SOIL NAIL RETAINING WALLS
Soil nailing is a stabilization process used to build retaining walls or shoring systems when space is limited. The process starts by installing threaded steel bars into slopes or cuts made into the soil. Grouted bars are installed to create a stable mass of soil. Then a temporary or permanent facing is applied to retain and support the existing soil.
The face is typically made of shotcrete and reinforced with woven mesh over steel plates. Permanent walls are usually built with a cast-in-place face over the wall surface. Often, the permanent walls are made of decorative stone or other facing added on top of the shotcrete surface.
Engineers and other experts familiar with this type of construction must analyze the site and develop a site-specific nail placement design, including their correct depth, angle, and frequency. This ensures that the structure remain stable through any forces acting upon it.
Soil nail walls should be constructed in ground where a 3- to 6.5-foot vertical slope can stand without support for up to two days during construction and is stable for the few hours it takes to drill and insert the nails. The depth of the cut layer depends on the soil's ability to stand unsupported while the nails are being inserted. Weathered rock, talus slope deposits, silts, clays with low plasticity that are not prone to creep, naturally cemented sands and gravels, heterogeneous and stratified soils, and some kinds of fine-to-medium homogeneous sand are suitable for soil nail construction. Soils not conducive to soil nail technology are soft plastic clays; peat/organic soils; loose, low-density, and/or saturated soils; and coarse sand and gravels that are uncemented or lack capillary cohesion.
Soil analysis is essential prior to soil nail construction. Among other considerations, experts must determine if the soil is "aggressive"; if it is, the nails need to be specially treated to prevent corrosion.
Drainage is a critical element in planning and construction. Most commonly, face drainage is used: a drainage element is placed behind the shotcrete wall covering the nailed structure. The drainage elements are installed from the top down as construction proceeds. Typically, synthetic strips or perforated pipes (8-12 in.) are installed, usually spaced about 5-6.5 feet apart. The water is collected at the wall base and channeled away. Alternatively, weep holes can be made through the face of the wall, used with or without perforated drainpipes. Whichever method is used, it’s vital to channel the water away from the wall so it doesn't collect behind it.
Soil nail construction has many advantages:
- Soil nail walls can be built to follow curved or zigzagged outlines.
- The equipment used is highly portable and can fit easily into small spaces.
- The process is flexible and makes modifications easy to carry out (e.g., nails can be moved as needed during construction).
- Construction causes less noise and traffic obstruction on highways.
- The process creates less impact on adjacent or nearby properties than do other construction methods.
- It generally requires less space and manpower.
Like everything, soil nail construction also has its drawbacks. Among the disadvantages:
- The method cannot be used at sites where groundwater is a problem.
- It is inappropriate for sites with soils having very low shear strength, in sand and gravels that lack cohesion, and on sites with other unsuitable soils.
- Soil must be able to stand unsupported while it is being nailed and before shotcrete application.
- Good drainage is essential, especially for permanent structures and in places prone to freeze-thaw cycles.
Finally, it is imperative that projects be designed, constructed, and monitored by specialists with experience in soil nail wall construction.
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