Design of Soil Nail Walls: Construction, Advantages and Disadvantages
Soil nail walls consists of installing passive reinforcement (i.e., no post-tensioning) in existing ground by installing closely spaced steel bars or sections (i.e., nails) and placing a front face support. Soil nails are later grouted if they are installed in drilled holes. Ungrouted soil nails are also possible if nails (or steel sections) are driven into the ground. Figure 1 shows typical details of a soil nail wall:
Figure 1: Typical soil nail wall arrangement
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Soil nail wall construction proceeds from the top to bottom, and head plates are installed on each nail. Shotcrete or concrete is typically applied on the excavation face to provide continuity when a soil nail wall is constructed. For a soil nail wall the general construction procedure involves:
a) Excavate for the first nail (soil must be sufficiently self standing)
b) Install the 1st nail.
c) Construct 1st phase shotcrete on soil face (optional if shotcrete is constructed) with wire mesh or other reinforcement if required.
d) Install soil nail head plate (with or without other attachments, Figure 2)
e) Construct 2nd phase shotcrete (depending on staging specifications).
f) Excavate to next soil nail level, and install next soil nail, shotcrete etc.
g) Repeat steps c) through f) until the final excavation level is reached.
h) Construct additional permanent facing if required.
* Drainage filters and drainage pipes are also commonly installed during construction.
Figure 2: Typical soil nail head plate and shotcrete details (US Practice)
Soil nailing is typically used to stabilize existing slopes or excavations where top-to-bottom construction is advantageous compared to other retaining wall systems. For certain conditions, soil nailing offers a viable alternative from the viewpoint of technical feasibility, construction costs, and construction duration when compared to ground anchor walls, which is another popular top-to bottom retaining system.
Soil nail walls are particularly well suited to excavation applications for ground conditions that require vertical or near-vertical cuts and have been shown to be particularly well suited in the following temporary or permanent applications:
- Roadway cut excavations.
- Road widening under an existing bridge end.
- Repair and reconstruction of existing retaining structures.
- Temporary or permanent excavations in an urban environment.
Soil nailing has proven economically attractive and technically feasible when:
- The soil in which the excavation is constructed is able to stand unsupported in a 1- to 2-m (3- to 6-ft) high vertical or nearly vertical cut for one to two days.
- All soil nails within a cross section are located above the groundwater table
- If soil nails are below the groundwater table, and the groundwater does not adversely affect the face of the excavation, the bond strength of the interface between the grout and the surrounding ground, or the long-term integrity of the soil nails (e.g., the chemical characteristics of the ground do not promote corrosion).
Soil nail advantages
Soil nail walls exhibit numerous advantages when compared to ground anchors and alternative topdown construction techniques. Some of these advantages are described below:
- Requires smaller right of wat than ground anchors as soil nails are typically shorter;
- Less disruptive to traffic and causes less environmental impact compared to other construction methods.
- Provide a less congested work place, particularly when compared to bracedexcavations.
- There is no need to embed any structural element below the bottom of excavation as with soldier beams used in ground anchor walls.
- Soil nail installation is relatively rapid and uses typically less construction materials than ground anchor walls.
- Nail location, inclination, and lengths can be adjusted easily when obstructions (cobbles or boulders, piles or underground utilities) are encountered. On the other hand, the horizontal position of ground anchors is more difficult to modify almost making field adjustments costly.
- Since considerably more soil nails are used than ground anchors, adjustments to the design layout of the soil nails are more easily accomplished in the field without compromising the level of safety.
- Overhead construction requirements are smaller than those for ground anchor walls because soil nail walls do not require the installation of soldier beams (especially when construction occurs under a bridge).
- Soil nailing is advantageous at sites with remote access because smaller equipment is generally needed.
- Soil nail walls are relatively flexible and can accommodate relatively large total and differential settlements.
- Measured total deflections of soil nail walls are usually within tolerable limits.
- Soil nail walls have performed well during seismic events owing to overall system flexibility.
- Soil nail walls are more economical than conventional concrete gravity walls when conventional soil nailing construction procedures are used.
- Soil nail walls are typically equivalent in cost or more cost-effective than ground anchor walls when conventional soil nailing construction procedures are used.
- Shotcrete facing is typically less costly than the structural facing required for other wall systems.
Soil nail disadvantages
Some of the potential disadvantages of soil nail walls are:
- Soil nail walls may not be appropriate for applications where very strict deformation control is required for structures and utilities located behind the proposed wall, as the system requires some soil deformation to mobilize resistance. Deflections can be reduced by post tensioning but at an increased cost.
- Existing utilities may place restrictions on the location, inclination, and length of soil nails.
- Soil nail walls are not well-suited where large amounts of groundwater seeps into the excavation because of the requirement to maintain a temporary unsupported excavation face.
- Permanent soil nail walls require permanent, underground easements.
- Construction of soil nail walls requires specialized and experienced contractors.
Soil nail wall construction sequence
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