Soil nail systems stabilize a section or block of retained ground by installing closely spaced grouted steel rods into the exposed face as the excavation proceeds downwards. This approach provides a flexible, cost-effective and efficient earth retention solution. The rows of soil nails are connected to structural shotcrete at the exposed face of excavation. As the excavation proceeds top-down in lifts, the soil will incrementally deform towards the excavation, mobilizing tensile strength of the nails to internally stabilize the retained soil. The nail layout and facing system are selected to create a stable retained soil mass, which is also designed as a gravity block to resist global rotation, sliding or bearing capacity failure.
Soil nail walls typically offer reduced cost and construction time compared to soldier pile walls. Temporary or permanent walls can be constructed, and nails can enhance stability of existing or re-graded slopes. The small, agile construction equipment can accommodate sloping ground profiles and complex geometries with interior and exterior corners. Selection of soil nail systems should recognize that some limited ground deformation is necessary to mobilize the passive resistance and that the high density of nails requires consideration of any adjacent subsurface utilities and facilities.
Soil nail walls are constructed from the top down in lifts of approximately 5 feet. Nails are drilled at each lift, with steel bars grouted into 4 to 6 inch diameter holes. The wall facing is constructed by embedding steel nail head plates into lightly reinforced shotcrete. Vertical drainage strips placed on the excavated soil face ensure drainage and prevent build-up of hydrostatic pressures on the wall. Shotcrete is placed directly against excavated soil face (covering drain strips and encasing steel mesh or bar reinforcing), and then rod-finished to complete the structural facing. For permanent walls, nails are corrosion protected and a second layer of shotcrete or cast-in-place concrete may be installed. Exposed permanent walls are often sculpted and colored to represent local rock formations. Nail pull-out tests verify design parameters at the start of construction. Shotcrete test panels and nail pull-out tests are performed throughout the production work to ensure compliance with quality control procedures.