Step-by-Step Soil Nailing Wall Construction Using Solid Steel Bars

The stages of constructing a soil nailed (or nailing) retaining wall using solid steel bars are explained below and illustrated in Figure 1.

Step 1 – Excavation

Initial excavation is performed to a depth where the excavation face remains stable without support for a short period (usually 24 to 48 hours). The excavation depth typically ranges between 1 to 2 meters, slightly below the final stabilized soil nailing wall depth. The excavation platform width must provide adequate access for installation equipment.

Step 2 – Drilling Holes

Drill holes are created from the excavation platform with predetermined length, diameter, angle, and horizontal spacing.

Step 3 – Placement of Steel Bars and Grout Injection

Steel bars are inserted into the drilled holes. The bars are mainly solid, though hollow steel bars can also be used. To maintain the bars centered within the holes and ensure sufficient grout coverage, spacers are attached to the bars before placement. Simultaneously, a tremie grout pipe is inserted into the hole. For corrosion protection, special corrugated plastic coatings may be applied. Cement grout is then injected through the tremie pipe, usually by gravity or low pressure. When hollow bars are used (which also serve as drilling tools), drilling and grouting occur simultaneously (only for temporary structures). Before Step 4 (temporary facing), geocomposite drainage strips are installed on the excavation face approximately between each group of adjacent nails and extend to the next excavation stage. The drainage strips continue down to the excavation base, where collected water is directed through a toe drain out of the stabilized wall.

Step 4 – Construction of Temporary Shotcrete Facing

A temporary facing is constructed to support the exposed excavation face before the next excavation stage. The common temporary facing in soil nailing walls consists of a roughly 100 mm thick shotcrete layer reinforced with light reinforcement, usually welded wire mesh placed near the mid-thickness (see lower part of Figure 2-1). The wire mesh length should overlap at least one full mesh cell with adjacent panels. After proper curing of the temporary facing, a bearing steel plate is installed over the protruding bar end. The bar is then carefully pushed into the fresh shotcrete layer. Next, the nail head is securely fastened to the bearing plate using a hexagonal nut and washer. After sufficient curing, the nut is tightened to the required torque, typically after at least 24 hours. If necessary, nail deformation under load is measured and capacity tests may be performed before the next excavation phase. Shotcrete must cure for at least 72 hours or achieve a minimum 3-day compressive strength (usually 10.5 MPa) before proceeding.

Step 5 – Subsequent Excavation Stages

Steps 1 to 4 are repeated for further excavation stages. At each stage, the vertical drainage strip extends downward to the next level. New wire mesh panels are installed with at least one full mesh cell overlap with previous panels. New shotcrete layers are cold-jointed to existing layers. At excavation completion, drainage strips connect to the toe drainage system.

Step 6 – Final Permanent Facing Construction

After the last excavation and installation of all nails, and after load testing, the final facing is constructed. This may include cast-in-place reinforced concrete, reinforced shotcrete, or precast panels. Permanent facing reinforcement generally consists of conventional steel bars or wire mesh. Horizontal joints between excavation stages are executed with less than maximum possible joint spacing when CIP concrete or shotcrete is used.

Note: In specific projects, modifications to these steps may be necessary. For example, shotcrete might be applied immediately after excavation and before drilling and nail installation, especially if excavation face stability is very weak.