Soil nailing is an economical, top-down earth retention method in residual or cohesive soils. Closely spaced reinforcing bars (nails) are installed in the face of the excavation. A structural shotcrete fascia bridges the nails and supports the soil/rock between nails. Soil nails are passive elements that become tensioned when the soil/rock deflects laterally as excavation depth increases.
Soil nail walls can be a temporary measure for support of excavation or structural underpinning, or they can function as permanent retaining walls or slope stabilization systems. This technique is most cost effective in difficult soil conditions such as till or weathered rock, where space or headroom is limited, and a permanent wall is required.
To ensure stability of the cut face, soil nailing is accomplished in a stage-down process. A vertical lift, typically four to six feet deep, is excavated along the wall alignment, with the face of the cut fully stabilized before excavation of the next level. The horizontal length of the excavation is directly related to the area of soil face that can be completed in one working day. Reinforcing material is placed across the cut face and shotcreted to create a structural face. The soil nails are then drilled in on a regular grid pattern and locked off.
In some cases, it may be advantageous to installing the soil nails before the shotcrete facing is placed. As one of the top soil nail contractors in construction, Moretrench evaluates the site-specific conditions to determine the most appropriate sequence.
Advantages & Limitations
- Rapid, economical installation
- For highway projects, less right of way required, and less disruptive to vehicular traffic than drilled shafts and soldier pile walls.
- Less congestion in the excavation compared to braced excavations.
- Easy field adjustment of nail placements if obstructions are encountered.
- Can readily be accomplished in restricted headroom situations.
- Typically not suited for installation in unstable/non-cohesive soils
- Pre-drainage dewatering is required for installation below the water table
Vertical Soil Nails
A vertical soil nail (VNAIL) is a drilled and grouted, near-vertical element designed to promote soil arching, increasing the stand-up time of an excavated face sufficiently to allow conventional soil nailing to be performed in marginal geotechnical conditions where there otherwise might be unacceptably high risk levels with soil nailing alone. VNAILs are typically, though not always, a temporary measure intended as a construction expedient. As such, they do not play a role with respect to the stability of the completed soil nail excavation support system.
Moretrench is acknowledged as one of the industry’s leading soil nailing contractors and has completed numerous projects for highway widening and deep excavations for new construction including the Route 7 Bypass and the Robert Wood Johnson University Hospital Proton Therapy Building.