggressive agents such as carbonates, chlorides, oxygen, and moisture from soil and air. Ensuring low grout permeability is crucial to effective encapsulation. The grout creates an alkaline environment that lowers corrosion potential. Minimum grout thickness between the steel and soil should be defined during design.

Epoxy Coating with Grout

Epoxy corrosion protection (see Figure 1) consists of fusion-bonded epoxy coating applied to the steel bars by the manufacturer before transport (especially sea shipment). Cement grout surrounds the epoxy-coated bars after installation. The minimum epoxy thickness required is 0.4 mm. Epoxy provides both physical and chemical protection, as it is a dielectric material. Handling and transport may damage the epoxy layer; in case of cracks, reapplication is recommended. Relevant standards can be found in ASTM A-775.

Galvanized Coating with Grout

A common corrosion protection method is galvanization, where steel bars and other metal components are coated with zinc. This coating offers both physical and chemical protection by forming a protective zinc oxide layer. Cement grout is also applied around galvanized bars.

Double Encapsulation (Dual Corrosion Protection)

For maximum corrosion protection in soil nailed walls, a corrugated sheath of at least 1 mm thickness made of chloride-resistant polyvinyl chloride (PVC) or high-density polyethylene (HDPE) may be installed around the grouted bar (see Figure 1). This method is commonly used in permanent anchors or ground anchors in corrosive or unknown environments.

The corrugated sheath transfers the nail load to surrounding soil while allowing grout injection. The grout must completely fill the annular space inside and outside the sheath. Minimum grout thickness between the sheath and bar is 10 mm, ensuring easy grout flow and physical protection. Minimum grout thickness between the sheath and hole wall is 20 mm.

Some systems inject the sheath in a factory before transport. The sheath must withstand handling and installation stresses, chemically stable, resistant to UV radiation, and impermeable. Typical sheath thickness ranges from 0.875 to 1 mm. Special sheathing techniques are proprietary to some companies.

Corrosion Protection of Nail Heads

Using galvanized steel components, a minimum 50 mm thick layer of permanent concrete or shotcrete covers bearing plates, washers, and nuts to protect against corrosion. Epoxy coating can also be applied to these elements.

Protection Against Stray Currents

When stray currents pose a risk, electrical insulation methods are used. This involves interrupting electrical current paths between the source and steel nails. Insulation can be applied along the current path or directly on the nails. PVC sheaths can be used for insulation. When sheaths are unavailable, bearing plates and nail heads must be insulated.